Brown-headed Cowbird | Life Histories of North American Birds | A.C. Bent
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Life Histories of Familiar North American Birds
A chapter from the electronic book: Life Histories of Familiar North American Birds

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Brown-headed Cowbird
Molothrus ater [Eastern Brown-headed Cowbird]

[Published in 1958: Smithsonian Institution United States National Museum Bulletin 211: 421-450]

The two most characteristic habits of this bird are indicated in the above names. The Greek word Molothrus signifies a vagabond, tramp, or parasite, all of which terms might well be applied to this shiftless vagabond and imposter. It deserves the common name cowbird and its former name, buffalo-bird, for its well-known attachment to these domestic and wild cattle. The species is supposed to have been derived from South American ancestors, to have entered North America through Mexico, to have spread through the Central Prairies and Plains with the roving herds of wild cattle, and to have gradually extended its range eastward and westward to the coasts as the forests disappeared, the open lands became cultivated, and domestic cattle were introduced on suitable grazing lands.

The cowbird is unique in a family of nest-building birds; the blackbirds all build strong, well made nests, and the orioles show remarkable nest-building ability; the bobolink builds only a flimsy nest of grass on the ground, but the cowbird builds no nest at all, relying on other species to hatch its eggs and rear its young. Whether the cowbird ever knew how to build a nest, and if it did, how it happened to lose the art and become a parasite, probably never will be known, though some interesting theories on the subject have been advanced. Much light is thrown on this subject by Herbert Friedmann (1929) in his study of the South American cowbird, to which the reader is referred. For the benefit of the readers who do not own this interesting and comprehensive book, we shall quote from it freely.

In his chapter on the origin and evolution of the parasitic habit he writes:

The evidence points unmistakably to the view that the Cowbirds originally bred in normal fashion and that parasitism is a secondarily acquired habit. The reasons for making this statement are:

1. The instincts of nest-building and incubation are so universally present in all groups of birds in all parts of the world that it seems likely that this is the primitive condition of the Cowbirds.

2. All the Cowbird's close relatives are nest-builders; in fact, its family, the Icteridae, is known as a family in which the nest-building instincts reach their pinnacle of development. . . .

3. Within the general Agelaioides and Molothrus we find several stages in the evolution of parasitism exhibited by different species. The Bay-winged Cowbird, A. badius, uses other birds' nests and lays its eggs in them but incubates and rears its own young. Sometimes it makes its own nest. The Shiny Cowbird, M. bonariensis, is parasitic but has the parasitic habit very poorly developed, wasting large numbers of its eggs. Rarely it attempts to build a nest but in this it is never successful. This indicates that originally it built a nest but no longer knows how. The North American Cowbird, M. ater, is entirely parasitic but is not wasteful of its eggs. . . .

4. The parasitic Cowbirds (Molothrus) have definite breeding territories and are more or less monogamous. Howard has shown that the territory precedes the nest in the evolution of the instincts of guarding associated with reproduction. If the Cowbirds were parasitic from the very beginning it would be very hard to explain their territorial instincts. . . . The fact that the Cowbirds are fairly monogamous indicates that they were monogamous originally and probably nested in normal fashion as all monogamous birds do.

5. The most primitive of the existing species of Cowbirds is. . .the Bay-winged Cowbird. This species is the only one of its group that is not parasitic and doubtless represents the original condition of the Cowbird stock. . . .

From the above it seems safe to assume that parasitism is not the original condition in the history of the Cowbirds. The problem, then is not whether the Cowbirds were always parasitic or not, but how they lost their original habits and became parasitic. . . .

The best theory advanced as yet, and one which my studies tend to support in part, is that of Prof. F. H. Herrick. This writer studied the cyclical instincts of birds and found that not infrequently different parts of the cycle are interrupted by various causes which result in a general lack of harmony between successive parts of the cycle. He suggested that the parasitic habit may have originated from a lack of attunement of the egg-laying and the nest-building instincts which resulted in the eggs being ready for deposition before a nest was ready for them. . . .

The first writer to see that one explanation would not serve for all the different groups of parasitic birds was G. M. Allen (1925). . . . Wisely refraining from offering an explanation of parasitism, he suggests several "possible ways of origin." One of the possibilities is that parasitism may have arisen from the occasional laying of eggs in strange nests by birds that are very sensitive to the ovarian stimulus provided by the sight of a nest with eggs resembling their own. This is substantiated by experimental evidence collected by Craig who found that in doves ovulation could be induced by comparable stimuli.

Otto Widmann (1907) offered the following interesting theory to account for the origin of the parasitic habit:

We know that fossil remains of horses, not much unlike ours, are found abundantly in the deposits of the most recent geological age in many parts of America from Alaska to Patagonia. It was probably at that period that the Cowbird acquired the habit of accompanying the grazing herds, which were wandering continually in search of good pasture, water and shelter, in their seasonal migrations and movements to escape their enemies. As the pastoral habit of the bird became stronger, it gave rise to the parasitic habit, simply because, in following the roving animals, the bird often strayed from home too far to reach its nest in time for the deposition of the egg, and, being hard pressed, had to look about for another bird's nest wherein to lay the egg. . . . By a combination of favorable circumstances this new way of reproduction proved successful, and the parasitic offspring became more and more numerous. In the course of time the art of building nests was lost, the desire to incubate entirely gone, paternal and conjugal affection deadened, and parasitism had become a fixed habit.

Dr. Friedmann (1929) disposes of this theory as "more interesting than suggestive," and adds: "It is somewhat surprising to find a naturalist of Mr. Widmann's ability advancing such a theory. Probably he meant it more as a suggestion to be taken for whatever it might be worth than as a real attempt at an explanation." The trouble with the theory is that we have no known facts on which to base it, there being no record of a cowbird leaving its nest to follow cattle, horses, or bison. Probably the parasitic habit was developed before the cowbirds invaded North America. And we do not know to what extent the primitive cowbirds, in South America, had developed the habit of following the wandering herds.

Dr. Coues (1874) makes the following suggestion:

Ages ago, it might be surmised, a female Cow-bird, in imminent danger of delivery without a nest prepared, was loth to lose her offspring, and deposited her burthen in an alien nest, perhaps of her own species, rather than on the ground. The convenience of this process may have struck her, and induced her to repeat the easy experiment. The foundlings duly hatched, throve, and came to maturity, stamped with their mother's individual traits--an impress deep and lasting enough to similarly affect them in turn. The adventitious birds increased by natural multiplication, till they outnumbered the true-born ones; what was engendered of necessity was perpetuated by unconscious volition, and finally became a fixed habit--the law of reproduction for the species. Much current reasoning on similar subjects is no better nor worse than this, and it all goes for what it is worth.

The weakness in this theory is that such cases of adventitious laying in alien nests must have been very rare at first, and the inherited tendency to repeat the experiment would soon disappear by cross-breeding with individuals of normal breeding habits, unless the habit proved to be beneficial to the species, and no such proof is evident. We frequently find fresh eggs of robins and other birds laid on the ground, but failure to reach their nests has never developed parasitic habits in these birds. ***

Spring.--The eastern cowbird has not far to go on its spring migration. It is one of the earlier migrants, leaving its winter range in the southern states during March and reaching the northern parts of its breeding range during the first 2 weeks in April, or sometimes before the end of March.

According to Friedmann (1929):

The Cowbird migrates by day, early in the morning and late in the afternoon. I know of no data to show that this species indulges in nocturnal migration, but it may do so to some extent. . . . The Cowbird commonly migrates with the Red-winged and Rusty Blackbirds and the Grackles; in fact these three are usually found together. Other less common associates are Meadowlarks and Robins in the east, and Brewer's and Yellow-headed Blackbirds in the west.

Doubtless many Cowbirds succumb annually to the perils attendant upon migration but so far as I have been able to find there are no definite records of such happenings. Because they migrate chiefly by day no Cowbirds have been picked up dead around lighthouses or the bases of tall monuments and buildings. Bendire (1895), however, tells of one that was blown out to sea and came aboard a vessel, "fully 1,000 miles east of Newfoundland."

During his studies of the cowbird at Ithaca, N.Y., Friedmann (1929) divided the spring migration into six more or less separate phases, much like similar phases in the migration of the red-winged blackbird. The first to appear were the vagrants, at dates ranging from March 1, 1919, to March 14, 1922; these were wandering individuals coming before the true migration, consisting mostly of males, which were usually not in song and did not display. The second group to come were the migrant males, passing through on their way to points farther north; they were arriving and departing at various dates ranging from March 20 to April 27. "These birds usually are seen scattered among flocks of Red-wings, not forming any solid flocks of their own kind. They come to the marshes to pass the night with the Red-wings and the Rusty Blackbirds but during the day scatter over the fields on the uplands, where, in small groups, they forage for food."

The third phase is marked by the arrival of the resident males, which come on the average between March 23 and April 8.

The resident males on arrival at once establish themselves on their posts and remain in their territories for a few hours early in the morning every day, spending the greater part of the day in foraging around for food, often going to stubble fields, and to plowed areas a little later in the season. As the season advances they spend more and more time in their respective territories and at the time the resident females arrive the males are spending at least half of each day within the limits of their areas. . . . The testes of these resident birds are noticeably larger than those of the migrants, and as all the birds collected at dusk in the marshes averaged smaller testes than those shot from their singing trees, it seems that the two groups do not associate in the marsh, the resident birds sleeping in their territories.

The migrant females are the first of that sex to arrive, passing to more northern points between March 23 and April 28. At first they appear as lone individuals among the flocks of migrating redwings, grackles, and cowbirds, but later they pass through in flocks of a dozen or more.

The fifth group is made up of resident females, arriving between April 3 and 11. "The arrival of these resident females acts as a spark to set off the pent-up energies and passions of the males and arouses them to an unbelievable frenzy. The persistence and determination with which the resident males pursue these females makes one wonder if either ever rest. The females evidently recognize the limits of the territories of their respective pursuers as they usually fly in wide circles closely followed by one or more males, but do not leave the general vicinity of their territories. The ovaries of these resident females are considerably larger than those of migrant birds of the same date."

The sixth and last phase of the migration marks the arrival of what are apparently the immature males and females. "About the beginning of May or even the last of April all the resident birds are established and no more migrants are to be found coming in to the marshes. There is a decided slump in the migration lasting until about the second week of May when suddenly there appear numerous flocks of Cowbirds of mixed sexes in the upland fields, around the cattle, and near barns. . . . The gonads of these birds are smaller than those of the only other Cowbirds then present (resident birds)." The males of this group do not seem to take much notice of the females, whereas the resident birds do, and, to some extent, so do the last of the migrant males. "These two facts," says Friedmann, "point to the conclusion that these birds are really the immature individuals."

Territory.--Friedmann (1929) proved to his own satisfaction that both the female and the male cowbird are confined to a definite breeding territory:

Not only has the female a definitely marked off breeding area, but the male has a definite post, entirely comparable to the "singing tree" that Mousley describes. During the summer of 1921 numerous individual male birds were seen daily on certain trees or on definite telegraph poles. From these perches they would sing and display; they might fly off but would soon circle around and come back. There was no question but that they were tied down to their respective singing trees. In one case the identity of the male was made certain because of a peculiar harshness of his song, and as this individual was to be found daily in the same tree, it seemed safe to assume that each day the same bird was seen at a given perch. Not only was a certain tree used by each male, but a certain part seemed to be preferred, usually the higher branches. . . .

That the female has a definite territory is not so easily noticed as she has no "singing tree," and is, as in most birds, less conspicuous and less often seen than the male. . . .

At Ithaca in the late spring and summer of 1921 I found that certain females (probably the same one in a given place each day) seemed to have definite territories. Just off the northeastern corner of the main quadrangle of the Cornell campus is a small body of water called Beebe Lake. One pair of Cowbirds stayed on the north shore of the lake, another pair on the south shore. I was sure that the birds I saw on the north shore were not the same as those of the south side because on several occasions I saw the pair on one side and simultaneously heard or saw the birds on the opposite shore. All the Cowbird eggs found in each territory were very similar to each other and uniformly different from those found in the other. . . .

The size of the territories is very variable, some being a mile or more long and comparatively narrow, others. . . much smaller. . . .

The Cowbirds do not make any very spirited attempts to defend their territories and consequently in regions of unusual abundance the territorial factor is much less noticeable. I have never seen Cowbirds fight and their method of defense is restricted to an intimidation display.

The females are probably not always confined to definite territories for their egg laying, for eggs evidently laid by two different females are often found in the same nest.

Mrs. Amelia R. Laskey, of Nashville, Tenn., has sent me notes on her 3-years' study of cowbird behavior. She remarks on territory and mating: "In the area about our home, in each of the three breeding seasons, one male and one female became dominant. This area may be called a 'domain' rather than a territory. The dominant male and the dominant female used this area in their pair formation and mating. They did not drive others from the food in the domain, carried on no boundary line defense, but tolerated both sexes in social contacts, feeding and flying together. I believe these birds displayed vestigial territory behavior in intimidating others so as to keep the domain for their own use in pair formation and mating, and this behavior perhaps may function to some extent in keeping other females from utilizing host nests within the domain of the dominant female.

"All evidence indicated that pairing and monogamous mating generally prevail. Although both sexes on many occasions throughout the breeding season associate in trios or larger groups, there was no indication of polygamous or promiscuous mating."

Courtship.--Friedmann (1929) noted three types of courtship display, the terrestrial, the aerial and the arboreal displays. He describes the first of these as follows: "The male would run alongside of the female, and when slightly ahead of her would turn a little so as to be placed somewhat diagonally to her, and would then ruffle the feathers of his neck and the interscapular region. Then he would bow or bend down his head a little and emit his squeaky, shrill note--pseeee. The wings and tail are not involved in the terrestrial mode of courting." Charles W. Townsend sent Friedmann the following note on this performance: "April 9, 1922--Three males and one female busily engaged in eating in a field. Every now and then a male would look up, puff up feathers, spread wings and tail and fall on head. This is evidently the bowing, as in trees, where he does not fall. Since this time I have seen the performance several times and it always impressed me as a falling and being stopped by his head and breast striking the ground. . . . It seems to me that the tree act is a low bowing, while the on the ground act is an actual fall, for the bird suddenly lets himself go and brings up against the earth, a comical procedure."

Of the aerial display, Friedmann (1929) says: "The display of the males in mid-air consisted of ruffling out the feathers of the neck, interscapulars and throat, bending down the head, and arching the wings more than in usual flight, and giving their squeaky song. During the instants when the wings were arched in display the flight seemed unsteady, a sort of half-hearted attempt at a glide during an unaccustomedly long interval between wing beats." Two males that he watched following a female "seemed to display and sing in unison. The two males and the female kept on flying back and forth, at an altitude of about two hundred feet, from 7:15 a.m., when they were first seen, until about 8:30 a.m., when they were last seen, without resting or alighting even for a second."

Of the arboreal display, which is the commonest and best known, Friedmann writes:

The display is often, but not always begun by the bird pointing its bill toward the zenith. This is usually done whenever another bird, especially another male, is very close to the displaying bird. Next it fluffs out the feathers of its hind neck, breast and sides and flanks. . . . It is during this part of the display that the bubbling guttural notes are given. Wetmore has written it 'bub ko lum' and I cannot improve on his description. These notes are quite low and not audible in the field at a distance of more than fifty feet. During this stage of the performance the bird sometimes rises and falls gently on its legs in a vertical direction, the rise hardly ever amounting to as much as the length of the tarsus.

After this the bird begins the display proper by arching its neck and spreading its tail. . . . Then it begins to raise its wings and bend forward. . . . All this time the feathers of the back are fluffed out just as are those of the underparts. Then the wings are brought out to their full expanse. . .and the toppling over proceeds from now on with accelerated rapidity, the tail being lifted before the body pivots and swings over. . . . The display ends when the wings are brought back to the body. . . . The bird then rights itself and is ready to repeat the whole performance.

The entire display lasts about three or four seconds and the 'tseee' note [see under "Voice"] usually has a duration of about a second or a little over. The frequency of display is extremely variable. I have seen a male display with almost clock-like regularity at intervals of five seconds for several minutes when no female was in sight, and I have also watched a male display once and not do it again for over an hour. Display becomes less and less frequent as the season wears on and is usually not indulged in to any extent after the middle of June, while song continues until a month later. Those displays that are given after the middle of June are usually incomplete. This incomplete display consists of spreading the tail, hunching the back and slightly arching the wings, but the bird does not fall forward.

A male bird, observed by Wetmore (1920) in New Mexico, "would sit quietly for a few seconds, then expand the tail and draw the tip slightly forward, erect the feathers of the back and to a less extent those of breast and abdomen, and then sing bub ko lum tsee. In giving the first three notes he rose twice to the full extent of his legs and sank back quickly."

C. J. Maynard (1896) describes the courtship flight as follows:

Two or more males often pay their attentions to one female, singularly, without attempting to quarrel, when she will suddenly take wing and all will start in pursuit. The flight of a female at this time is exceedingly swift, for she will usually manage to keep ahead of her followers who ardently press on, giving a rather sharp, prolonged cry as they dart through the air. All the males within hearing join in, and it is not unusual to see a half dozen at a time after one of the other sex who will lead them a long chase, now darting upward to a considerable height, then doubling, will glide through the tangled branches of a clump of trees, emerging on the opposite side with great rapidity. This exciting race is evidently maintained merely as a matter of sport, for when the object of chase becomes weary she will quietly settle on the branch of a tree, and her admirers gather around her, calmly arranging their feathers. After resting for a time one will commence his gallantries once more, when the female darts into the air again and the males dash vehemently after her as before.

In this connection, it may be well to consider the sex ratio and the sex relations. The prevailing impression that the males far outnumber the females is probably more apparent than real, for the males are more conspicuous and less retiring; Friedmann (1929) says: "From my observations I would put it as about three males to two females." The sexual relations of the cowbirds may not be above criticism, but they are probably not as bad as they are often painted. Cowbirds have been called monogamous, polygamous, polyandrous, and even plain promiscuous; probably any one of these terms could be applied to certain individuals under certain circumstances; but there is much evidence to indicate that the cowbird was originally monogamous and is so by preference today in most cases.

Friedmann (1929) writes:

At Ithaca I have found that each male and each female has a definite territory. . .and that there is a more or less definite pairing between the birds. My experience has been that if the birds are not strictly monogamous, at least the tendency towards monogamy is very strong. My observations have been supported by those of Dr. Alexander Wetmore, of the Smithsonian Institution. He informs me that in Utah he had exceptionally favorable conditions for observing the sexual relations of the Cowbirds, and that in a relatively small area (which was quite open and made observation of the birds an easy matter), he watched six pairs of Cowbirds, each pair having their own territory, and the birds remaining true to their mates. The male of pair A stayed with female A, and did not consort with any of the other five females.

At Lake Burford, N. Mex., Wetmore (1920) noted that a pair of cowbirds mated on June 2, "remained constantly nearby for ten days or more. On June 5 and 6 a second female appeared and fed with the others. The male was seen running at them with his bill pointing straight in the air and then pausing to sing and display. The second female disappeared at once while the pair remained together until June 13."

Cowbirds are often seen in small flocks even during the breeding season, which might give the impression of loose sexual relations, and it is well known that, if one of a pair of mated birds is killed, the survivor secures a new mate in a surprisingly short time, showing that there is always an available supply of unmated birds ready to fill in the gap. These flocks are probably made up of such unmated, surplus individuals and are usually seen in places where there are few or no nests; they are not, therefore, breeding birds. Moreover, these flocks may consist of immature, 1-year-old birds, which cannot be distinguished in the field from adults, which have arrived later in the season than the adults and have not mated.

As evidence of polyandry, Friedmann (1929) relates the following experience:

A pair with whose territory I was fairly familiar was noted several times and each time there was just a single male and a single female. The male used to stay in his singing tree and so was easy to find. The female, when wanting her mate, would fly into the open and give her flight rattle. The male would quickly take off after her. One day it was noted that when the female called for her mate he came directly from his favorite perch as always but another male, new to the territory, also answered her summons. This interested me not a little and I went back there the next day and waited for the female to call for her mate. Again both males answered her summons, the original male coming as always from his singing tree and the new one from a tall tree near a railroad track. Some time later in the afternoon the original male was seen again in his singing tree and the second male was noted in the tree from which he had flown in answer to the call of the female. On the next two successive days this male was seen in or near this tree and it certainly looked as though he had established himself there. A week later the place was revisited and both males were found, each in his own tree and both again answered the summons of the female.

This apparently was a case of polyandry--an unmated male established himself in the territory of a mated pair. He was not there originally as the pair had been watched considerably before his advent. The original male seemed not to mind the presence of the other. However, no actual intercourse between any of the birds was observed.

Mrs. Nice (1937) writes: "With a small population of Cowbirds, this investigator found the species predominantly monogamous, with some tendency towards polyandry. But here on Interpont, with an abundance of Cowbirds, promiscuity prevails just as the older writers maintained. A banded male has been seen with three different females and one unbanded female, while banded females are seen with varying numbers of males from one to five." And Forbush (1927) says: "Cowbirds are free lovers. They are neither polygamous nor polyandrous--just promiscuous."

Nesting.--The remote ancestors of the cowbirds may have been and probably were nest builders, incubating their eggs and rearing their own young, as other birds do. It is difficult to imagine how they could have evolved otherwise. I once saw a poor apology for a nest that I thought might have been built by a cowbird. While driving across the North Dakota prairies, on June 14, 1901, we saw a crude bunch of straws and dried grasses lodged in a bush; it had the appearance of a roughly built nest, but it was too large and bulky and too loosely and carelessly put together to have been built by any other bird in that region; a hollow in the center held a single egg of a cowbird. As cowbirds were abundant in that section and other nesting birds were scarce, it occurred to me that perhaps a cowbird, being unable to find a suitable host, had made an attempt to build its own nest. It is more than likely, of course, that some small animal may have placed the material there and the cowbird had mistaken it for a bird's nest. We were too far away from any human habitation for any man or boy to have put it there, so I will let the reader decide how it got there; I offer it only as an interesting suggestion.

Regardless of what significance the above suggestion may have, our North American cowbirds, as we know them today, are all wholly parasitic, laying their eggs in the nests of other birds and leaving their young to be raised by their foster parents. In the long list of birds thus imposed upon, the vireos, the wood warblers, and the small sparrows figure most prominently. No attempt will be made to list here all of the victims of the cowbirds; this has been well done by Bendire (1895) and more thoroughly done by Friedmann (1929). The latter makes the following general statements as to the families afflicted:

Most of the victims of the Cowbird are contained in four families--the tyrant flycatchers, the finches, the vireos, and the warblers. Of the thirty-six species and subspecies of tyrant flycatchers in the North American fauna eleven are known to be parasitized, while of the remaining twenty-five, seven do not breed within the breeding range of the Cowbird. . . . The Cowbird is known to victimize sixty-two species and subspecies of finches. The total number of North American forms of this great family is one hundred and ninety-four, of which about a hundred are not known to breed within the range of the Cowbird. Yet the family is one of great importance to the parasite as some of its component species are very frequently victimized. The Cowbird is probably one of the chief factors in checking the increase in the smaller Sparrows and Finches.

The Vireos, while relatively few in species, are nevertheless a very important factor in the natural economy of the Cowbird, and the latter is undoubtedly the most serious single enemy of the birds of this family. No birds are more frequently affected, either absolutely or relatively, and none make less protest at the frequent impositions of the parasite. Of the twelve species of Vireos in the North American Check-List, nine are known to be victimized. Including subspecies, twelve of the twenty-five forms are included in the present list of victims. Of the remaining thirteen, five do not breed within the Cowbird's range and six others have ranges which only slightly coincide with that of Molothrus.

Of the fifty-four species of Warblers in the North American fauna thirty-six are known to be more or less imposed upon by the Cowbird, and, of the remaining eighteen, at least ten do not breed in any part of the Cowbird's range, or are, at most, so rare that the absence of records means nothing. The other eight are still little known and very few of their nests have been found. Including subspecies, forty-four, of the seventy-three kinds of Warblers, are included in the list of the victims of the Cowbird.

Of other families of less importance in the economy of the cowbird, he says that five species of mockingbirds and thrashers are rarely victimized by the Cowbird; with only one of the five is the Cowbird definitely known to be successful. He continues:

The Wrens are almost negligible factors in the ecology of the Cowbird, and the latter is of no great consequence in the life histories of these birds. Four of the fourteen species in the A.O.U. Check-List are known to be victimized by the Cowbird, all of them very infrequently. Counting subspecies, six, of the thirty-six in the North American fauna, are included in the roll-call of the Cowbird's victims.

Paridae are of little importance in the economy of the Cowbird, and the latter plays an inconsequential role in the lives of the Titmice. Five species of the eleven in the A.O.U. Check-List are recorded as victims of the Cowbird; all of them uncommonly.

The kinglets and gnatcachers, are, with one exception, infrequently molested by the Cowbird. They are interesting in that they are among the smallest birds definitely known to be affected by the parasite. Three of the six species in the North American fauna are included in the list of hosts: one of the three being represented by two geographic races.

The Thrushes are of considerable importance in the natural history of the Cowbird, and are among the largest birds commonly and regularly parasitized. Not only do we often find Cowbirds' eggs in the nests of some of these birds, but frequently they may be seen caring for the young Cowbirds. Even here, where the rightful young are of approximately the same size as the young parasites, it is rather unusual to find any but the Cowbird surviving in a victimized nest. . . .

Seven of the fifteen North American species in this family are more or less imposed upon by the Cowbird, two of them being represented in the present list by two races each.

Bendire (1895) listed 91 species and subspecies victimized by the eastern cowbird. *** Friedmann (1929) listed 114 species and subspecies as victims of the eastern cowbird, adding three species, the ruby-throated hummingbird, Nelson's sparrow, and the brown creeper as hypothetical. In subsequent papers (1934, 1938, 1943, and 1949), he increased the known hosts of the eastern cowbird to 149 forms.

Cowbirds' eggs are sometimes found in nests of birds that are wholly unfitted to become foster parents for the young, in which cases the eggs never hatch or the young never survive. If the eggs of the owner of the nest are much larger than those of the parasite, the cowbirds' egg will not receive enough warmth from the body of the incubating bird to hatch. If the young of the selected foster parents are fed on food unsuitable for the young cowbird, the latter cannot be expected to thrive on it; one can hardly conceive of a mourning dove, which is listed as a victim, feeding a young cowbird on "pigeon milk," or of a kingfisher feeding it on fish. A swallow might hatch a cowbirds' egg and feed the young one in the nest, but not afterward, as young swallows are taught to feed on the wing. It is quite important for the cowbird to select an open nest of some altricial bird that feeds its young in the nest until they are nearly able to fly; the young of precocial birds leave the nest soon after they are hatched and the young cowbird would be deserted; the egg of a cowbird has been found in a killdeer's nest, but, if the egg ever hatched, the young must have been left in the nest to starve or die of exposure.

The temperament of the host species is also of importance; the hawklike character of the shrikes makes them absolutely free from cowbird molestation; and the pugnacious kingbirds are seldom imposed upon. Birds nesting in holes are mostly free from cowbird interference; Friedmann (1929) says that woodpeckers, house wrens, nuthatches, chickadees and bluebirds are "very seldom molested, in fact the Bluebird is the only one of these birds for which I have found more than a very few records." Some birds are intolerant of cowbirds' eggs; he mentions the robin, catbird, and yellow-breasted chat as "examples of absolutely intolerant species. Others such as the Yellow Warbler are intolerant to a certain extent." I should except the catbird, as I once found a catbird sitting calmly on a nest that contained four eggs of the cowbird and only one of its own! Friedmann adds: "Birds react to Cowbirds' eggs in several ways. The great majority of species seem not to mind the strange eggs in the least and accept, incubate and hatch them. Of these birds, some occasionally cover over the parasite eggs by building a new floor over them if they have no eggs of their own at the time. This is true of such birds as the Red-eyed, Warbling, Blue-headed and Yellow-throated Vireos, the Prothonotary, Yellow and Chestnut-sided Warblers and the Redstart. This has also been recorded in the following species--Meadowlark, White-crowned Sparrow, Cardinal and Indigo Bunting, but only a single time in each case."

He conducted experiments with a robin's nest and a catbird's nest to learn what the reaction of these birds to strange eggs would be. The robin threw out a song sparrow's egg, which was spotted much like a cowbird's, but accepted a chipping sparrow's, which was mainly blue like the robin's, though much smaller. The catbird ejected both the song sparrow's and the chipping sparrow's eggs. Of the yellow-breasted chat, he says: "The eggs of the Chat are very similar to those of the Cowbird, but nevertheless the nest is almost invariably deserted if a parasitic egg is laid in it. This is doubtless due to the extreme shyness and nervousness of the Chat, rather than to any superior ability to distinguish the strange eggs from those of its own. . . . Nevertheless on at least two occasions Chats have hatched and reared Cowbirds." Some other birds, perhaps more than we know about, eject the alien eggs from the nests. B. H. Warren (1890) says: "I have twice found broken eggs of Cowbirds on the ground near nests of the Yellow-breasted Chat, and on three occasions have discovered the shattered remains of these eggs directly beneath the pendant nests of Baltimore Orioles."

Among the birds that show their intolerance by burying the cowbird's egg in the bottom of the nest, or by building a second story over it, so that the alien egg fails to hatch, the yellow warbler is the star performer. Two-story nests of this warbler are fairly common, where cowbirds are numerous. . . three-storied nests are not very rare, and as many as four or even five stories have been built. In addition to those birds mentioned above as addicted to this habit, R. M. Anderson (1907) reports a "Traill Flycatcher's nest with a Cowbird's egg imbedded." C. R. Keyes (1884) found a scarlet tanager's "nest with a Cowbird's egg embedded in the bottom." Amos W. Butler (1898) reports a nest of the Maryland yellowthroat, containing three stories. "Two additional nests were built upon the original structure, burying beneath each the egg of a Cowbird." E. A. Samuels (1883) claimed to have a double nest of the American goldfinch in his collection, but this seems open to question, as the goldfinch usually nests later in the season than the cowbird.

The female cowbird is an expert nest hunter; in fact she has to be. Coues (1874) describes her nest hunting graphically:

It is interesting to observe the female Cow-bird ready to lay. She becomes disquieted; she betrays unwonted excitement, and ceases her busy search for food with her companions. At length she separates from the flock, and sallies forth to reconnoitre, anxiously indeed, for her case is urgent, and she has no home. How obtrusive is the sad analogy! She flies to some thicket, or hedge-row, or other common resort of birds, where, something teaches her--perhaps experience--nests will be found. Stealthily and in perfect silence she flits along, peering furtively, alternately elated or dejected, into the depths of the foliage. She espies a nest, but the owner's head peeps over the brim, and she must pass on. Now, however, comes her chance; there is the very nest she wishes, and no one at home. She disappears for a few minutes, and it is almost another bird that comes out of the bush. Her business done, and trouble over, she chuckles her self-gratulations, rustles her plumage to adjust it trimly, and flies back to her associates.

Russell T. Norris (1944) gives the following account of a cowbird laying in a song sparrow's nest, which was photographed by Hal H. Harrison:

Just before 4:30 a.m., about 22 minutes before sunrise, we heard the sputtering note of a Cowbird, and a few seconds later a female Cowbird alighted on the camera. After looking around cautiously, she flew to the ground at the base of the tripod and began to walk nervously toward the nest. As she reached the rim of the nest, she paused and carefully surveyed the surrounding territory, then stepped into the nest, and turned about several times. Finally she settled down, and Harrison pressed the button on the battery. As the flash went off, the Cowbird flushed. She had been on the nest no more than 15 seconds and had not deposited her egg. . . .

At 4:38 a.m. I noticed a movement in the grass behind the nest, and after a few seconds the Cowbird appeared. She approached the nest warily, stepped up onto the rim, and paused there. Then she entered the nest and began to turn about as she had on her previous visit. After a few seconds, she stepped back onto the rim and looked around. She three times repeated this procedure of standing on the rim, then uneasily turning about in the nest. In one instance she mounted the rear rim and looked back into the grass. At approximately 4:40 a.m. she settled on the nest and Harrison released the shutter. The Cowbird raised herself slightly but remained a few seconds before flying away. Upon examining the nest I found a fresh Cowbird egg. Undoubtedly the egg was being laid as the picture was taken.

He ends the story with the statement that: "Two sparrows and the Cowbird hatched and were reared successfully until the Cowbird was seven, the sparrows six days old, when the nest was destroyed by a predator."

Harry W. Hann (1941), in the studies of the cowbird at the nest of the ovenbird, came to the following conclusions:

1. The female Cowbird regularly finds the nest of the host by seeing the birds building.

2. She sometimes watches the building process intently and this doubtless stimulates the development of eggs, which are laid four or five days later. This theory, first suggested by Chance for the Cuckoo, accounts for the delicate synchronization of the egg-laying of the Cowbird with that of the host, and does not preclude the possibility of laying several eggs on successive days.

3. The eggs of the Cowbird are usually laid during the egg-laying time of the host, but exceptions are common. Extremes noted during the study were three days before the first Ovenbird's egg was laid, and three days after incubation began.

4. A Cowbird lays but one egg in a nest unless nests are scarce; in that case it lays more.

5. The female Cowbird makes regular trips of inspection to nests during the absence of the owners, between the times of discovery and laying, and knows in advance where she is going to lay.

6. Her regular time for laying is early in the morning before the host lays, and she will frighten the owner from the nest if she happens to be there first. . . .

7. The Cowbird is both alert and determined when she comes to the nest to lay. She moves about in the vicinity of the nest and looks carefully for as much as three minutes before entering, but will return to the nest if she is frightened away.

8. She spends from a few seconds to a minute in the nest when laying and flies directly from the nest as soon as the egg is laid.

9. The Cowbird disturbs the nest of the Oven-bird but little when she enters to lay, and I have found no broken eggs which were attributable to her entering.

10. Parasitized nests regularly have one or more eggs removed by the female Cowbird. These are not removed at the time of laying, but during the forenoon of the previous day, or the day of laying, or rarely on the following day. . . .

11. Eggs removed are eaten by the Cowbird, but are not removed for that purpose alone or their disappearance would not be correlated so closely with the laying of her own eggs. The number of eggs removed from parasitized Oven-birds' nests was eighty-five percent of the number of eggs laid [by the cowbird] and included four eggs of the Cowbird itself. From nonparasitized nests of the Oven-bird only a single egg disappeared during the study.

12. The statement by Burroughs that a Cowbird takes an egg from a nest only when two or more eggs are present is borne out by this investigation.

Mrs. Nice (1939) has twice seen a cowbird remove an egg from a song sparrow's nest, "The thief eating the egg and shell," in one case. And T. S. Roberts (1932) has seen one remove an egg from a scarlet tanager's nest and from the nest of a chipping sparrow. W. V. Crich, of Toronto, has sent me a photograph of a cowbird's egg in a last year's nest, showing that this clever nest hunter sometimes makes a mistake.

Eggs.--To determine the number of eggs laid by the cowbird during a season is a question that cannot be answered with certainty, but we have some data indicating that it lays no more eggs than many other passerine birds. Friedmann (1929) made a careful and thorough study of three well-known cowbird territories in which all or nearly all of the nests were located and in which the eggs of the three different females could be recognized. Of these he says:

Two of the birds laid five eggs each and the other laid four. In one of these cases where five eggs were laid (Terr. A.), I found no more after the fifth one although a great deal of time was spent in this breeding area. It was just because no egg was found on the sixth day that I kept very close watch of the bird and its territory on that and the following day. On the day this individual laid its fifth egg the other "five-egg" bird, (B),  laid its first. For four days thereafter this bird (B) laid an egg daily and no more were found for individual A. On the day Cowbird B laid its fifth egg a heavy storm broke out and for a month and a half thereafter it rained more or less violently every day. As fast as nests were found, they were destroyed or washed away by the heavy rains, and, of course, it became impossible to keep any check on the actions of the Cowbirds. . . .

Above it was stated that no eggs were found in the territory of Cowbird A after that bird had laid its fifth. Of course the mere fact that none were found is no indication that none were laid. However, the second of the "five-egg" birds (individual B) was collected five days after it had laid its fifth egg. Only five discharged egg follicles were found in its ovaries and the oldest of these follicles was still very prominent so that if any more eggs had been laid, follicles would have betrayed the fact. This shows pretty conclusively that only five eggs were laid in the case of this bird. This, together with the fact that bird A was known to lay at least five eggs and, judging by the four-day rest (?) after the laying of the fifth egg, probably did not lay any more, suggests the idea that five eggs may possibly represent what in other birds would be called a clutch, although this is doubtful. We cannot be certain that Cowbird A laid only five eggs although I feel that I would have found at least one more egg in the four days between the fifth egg and the stormy season, if the bird had kept on laying. . . .

A record of the eggs laid by each bird and the nests used may be of some interest. Cowbird A laid its first egg on May 23 in the nest of a Chestnut-sided Warbler; its second egg, May 24, in a Veery's nest, its third, May 25, in another Veery's nest, its fourth, May 26, in a nest of a Redstart, and its fifth, May 27, in the same Redstart's nest.

Cowbird B laid its first egg May 27 in a nest of a Redstart, its second egg May 28 in the same nest, its third, fourth, and fifth on May 29, 30, 31, respectively, all in one nest of a Red-eyed Vireo.

Cowbird C laid its first egg May 22 in a Veery's nest, its second, May 23, in a nest of a Redstart, its third, May 24, in the same nest, and its fourth and last recorded egg, May 25, in a Red-eyed Vireo's nest.

From the above it may be seen that the cowbird lays one egg each day, and that it is not specific in its choice of hosts. As evidence that the cowbird may sometimes lay more than five eggs, Friedmann quotes F. L. Rand, who had kept cowbirds in captivity, as saying that "a little hen Cowbird that had the liberty at all times in a suite of rooms, was tempted by me to enjoy as nearly as possible its natural bent in the direction of egg laying and the results obtained in the way of information were somewhat surprising. Eight or ten last year's nests were placed around the room, with dummy (candy) eggs in them; each morning about six o'clock the little hen would seek some nest or other in which it would drop her egg, but not always in the same nest; often times, the candy egg would be found on the floor; so, in fourteen successive days, the little hen had laid thirteen eggs; this would indicate apparently that the destructive nature of the bird is even greater than it has been thought to be."

Lawrence H. Walkinshaw (1949) concluded during his studies in Michigan, that 25 eggs were laid between May 25 and July 20, 1944, by a single cowbird "because (1) they were very similar in coloration (2) no two were laid on the same day (3) the length of 11 similarly colored eggs had significantly less variability than the length of 22 non-similarly colored eggs."

As to the number of eggs laid in any one nest, we have plenty of positive information; Friedmann (1929) writes on this subject:

In order to determine definitely whether or not the Cowbird normally lays but a single egg in a nest, data on approximately nine thousand victimized nests of one hundred and ninety-five kinds of birds were assembled, and it was found that in over two-thirds of the cases only one parasitic egg was found in a nest. This shows pretty clearly that the normal, the usual, the characteristic thing is for a Cowbird to deposit one egg in a nest.

Nevertheless, about a third of the nests held more than a single Cowbird's egg apiece. This is no inconsiderable number or percentage of exceptions to the above rule, and calls for an explanation. It has been shown that under normal conditions each pair of Cowbirds has a more or less definite territory and that the female tends to restrict herself to nests within that particular area. However, the cyclical instincts of the female are so aborted that she is probably quite easily induced temporarily to forsake her territory long enough to deposit an egg in a near-by nest. . . . Again, in regions where the Cowbird is very common (and this applies to a great part of its range) and territories are apt to overlap in this way two Cowbirds may make use of the same nest. In this way I believe we can account for the fact that not infrequently eggs of two, or more rarely even three, different individuals are found in the same nest."

Furthermore, a cowbird may lay more than one of her own eggs in the nest, provided she can find at the proper time no nest in which she has not already laid.

Mrs. Nice (1939) says: "During seven years' study on Interpont, 98 of the 223 Song Sparrow nests located contained Cowbird eggs; 69 held one egg, 26 held two eggs and three held three. Only once did I find four Cowbird eggs in a single nest; this happened in June, 1928, and the nest belonged to a Maryland Yellowthroat. ***

One cowbird's egg in a nest is evidently the prevailing rule, but two eggs, often laid by two different females, are not a rare occurrence and we have numerous records of three or more in a nest. A. C. Reneau tells me that he has twice found three eggs in the nest of a phoebe and once in the nest of a Bell's vireo. A. D. DuBois' list contains a record of three eggs in a cardinal's nest, three in a wood thrush's nest and five in another nest of a wood thrush. Frank R. Smith writes to me of three eggs in the nest of a wood thrush, deposited before the thrush had laid any eggs, and apparently laid by different females. T. S. Roberts (1932) says: "Three or four are uncommon, though Mr. Kilgore and the writer once found a Wood thrush's nest containing two eggs of the Thrush and six of the Cowbird, the latter of two distinct patterns, suggesting that two Cowbirds had laid three eggs each in this nest. . . . At Mille Lacs on July 7, 1934, Mr. Marius Morse found a Willow Thrush's nest containing two eggs of the owner and eight of the Cowbird. . . . Apparently four different Cowbirds had laid one or more eggs each."

J. H. Langille (1884) writes: "I have frequently found more than one in the same nest; once not less than four in the nest of a Scarlet Tanager, which had only room enough left for two of her own. Mr. Trippe once found a Black-and-white Creeper's nest with five of the eggs of the interloper and three deposited by the owner." Isaac E. Hess (1910) mentions a scarlet tanager covering four eggs of the cowbird and an oven-bird's nest that contained seven eggs of the parasite. I have already mentioned the case of a catbird sitting on four cowbird's eggs and one of her own. F. A. E. Starr writes to me: "I once found a red-eyed vireo's nest with the vireo sitting on six cowbirds' eggs and none of her own. A nest of the Wilson thrush was found containing one egg of the thrush and four eggs of the cowbird." Sanborn and Goelitz (1915) report a towhee's nest that "contained one Towhee egg and eight Cowbird eggs."

Bendire (1895) describes the eggs as follows:

The shell of the Cowbird's egg is compact, granulated, moderately glossy, and relatively much stronger than that of its near allies, the Icteridae. The ground color varies from an almost pure white to grayish white, and less often to pale bluish or milky white, and the entire surface is usually covered with specks and blotches varying in color from chocolate to claret brown, tawny, and cinnamon rufous.

In an occasional specimen the markings are confluent and the ground color is almost entirely hidden by them; in the majority, however, it is distinctly visible. These markings are usually heaviest about the larger end of the egg, and in rare instances they form an irregular wreath. The eggs vary greatly in shape, ranging from ovate to short, rounded, and elongate ovate, the first predominating.

The average measurement of 127 eggs in the U.S. National Museum collection is 21.45 by 16.42 millimeters, or 0.84 by 0.65 inch; the largest egg measures 25.40 by 16.76 millimeters, or 1 by 0.66 inch; the smallest, 18.03 by 15.45 millimeters, or 0.71 by 0.61 inch.

Incubation.--Friedmann (1929) says that the incubation period of the cowbird "is ten days, about the shortest of any of our passerine birds." But Mrs. Nice (1939) says: "On Interpont with the Song Sparrow as host the Cowbird egg has never hatched in ten days. Sometimes it hatches in eleven days, sometimes in twelve, and occasionally in thirteen or even fourteen days. It requires about one day less of incubation than the Song Sparrow egg, hence it normally hatches first and the bird gets an advantage from the start. Some eggs have been laid after incubation has started; these have hatched from one to five days later than the Song Sparrows, and most of the little birds perished."

Hervey Brackbill gives me the following personal observation on the subject: "The incubation period that I observed for one egg at Baltimore was about 11 1/2 days. This egg was laid on May 18 before 8:45 a.m. (studies by other investigators indicate that laying is usually done at about 5 a.m.), and after steady incubation by a Wood Thrush hatched on May 29 at 1:25 p.m.; that is, at that hour I found the bird enclosed in only half of the shell and, when I touched it, it wriggled free of that. The incubation period of the thrush's own eggs was 12 to 13 days."

Young.--It will be seen from the above that the young cowbird usually hatches at least 1 day ahead of the young of its foster parents. It does not, apparently, make any effort to oust its nest mates, as the European cuckoo does, but it is so much larger than the young of the smaller foster parents, and grows so much faster, that some of the smaller young are often crowded out of the nest; also, it gets more than its share of the food brought to the nest, a condition that sometimes proves fatal to its nest mates. Mrs. Nice (1939) says on this subject:

Many writers assert that each Cowbird is raised at the expense of a brood of young. This is not true with Song Sparrows. Sixty-six successful nests without Cowbirds on Interpont raised an average of 3.4 Song Sparrows, while twenty-eight successful nests with Cowbirds averaged 2.4 Song Sparrows. So, taken by and large, each Cowbird was reared at the expense of one Song Sparrow. . . .

Song Sparrows often raise all of their own young that hatch along with a pensioner, anywhere from one to five Song Sparrows having been fledged in such nests. With two Cowbirds of like age in the nest, the Song Sparrows have been able to bring up only one or two of their own children. Smaller birds undoubtedly suffer more than do Song Sparrows, but there is little information on this subject. Once I found a nest containing three young Maryland Yellowthroats and a Cowbird just ready to leave.

T. C. Stephens (1917) studied the feedings of a young cowbird and two young red-eyed vireos in a nest of the vireo; during a period of a day and a half, the young cowbird received 58 percent of all the food; the cowbird was both older and larger than the vireos; the older vireo received 27 percent of the food and the younger one only 15 percent.

Alexander F. Skutch writes to me: "On May 20, 1931, I found under a bridge near Ithaca, N.Y., a phoebe's nest containing four of its own eggs and one newly hatched cowbird, half of its shell still remaining in the nest. On May 22, there were two young phoebes, hatched since the preceding day. The cowbird appeared to be fully four or five times their size. Its eyes were partly open and the sheaths of its remiges were sprouting. By May 25, one phoebe nestling and one of the unhatched phoebe eggs had mysteriously vanished. By May 28, the cowbird was well feathered, could perch, and showed fear. The young phoebe, six days old, was still blind and was very weak and helpless; its pin feathers were just sprouting. On May 29, I found the cowbird on the rim of the nest. The young phoebe's eyes were just opening. By May 30, the cowbird had left the nest, aged ten days. By June 3, the phoebe was well feathered. By June 6, the young phoebe had departed, at the age of fifteen days. The parent phoebes were feeding the young cowbird nearby, and the female had already laid a new egg in the nest! Returning on June 12, I found her incubating five eggs of the second brood in the old nest.

"On July 29, also near Ithaca, I found a nest of the red-eyed vireo with two young vireos and one cowbird, all about a week old. By August 2, the cowbird had left the nest; the vireos, still inside, were in a thriving condition and seemed about ready to depart."

Friedmann (1929) made a careful study of the growth and development of the young cowbird in the nest; it is not essentially different in pattern from the development of other young passerine birds, except for its increase in weight--the really important factor in its survival: "This Cowbird, probably less than an hour old, weighted 2.5 grams. . . . The average weight of a day old Cowbird is 4.5 grams. . . . At the end of the second day the young Cowbird may weigh from 7.5 to 8.5 grams. . . . For the first two days, the daily increase is close to 100 percent, but from then on the rate is slower, averaging about 50 percent on the third day and gradually lessening until it comes to be about 10 percent on the eighth day and only 5.5 percent on the ninth. When the Cowbird leaves the nest it averages about 33 grams or approximately 13 times its weight on hatching."

The rate of growth varies considerably, depending largely on the kind and amount of food furnished by different species of foster parents. A lone cowbird, in a nest by himself, grows faster than one that has competition from other nest mates.

Probably all the altricial species that successfully hatch a cowbird's egg feed the young imposter, or attempt to do so, while it is in the nest, but the larger species are not always successful in rearing it to a nest-leaving age. The smaller flycatchers, the vireos, the wood warblers, and the smaller sparrows are the most successful in this and therefore make the best foster parents. As Friedmann (1929) remarks: "Obviously only those species that serve as foster parents of the young Cowbird are important in the economy of the parasite. Of the 195 birds on the list, 91 have been definitely recorded as rearing young Cowbirds. Of the remaining number, a large number doubtless could, and do, act in this capacity but are less commonly victimized and so have been less often recorded."

In order successfully to rear a young cowbird until it attains its growth and is able to shift for itself, the foster parent must feed it for some time after it leaves the nest. The smaller flycatchers, the vireos, the wood warblers, and the smaller sparrows that have acted as hosts usually do this, and most of them have been definitely recorded as doing so, as have also the house wren and the Carolina wren. Milton B. Trautman (1940) lists, among the larger birds observed feeding fledgling cowbirds out of the nest, the catbird, eastern robin, wood thrush, starling, yellow-breasted chat, eastern cardinal and red-eyed towhee; most of these were observed in the act only once or twice, but the cardinal was seen more than 13 times. Baird, Brewer, and Ridgway (1874) mention that J. A. Allen saw a brown thrasher feeding a nearly full grown cowbird. Probably the above lists could be considerably enlarged.

In at least three cases, a female cowbird has been seen to feed a young cowbird that was supposed to be its own young, but in no case could the relationship be proven. J. R. Bonwell (1895), of Nebraska City, Nebr., reported seeing a female cowbird feed a young cowbird in a nest with young rose-breasted grosbeaks. "Nearly every evening she would come and feed the young Cowbird, but if the young Grosbeaks would open their mouths for food she would peck them on the head and refuse them food." Forbush (1927) mentions two other cases. He knew Mason A. Walton, of Gloucester, well enough to accept his report of seeing a female cowbird feed a young cowbird in a yellow warbler's nest, but he had no way of knowing that she was the parent.

The other case cited was based on a careful observation by Laurence B. Fletcher (1925), who trapped a female cowbird with a young one, saw the female feed it, banded both birds and saw the two together afterward, the same adult still feeding the young. The adult, which bore Biological Survey band No. 64782, continued to feed the banded fledgling and no other, although there were other young cowbirds near.

None of these observations prove that the female cowbird recognizes its own young; but they do indicate a lingering vestige of the lost maternal instinct.

Since the above was written, Russell T. Norris (1947) has published the results of his extensive study of the cowbirds of Preston Frith, to which the reader is referred. The conclusions he arrived at are not far different from what is indicated above, but the following paragraphs are of special interest:

Of 19 Cowbird eggs, one hatched four days before the host; 4 hatched one day before; 10 hatched the same day as the host; 3 hatched one day later than the host; and one hatched five days later than the host. . . . .

In the 237 observed nests, the hosts laid 668 eggs, of which 383 (57.3 percent) hatched; the Cowbirds laid 108 eggs, of which 46 (42.6 percent) hatched; 37.7 percent of the host eggs, 26.8 percent of the Cowbird eggs produced fledglings. Of the host eggs that hatched, 64 percent produced fledglings; of the Cowbird eggs that hatched 63 percent produced fledglings.

With four exceptions all parasitized nests that produced young produced at least one host young.

The 35 non-parasitized (successful) nests produced 2.94 fledglings per nest; 19 parasitized (successful) nests fledged 2.05 host young per nest, indicating that each parasite was raised at the expense of about one host young.

Plumages.--The plumage changes of the cowbird are simple. The natal down is described by Dwight (1900) as olive-gray. He describes the juvenal plumage in which the sexes are alike, as "above, including sides of head and neck, wings and tail, dark olive-brown, the feathers edged with pale buff, whitish on the primaries. Below, dull white, buffy on throat, breast and flanks much streaked with olive brown. Chin white or yellowish."

A complete postnuptial molt occurs in August or early September, producing a first winter plumage which is indistinguishable from that of the adult. In the male, the head, throat, and nape are purplish "clove-brown," but all the rest of the plumage, including the wings and the tail, is clear lustrous black with green and purple reflections. The female assumes at this molt the "mouse-gray" plumage of maturity. The nuptial plumage in both sexes is acquired by wear, which is not conspicuous. Adults have a complete molt in September and no prenuptial molt. The seasonal changes are inconspicuous.

Food.--Beal (1900) reports on the contents of 544 stomachs of the cowbird, taken in 20 states during every month in the year, examined by the Biological Survey:

The total food in these stomachs was divided as follows: Animal matter, 22.3 percent; vegetable, 77.7 percent. . . . The animal food consists almost entirely of insects and spiders, a few snails forming the exceptions. The insects comprise wasps and ants (Hymenoptera), bugs (Hemiptera), a few flies (Diptera), beetles (Coleoptera), grasshoppers (Orthoptera), and caterpillars (Lepidoptera). . . . Grasshoppers appear to be the cowbird's favorite animal food, and compose almost half of the insect food, or 11 percent of the whole. . . .

The vegetable food of the cowbird exceeds the animal food, both in quality and variety. When searching the ground about barnyards or roads the bird is evidently looking for scattered seeds rather than insects, though the latter are probably taken whenever found. Various other substances are also eaten, but they are mostly of the same general character, such as hard seeds of grasses or weeds, with but little indication of fruit pulp or other soft vegetable matter.

In his list of vegetable matter the following items are the most prominent: corn was found in 56 stomachs, wheat in 20, oats in 102, and buckwheat in one, as against seeds of ragweed in 176 stomachs, barngrass in 265 and panicgrass in 133. Grain as a whole amounted to 16.5 percent, or about one-sixth of the total food for the year, and probably one-half of this was waste grain. "In summing up the results of the investigation," he says, "the following points may be considered as fairly established: (1) Twenty percent of the cowbird's food consists of insects, which are either harmful or annoying. (2) Sixteen percent is grain, the consumption of which may be considered a loss, though it is practically certain that half of this is waste. (3) More than 50 percent consists of the seeds of noxious weeds, whose destruction is a positive benefit to the farmer. (4) Fruit is practically not eaten."

Dr. B. H. Warren (1890) says that cowbirds eat blackberries, huckleberries, cedarberries, wild cherries, and summer grapes (Vitis aestivalis). E. R. Kalmbach (1914) includes the cowbird among the birds that eat the alfalfa weevil; from the first of May to the middle of July, the weevil forms more than half of the bird's food. A. H. Howell (1907) credits the bird with feeding on the cotton boll weevil. And Hervey Brackbill sends me the following note: "One afternoon I came upon a female cowbird eating dandelion seeds from a full-blown seed-head. It must have found feeding from the upright stem inconvenient, for after I had seen it take a few billfuls, it suddenly thrust out one foot and pinned the stem to the ground and finished eating that way."

Economic Status.--It appears that in its food habits the cowbird is decidedly more beneficial than harmful, doing very little damage to the farmer's crops and destroying many destructive insects. The chief cause of its unpopularity is in the harm that it does through its parasitic habits for it undoubtedly interferes with the successful hatching and rearing of large numbers of small insectivorous birds.

Paul Harrington, of Toronto, has sent me his notes on the study of 100 nests of various birds that contained cowbird eggs or young. Most of these were nests of small birds, 48 of warblers, 37 of finches, 5 of vireos, 5 of flycatchers, and 5 of various other birds. Cowbirds had deposited 115 eggs in these nests, only a single egg in 80 of them. He estimated from his records that, approximately for every cowbird raised to a self-sustaining age there is a loss of three and one-third birds of some smaller species.

Before we condemn the cowbird for its parasitic habits, however, it must be shown that the young birds sacrificed for the cowbirds have more economic value than the parasites. Beal (1900) comments on this point: "When a single young cowbird replaces a brood of four other birds, each of which has food habits as good as its own, there is, of course, a distinct loss; but, as already shown, the cowbird must be rated high in the economic scale on account of its food habits, and it must be remembered that in most cases the birds destroyed are much smaller than the intruder, and so of less effect in their feeding, and that two or three cowbird eggs are often deposited in one nest."

Behavior.--Cowbirds are highly gregarious at all seasons: although the females and mated males scatter out in their breeding territories, in the vicinity there are generally to be found flocks of unmated or promiscuous birds with which the breeding birds associate more or less. They are sociably inclined toward each other and there seems to be no jealousy among them. Friedmann (1929) never saw them fighting, but Mrs. Nice (1937) has seen it five times, "the occasion being disagreements between males during communal courting parties."

The outstanding feature of the cowbird's behavior is its well-known fondness for, or association with, grazing cattle from which it derives its appropriate common name. In its association with these animals it is quite fearless, searching for food about the heads of the grazing animals or even between their feet, sometimes even alighting upon their backs, where they are supposed to relieve the animals of annoying insects. The movements of the cattle undoubtedly stir up grasshoppers and other insects, making them more easily available for the birds. The statements by some earlier writers that cowbirds search the droppings of cattle to feed on intestinal worms is not substantiated by stomach analysis.

On the ground the cowbird walks or runs, but seldom hops; while feeding it often holds its tail erected high in the air, with the wings drooping below it. Its flight in the air seems rather unsteady, much like that of the red-winged blackbird, from which it can be distinguished in the mixed flocks by its smaller size. Cowbirds seem to be on good terms with other blackbirds and starlings, associating with them in enormous mixed flocks on their feeding grounds or roosting with them at night. They also, sometimes, join with swallows or martins in their night roosts. Forbush (1927), however, says that "in New England Cowbirds usually roost by themselves; often they choose thick coniferous trees or other thickets in the shelter of which they pass the night in great numbers. Another favorite roosting place is in the grass and reeds far out on wide meadows."

John E. Galley has sent me some notes on a large winter roost of cowbirds and starlings at Midland, Tex. At the height of their abundance, he estimated that the roost contained "between 10,500 and 11,000 individuals, of which 2,000 or 2,500 were cowbirds." They were roosting in Chinese elms around the courthouse. "The starlings were grouped in the topmost branches, the cowbirds below them."

Voice.--Aretas A. Saunders contributes the following study: "Some of the sounds produced by the cowbird are distinctly seasonal, produced mainly, if not entirely, by the male, and therefore should be considered songs, though they are not particularly pleasing musically. The commonest of these consists of a prolonged, high-pitched, squeaky note, followed by two or three shorter, lower-pitched, usually sibilant notes. This song may be written wheeeee tsitsitsi. My records are all somewhat different in details, but the first note is pitched from C'''' to F'''', and the notes that follow are from one and a half to two and a half tones lower, the lowest note in all the records pitched on G'''. The first note may be all on one pitch or slurred slightly upward. A not uncommon variation of this has the first note followed by a downward slur which is explosive and sibilant at its beginning; this sounds like wheeeee tseeya and is strongly suggestive of a sneeze.

"During courtship, when the male is going through his bowing and his wing and tail spreading, another kind of song is produced, something like that described for the grackle under similar circumstances, but it usually goes from low to high pitch abruptly--two or three low notes and then a few high, squeaky ones. The low notes are not harsh, but gurgly. It sounds like glub-glub-kee-he-heek. The interval between them is three and a half tones to an octave, and the pitch between them varies from C''' to C''''.

"The seasons for these songs last from the arrival of the birds to early July, when the egg laying is over and the birds gather in flocks to feed in fields for the rest of the summer. Call notes are a short chuck, a slurred preeah, and a loud, harsh rattle."

Eugene P. Bicknell (1884) says: "There seems to be no regularity about singing in the fall; but I have heard imperfect songs and half-songs at different times within a month after the middle of September. Sometimes, in the autumn, when Cowbirds are assembled in small flocks, they become garrulous, when their commingled utterance of low notes produces a sound as of subdued warbling."

Field marks.--The cowbird is the smallest of our blackbirds and can generally be recognized in the mixed flocks by size alone, even if the brown head and glossy black body of the male and the plain dark-gray coat of the female cannot be distinguished.

Enemies.--Some of the host species are hostile to the cowbird, but few succeed in driving it away. Some remove the eggs of the parasite and some bury them; many eggs are removed by human observers, and the total egg loss must be considerable. An interesting demonstration of hostility is mentioned by Dr. George M. Sutton (1928). He noted that in Pymatuning Swamp the swamp-nesting small species were nearly immune from cowbird parasitism, because the red-winged blackbirds ganged up against the cowbirds and drove them out of the swamp. He "saw a flock of Red-wings once pursue a female Cowbird until she was utterly exhausted and plunged into the water to escape. Her pursuers chased her to the edge of the Swamp then headed off and forced her back to the opposite bank."

While roosting in the swamps, cowbirds are preyed upon by mink and weasels and perhaps owls; during the day, they are subject to attack by hawks and falcons.

Harold S. Peters (1936) lists one louse, two flies, two mites, and one tick as external parasites on the eastern cowbird.

Fall.--As soon as the egg-laying season is over, the cowbirds begin to gather into large flocks and wander about over the country, feeding in the fields and pastures. The young birds join these flocks as soon as they are able to fly. While moulting, in August, the young males are quite conspicuous, the glossy black feathers of the new plumage being scattered among the old brown feathers of the juvenal dress and giving them a curious, mottled appearance.

The enormous flocks are often quite spectacular as the great, black clouds of birds swoop down into the fields to feed or pour into their roosts at night, sometimes in association with other blackbirds or starlings. Elon H. Eaton (1914) says: "The flocks of cowbirds found during September in the grain fields and pastures are so large that on one occasion after discharging my gun into a flock which was passing I picked up 64 birds from the two discharges of the gun, which will indicate the density of the flock. My estimate of the flock referred to was that there were between 7,000 and 10,000 birds. The usual flock in the fall, however, consists of from 50 to 200 birds."

The fall migration gets under way in September, but is mainly conducted during October, some individuals lingering well into November.

Winter.--A few cowbirds sometimes remain to spend the winter as far north as Massachusetts and southern Ontario. During mild winters considerable flocks sometimes spend the winter on Cape Cod, which is usually free from snow, and along the coastal marshes of southern New England. On February 12, 1935, I was surprised to see seven male cowbirds on my window feeding shelf in Taunton, Mass., all fighting for food. The weather, below freezing, was clear and cold, and the ground was covered with deep, hard-frozen snow, as it had been for the past few weeks of unusually cold weather. The males continued to visit the feeding shelf all through that month, and on the 28th three females appeared.

Thomas McIlwraith (1894) says: "In Southern Ontario nearly all the Cowbirds are migratory, but on two occasions I have seen them located here in winter. There were in each instance ten or a dozen birds which stayed by the farm-house they had selected for their winter residence, and roosted on the beams above the cattle in the cow-house."

Milton B. Trautman (1940) noted wintering cowbirds in Ohio for nine winters. "Usually, only a few individuals or a few small flocks totaling less than 20 birds were noted in winter. In 2 years 50 to 300 wintered. The birds remained throughout the day about barnyards and adjacent fields where cattle were kept. Some roosted at night in brushy inland marshes or in cattail swamps, and when only a solitary individual or a few were present, they most frequently roosted and associated with English Sparrows."

The regular winter range is south of the Potomac and Ohio River Valleys and extends to Florida and the Gulf coast. Here they join in large mixed flocks with redwings, rusty blackbirds, starlings, grackles, and meadowlarks, feeding in the stubble fields and rice fields.

Brown-headed Cowbird* Molothrus ater [Eastern Brown-headed Cowbird]

*Original Source: Bent, Arthur Cleveland. 1958. Smithsonian Institution United States National Museum Bulletin 211: 421-450. United States Government Printing Office