[Published in 1953: Smithsonian Institution United States National Museum Bulletin 203: 160-182]
The familiar yellow warbler, also commonly called the summer yellow bird or wild canary, is the best known and the most widely known of all of our wood warblers. It is one of the few birds that almost everybody knows by one of the above names. It is universally beloved as it comes to us in the flush of budding spring, gleaning the shrubbery, like a rich yellow flame among the freshly opening leaves, or bringing to the apple orchards a flash of brilliant sunshine to mingle with the fragrant blossoms. As Dr. Chapman (1907) says: "In his plumes dwells the gold of the sun, in his voice its brightness and good cheer. We have not to seek him in the depths of the forest, the haunt of nearly all his congeners, he comes to us and makes his home near ours."
The yellow warbler, as a species, is also the most widely distributed member of its family. Its breeding range extends from the Atlantic to the Pacific in both Canada and the United States (110 degrees of longitude), and from the Barren Grounds in northern Canada to Mexico and the Gulf States (40 degrees of latitude). Its winter range covers 54 degrees of longitude and 31 degrees of latitude in Central and South America. Professor Cooke (1904) says: "The extreme points of the yellow warbler's range--northern Alaska and western Peru--are farther separated than the extremes of the range of the black-polled warbler, which is considered the greatest migrant of the family." But it must be remembered that the yellow warbler breeds much farther south than the blackpoll.
Spring.--The spring migration of the yellow warbler is long and partially circuitous; eastern yellow warblers that winter as far east as British Guiana probably make a roundabout flight to Central America, as there seem to be no springtime records for this bird in the West Indies and few for it in Florida. These birds may fly across the Gulf from Yucatan to Cuba and Florida, but the main flight is probably directly north from Yucatan to Louisiana and other points on the Gulf coast; they have been repeatedly seen flying northward in the middle of the Gulf. There is also a considerable migration along the coast of Texas, which I have personally observed.
The migration is also prolonged or very irregular, for according to the dates of departure given to me by Alexander F. Skutch (see under Winter), the last of these warblers do not leave Central America until the very last of April, or the first of May, after the first arrivals have reached New England; some of these records, however, may apply to one of the western races. After the birds reach the United States, the migration fans out northward and northeastward and seems to be more rapid. Of this Frederick C. Lincoln (1939) says: "Coming north from the Tropics these birds reach New Orleans about April 5, when the average temperature is 65o F. Travelling on northward much faster than does the season, they reach their breeding grounds in Manitoba the latter part of May, when the average temperature is only 47o. Encountering progressively colder weather over their entire route, they cross a strip of country in the 15 days from May 11 to 25 that spring takes 35 days to cross. This 'catching up' with spring is characteristic of species that winter south of the United States and of most of the northern species that winter in the Gulf States."
Territory.--Soon after their arrival on their breeding grounds the males begin to select their territories and to defend them. Dr. S. Charles Kendeigh (1941) made a study of the territories of birds in a prairie community in northwestern Iowa, and writes:
A special study of the Yellow Warbler indicated that territorial requirements included suitable nest-sites, concealing cover, tall singing posts, feeding areas in trees, and space, and that when certain of these factors were lacking, territorial relations became confused and the behavior of the birds was modified. . . . These warblers possessed territories that averaged about 150 feet in diameter, or approximately two-fifths of an acre. Even in locations where trees were included, the territories appeared to be of about the same size. The limits of the territory often did not coincide with the boundaries of the thicket in which the nest was located but extended over the neighboring grassland and often included parts of neighboring thickets. These territories were defended by the males partly by singing, although in shrubby areas lacking trees they were handicapped by lack of singing posts from which to proclaim their ownership and to advertise themselves. A few made use of fences from which to sing and also of tall posts and wire from an abandoned electric line that extended through the area. The role of the female in defense of territory was not determined.
Probably due to this lack of singing posts and to the unusual abundance of birds, chasing was also extensively used as a defense measure, and during the height of the nesting season squabbling birds were a common sight all over the area. . . . Neighboring males seemed to lack any conception of the limits of each other's territories and moved about indiscriminately until chased out. No actual fighting was observed. . . . In other parts of the area where trees were available, the males commonly sang at a height of 18 feet, often up to a height of 45 feet, and chasing was not often observed.
For yellow warblers observed by Wendell P. Smith (1943) at Wells River, Vt., "territorial exclusiveness scarcely existed. In one season a Chestnut-sided Warbler's nest was located within five feet of that of the Yellow Warbler. The following species were represented by one nesting pair within a radius of thirty feet: House Wren, Catbird, Black and White Warbler, Chestnut-sided Warbler, Northern Yellow-throat and Indigo Bunting. Unless another individual came very close to the nest, no hostility was shown by either male or female. Too close an approach would bring a swift attack by one or the other, however, but for only a short distance when the pursuer would give up the chase."
A. D. DuBois mentions in his notes a nest that was about 6 feet from the door of a screened porch in daily use and tells the following story about the territory involved: "Twelve yards south of this nest was a spruce tree. On several occasions the male met another male at this tree or beyond it. Both alighted at times in the treetop. Their boundary arguments had the appearance of pushing contests in the air; and sometimes the contestants revolved in the air, about an imaginary axis between them. Once, while one of the warblers was in the tree, the other was seen to poise near the tree on fluttering wings, remaining for two or three seconds as nearly stationary in the air as a hummingbird. Twelve yards beyond the spruce I found a nearly completed nest in tall lilacs; but this nest was not finally occupied." Apparently, a second pair of warblers had tried to build a nest too near the territory of the first pair and had been driven out of the territory.
Courtship begins soon after arrival of the species. Within a period of from four to six days greatly increased singing is noted which marks its inception. Persistent and lively pursuit of the female by the male was observed, taking place within a restricted area (once within a radius of thirty feet). From one to four days elapsed before courtship was completed. Sexual union may not take place until nest building begins as the following observations in 1938 tend to show. Pursuit of the female began on May 23, continued on the 24th but frequent attempts at intercourse on the part of the male were unsuccessful. On the 26th copulation was seen to take place and on that date the nest was completed. . . . A period of several days intervened between nest completion and egg laying. During two seasons of rather intensive observation, this was two days.
Nesting.--Although we have come to regard the yellow warbler as a sociable and friendly little bird that seeks our company and builds its nest in the shrubbery about our homes, often close to our houses or in the bushes under our windows, such were not the original nesting sites and even now are far from being the commonest situations chosen, although they may seem the most evident.
The favorite nesting sites in southern New England are along small streams and brooks, around the borders of swamps and ponds and lakes, or in the more open brushy swamps (where the land is moist but not too wet) among willows, alders, elderberry and blueberry bushes, and other moisture-loving shrubs and small trees. They also nest in drier situations, in shrubbery about open spaces, along brush-grown fences and hedgerows and roadside thickets, or in cut-over lands grown up to sprouts and to thickets of wild raspberry, blueberry, and other bushes.
In such situations the nest is built in an upright fork or crotch of a bush or sapling, seldom over 6 or 8 feet from the ground or less than 3. Nests are sometimes built at higher levels in apple trees in orchards or in small trees about houses but rarely as high as 30 or 40 feet. Near human habitations, clumps of lilac bushes, often close to windows or doors, are decided favorites, while various kinds of ornamental shrubs about our gardens or grounds also provide suitable nesting sites.
Mr. DuBois has sent me the data for 30 nests of the eastern yellow warbler found in Minnesota, Illinois, and New York. Among these, 4 were in willows, 3 each in lilacs and alders, 2 each in elms and box-elder saplings, and 1 each in a grapevine, an ash sapling, a spirea bush, and a currant bush. One of these latter, in an unspecified bush, was 14 feet from the ground, and another, in a wild grapevine climbing on a tree beside a coal bin, was 8 feet from the ground; those in the elms were 12 and 14 feet, respectively, from the ground. The remainder were mostly 5 feet or less above ground, the lowest being at a height of 2 feet, in a currant bush near a vegetable garden. He tells of a nest that was built in a wild rose bush at the edge of a small run near his vegetable garden; "this nest was so compactly fabricated as to hold water for some time; I saw about one-fourth inch of water standing in the bottom of it after a heavy rain."
On two occasions, he has found the new nest to have been built on top of the old nest of the previous year.
F. G. Schrantz (1943) has published the results of a careful study of 41 nests of the eastern yellow warbler in Iowa, during 1938 and 1939, on the restricted grounds of the Iowa Lakeside Laboratory. Among those at heights from 1 1/2 to 5 feet from the ground were 27 in wolfberry bushes (Symphoricarpos occidentalis), 8 in young saplings of boxelder (Acer negundo), 2 in wild gooseberry (Ribes gracile), 1 in wild currant (Ribes floridum), and 1 in an introduced species of honeysuckle. One nest was in a cottonwood at a height of about 10 feet, and another in a boxelder about 15 feet above ground.
Dr. Roberts (1936) says that in the prairie regions of Minnesota, where the underbrush is scarce, the yellow warblers build their nests in the cottonwoods in the tree-claims, "against the trunks of the large trees, supporting them on small lateral branches and twigs. . . . These arboreal nests are often fifteen to twenty-five feet from the ground and occasionally still higher." And in the huge cottonwood trees along the river, he as seen nests placed at elevations of 40 to 60 feet. Others have also recorded nests at heights of 40 to 60 feet.
In Dr. Kendeigh's (1941) prairie community, "twenty out of twenty-nine nests were placed in buckbrush, with the rest in boxelder, lilac, willow, or currant. The buckbrush is a low bush usually three or four feet high, growing in rather dense thickets in the open, especially grassy areas of Poa and Agropyron. Nests placed here varied between two and three feet above the ground. The nest found closest to the ground (18 inches) was, however, in a small boxelder. In taller shrubs and trees, the nests were found up to about seven feet above the ground."
Mr. Schrantz (1943) watched the building of a nest from the first stages of construction to its completion and the laying of the first egg, covering a period of 4 days.
Construction was first observed at 7:45 a.m. on June 12, 1939, when a female Yellow Warbler was seen carrying a tuft of plant-down into a small boxelder sapling. Upon examination, a mass of plant-down about one and one-half inches in diameter was found at a measured height of two feet three inches from the ground in the fork of the sapling. During an hour of observation the female continued to carry plant-down at intervals of about four minutes, although once it did not bring any material for twenty minutes. At noon the plant-down mass had increased to about three inches in diameter and was more compactly pushed into the fork. By 6:45 p.m., there were many strands of plant fibers and grasses woven around and through the plant-down in such a way as to wrap and bind the plant-down around the small twigs of the fork. The nest was just assuming a cup-shaped structure. The female was now bringing large loads of a mixture of grasses and plant fibers and working at a rate of about one trip every four minutes. The first day's building was completed at 7:55 p.m. The nest was now partially surrounded with woven plant fibers and grasses with a slight formation of a rim.
On the second day the work continued and the "rim consisted of plant fibers and grasses woven partly into the original down but mostly into the sides and around the top. At 6:45 p.m., the nest appeared completed with a well-formed cup, plant-fiber and grass rim, and a plant-down floor." The third day was partly rainy and little was accomplished but "by 8:00 on the fourth morning, the plant-down inside the nest was smoothed out and contained a few strands of fine grasses. . . . During all the observations on the building of this nest the male at no time was seen to bring any nest material. However, since there were many hours during the day when no observations were made, it is possible that he might have helped at some time. . . . At 6:30 a.m., the following day, one egg was found in the nest. . . . The dates of the beginning of construction and the dates the first eggs were laid were obtained for two other nests, and the time which elapsed in both cases was four days."
Only the female was seen to take part in the building of the nest that Mr. Smith (1943) watched, but my experience was somewhat different. On May 10, 1942, I found a pair of yellow warblers building a nest in the top crotch of a blueberry bush, close to the side of a country road. They were very tame and gave me an unusual opportunity to watch them for over an hour at short range. I parked my car within 5 feet of the nest and took motion and still pictures, with cameras even nearer. The nest was nearly done and they were putting in the lining. Both birds helped in the work, but the female did nine-tenths of it. She came at frequent but rather irregular intervals, bringing a billful of soft plant down that looked like the down from ferns, some of which I found growing nearby, and to which I saw her making frequent trips; the fronds of the cinnamon ferns were just unfolding. This material she deposited in the cup of the nest and settled her body down into it, smoothing the lining into place by turning her body around in different directions, pressing it down with her body and up against the sides of the cup in a sidewise motion of the wings. Occasionally she reached over the rim of the nest, smoothing it with her neck and tucking in the loose ends with her bill. The bottom and outside of the nest seemed to be about finished; one side of it, opposite the most exposed side, was anchored to a nearby twig with strands of plant fiber. The female seemed utterly fearless; the male was more shy, but his streaked breast was occasionally seen at the nest.
Robie W. Tufts tells me that he has seen the male at the nest; he saw a male come to a partly finished nest sit in it for over a minute as though testing the workmanship and sing twice while sitting there. The male is always very attentive during nest-building, following his mate back and forth on her trips for material and keeping close to her most of the time. His interest in the nest is so keen that it would be strange if he did not sometimes help.
The eastern yellow warbler builds a neat, strong nest, the materials being firmly and smoothly interwoven and the lining compactly felted. Five local nests before me show quite a variation in the materials used and in their arrangement. The most obvious material, occurring more or less in all of the nests, consists of the silvery-gray strands from the last year's stalks of milkweed, Indian hemp, or other similar dead weeds. One nest has a great mass of such material below it on one side, evidently to fill in space in the fork that supported it; mixed with this material are a few strands of grasses, other shredded weed stems, bits of wool, and gray fur. Although this nest is far from neat externally, the cup of the nest is well and firmly made of finer silvery fibers and fine grasses, cinnamon fern down, with which it is profusely lined, and a few fine white hairs. The rim is strongly reinforced with horsehair and decorated with the cinnamon down. This nest, the largest of the lot, measures nearly 5 inches in height and 3 inches in diameter, externally. The smallest and the neatest of the five is made of finer strands of similar materials without a trace of the cinnamon fern down, the whole being very firmly and smoothly woven into a compact little nest; the rim is neatly made of very fine grasses, and it is smoothly lined with white plant down; it measures only 2 inches in height and 2 1/4 inches in diameter, externally. Grasses enter largely into the construction of all the nests. One in particular is lined with both white and buff plant down and a little very fine grass, and has a solidly built rim of strong grasses very firmly interwoven; the foundation consists of dry brown and gray lichens, or mosses, and a lot of cotton waste, such as is used to clean machinery. A two-story nest, which measures 4 inches in total height, is profusely lined with white cotton in both stories. There is little difference in the internal measurements, which vary from 1 3/4 to 2 inches in diameter, and from 1 1/4 to 1 1/2 inches in depth.
None of my nests contain any feathers, but Dr. Roberts (1936) tells of a nest that was made entirely of chicken feathers, with "not a bit of material of any other kind." It was built in a jewelweed (Impatiens capensis), but after a brisk wind and a sharp shower both nest and weed were completely wrecked. He shows a photograph of a nest built almost entirely of sheep wool, and speaks also of the use of fine strips of inner tree bark, which probably occur in many nests, of quantities of fine, white, silky pappus from various plants, and a few feathers. DuBois mentions in his notes a nest in which five soft, white chicken feathers were woven into the lining, the largest one when stretched out measuring 3 3/4 inches; there were also two or three feathers in the body of the nest. In my collection is a beautifully camouflaged nest that was built in the upright crotch of a small poplar and seems to be made very largely of white cotton mixed with fine, light-colored fibers. It is lined with cotton, and with a few green poplar leaves fastened to the exterior, the whole being firmly bound with some of the finest fibers and with spider silk; the light-colored material matches the bark of the tree so closely that it might easily be overlooked.
T. E. McMullen has sent me his data for over 40 nests of the eastern yellow warbler found in New Jersey and Pennsylvania. The lowest nest was only 1 foot from the ground in a small bush, and the highest was 30 feet up in an elm. In addition to the shrubs and trees mentioned above he lists arrowroot, blackberry briers, elder, holly, Osage-orange and button bushes, birch, wild cherry and oak saplings, and a pear tree.
The well-known habit of building nests one or more stories over cowbirds' eggs will be discussed under enemies of the yellow warbler.
Eggs.--Four or five eggs make up the usual set for the eastern yellow warbler; sometimes as many as six are found, or as few as three. In shape, they vary from ovate to short ovate, or rarely show a tendency to elongate ovate. They are only slightly glossy. These handsome eggs show a great variation, both in ground color and in markings. The most common ground colors are grayish white or greenish white but some eggs have a bluish white or even a soft, pale green ground color. The spots and blotches show an even greater variety of colors. Shades of "fuscous," "olive-brown," "citrine drab," "buffy brown," "buffy olive," "light brownish olive," "raw umber," "metal bronze," or "tawny olive" are intermingled with undertones of "deep dull gray," "neutral gray," "purplish gray," "pale purplish gray," "mouse gray," or "buffy brown." The markings tend to form a wreath around the large end where, on the heavily-marked types, the blotches overlap the undertones and an almost endless number of shades are formed. Sometimes a few spots or scrawls of dark "mummy brown" or "olivaceous black" stand out in sharp contrast to the other markings. Although the eggs are usually well marked, sometimes with blotches a quarter of an inch in diameter, often they are only finely speckled with gray undertones.
The measurements of 50 eggs average 16.6 by 12.6 millimeters; the eggs showing the four extremes measure 17.8 by 13.2, 17.8 by 13.7, 15.2 by 12.7, and 15.8 by 11.7 millimeters (Harris).
Young.--The incubation period for the eastern yellow warbler has been recorded as from 8 to 11 days (most observers place it as about 11 days for each individual egg); often, but not always, it begins before the set is complete, making the period appear shorter for the first egg laid. Eggs are generally, though not always, laid on successive days, but at times 1 or 2 days intervene between layings. Incubation is performed wholly by the female. The male stands guard near the nest and feeds the female while she is sitting, but she also leaves occasionally to feed herself. The young remain in the nest from 8 to 15 days, according to several observers, but here again the normal time is probably between 9 and 12 days, if they are undisturbed.
Harry C. Bigglestone (1913) describes the hatching process as follows:
At about 5:30 a.m. on July 3 the writer was attracted by a peculiar rolling motion of the egg in the nest, and noticed upon observation, that the shell bulged out in a ring around the middle or a little nearer the smaller end; and soon it began to crack at this place. The egg raised on the small end, leaning against the side of the nest, and the young bird freed himself from the shell by a series of pushes and kicks by the head and feet, respectively. The head escaped from the larger part of the shell and the lower part of the body from the smaller end. The crown of the head and the median line of the back of the nestling were downy. This entire process covered a period of less than four minutes.
The empty shells were broken up and eaten by the parents. He says that brooding was carried on entirely by the female, except that he once saw the male brooding for 7 minutes, and adds:
The female was more careful in brooding the young during the first few days. She would stop for intervals throughout the day, while feeding, and brood the young. Her way of completely covering the brood was to fluff out the under coverts against the rim of the nest and bring the wings down, just inside, so as to effectually close the nest.
. . .The female had different brooding attitudes for the varying circumstances. For protection against the cold of early morning she brooded in the manner described above, completely covering the young. Through the rains she brooded in much the same way as for cold, sheltering the young so that after an unusually heavy downpour, the nest remained perfectly dry inside. During the heat of midday she usually stood in the nest with wings spread, shielding the young, but without shutting off the circulation of the air. On the contrary, at times she gently flapped her wings, as if fanning the young. During the strong winds she stood in the nest with wings outstretched, and leaned in the direction of the wind, so as to secure a delicate balance and at the same time keep the young in the nest.
Feeding the nestlings was carried on about equally by both male and female parents for the first 7 days, after which the male was frightened away by a snake and did no more feeding, the female carrying on for the next 4 days. During observations covering nearly all of 10 full days and part of another there were 2373 feedings, 813 by the male and 1560 by the female, there being only 33 feedings during the whole of the last day. "During the first three or four days when the female was brooding, usually the male gave her the food, which she distributed to the nestlings." Some of the food had to be broken up before it was given to the young; and sometimes it had to be thrust down their throats. There were 331 feedings of unrecognized food, and 553 of identified insects. The identified food consisted of 659 green worms, 326 fly worms, 162 other worms, 147 May flies, 103 moths, 75 millers, 65 mosquitoes, 26 larvae, 25 grasshoppers, 23 spiders, 18 ants, 14 grubs, 8 beetles, 4 damsel flies, 2 tree hoppers, and 1 bee. Feeding began at from 4:29 to 4:50 a.m., and ended at from 7:36 to 8:04 p.m., the average feeding period being 15 hours and 30 minutes per day. The parents were not seen to follow any system of rotation in feeding the young. "At no time while the nest was under observation did the parents feed by regurgitation," though the parents on several irregularly occurring occasions were seen to insert an apparently empty bill into the mouth of the nestling, but it was long after hatching.
The excreta were removed by both parents; they were eaten during about the first half of the nest life and carried away after that; the female did most of this. The parents were very watchful of the young, and were seen to drive away such birds as the cowbird, blue jay, wren, chickadee, brown thrasher, kingbird and blackbird, if they came too near the nest; the only bird that was not driven away was a catbird. The presence of a garter snake at the base of the bush caused great excitement; the snake was seen to climb up into the bush and carry off one of the young when it was about six days old; the young bird was dead before it could be rescued.
Schrantz (1943) writes: "The Yellow Warblers are hatched naked except for a scanty amount of down and are an interesting sight with their large bulging eyes and abdomen. It was observed that the eyes were commencing to open on the third day after hatching. By the fifth day the young can completely open their eyes, but in many cases would immediately close them when the nest was approached. At this age they would also duck down in the nest as if trying to hide. A slight tapping on the nests would cause a rapid outstretching of necks with wide open mouths." Bigglestone (1913) found that almost any slight noise near the nest would produce the same results. Studies of weights by Schrantz showed that--
the young averaged, when hatched, 1.27 gms.; at one day old, 1.87 gms.; at two days old, 2.95 gms; at three days old, 4.36 gms.; at four days old, 5.57 gms.; at five days old, 7.26 gms.; at six days old, 8.20 gms.; and at seven days old, 8.78 gms. . . .
Of the 168 eggs in forty-one nests, 119 eggs, representing 70.83%, hatched. Thirty-four eggs, representing 20.24%, disappeared due to wind, abandonment of nest, and unknown causes. Fifteen eggs, representing 8.93% were addled, two of which were buried with a Cowbird's egg. Of the 199 nestlings, twenty-eight disappeared. This represents 16.66% of all eggs laid. Four of them were seen dead in the nests. The others disappeared from unknown causes. Therefore a total of 91 fledglings, representing 54.17% of the original 168 eggs, left the nest. . . .
After all the young left a nest, the parent birds could be found feeding them in the immediate vicinity of the nest for a period of about three days. After this time the birds became more dispersed from the nesting site, but could still be found in the vicinity for a week or ten days.
An unusual casualty is recorded in the following note sent to me by Dr. Harrison F. Lewis: "A nest of this species which I found in a sheep pasture, was largely built of wool, presumably gathered from the neighboring bushes, where it had been left by the sheep. One of the young birds in this nest died as a result of having threads of the wool in the nest become entangled about its tongue and bill. Another member of this brood became entangled in a similar fashion, but I released it."
Plumages.--Dr. Dwight (1900) calls the natal down "mouse-gray," and describes the juvenal plumage, in which the sexes are alike, as "above, pale olive-brown. Wings clove-brown broadly edged with bright olive-yellow paling at tips of the quills, the edge of the outer primary bright lemon-yellow. Tail pale clove-brown, the inner webs of the rectrices lemon-yellow, the outer edged with olive-yellow. Below, pale sulphur-yellow, unstreaked."
The first winter plumage is acquired by a partial postjuvenal molt early in July that involves the contour plumage and the wing coverts but not the rest of the wings or the tail. He describes the young male as "above, pale yellowish olive-green, the edgings of the wing coverts paler. Below, dull lemon-yellow obscurely, narrowly and sparingly streaked on the throat and sides with pale chestnut." The female is paler throughout and has no streaking.
The first nuptial plumage is acquired by a partial prenuptial molt in early spring, "which involves most of the body plumage, the wing coverts and the tertiaries, but not the primaries, the coverts, the secondaries, nor the tail. The whole plumage becomes golden lemon-yellow, greener above and [in the male] brightly streaked on the throat, breast and sides with pale chestnut, somewhat veiled by the feather edgings. The forehead and crown are yellower than the back and usually chestnut tinged. The tertiaries and wing coverts are broadly edged with bright lemon yellow." The female in this plumage is yellower than in the fall and has a few obscure chestnut streaks below. Old and young birds are now very much alike, often practically indistinguishable, except for the worn juvenal wings and tail.
Adults have a complete postnuptial molt in July before or while they migrate and a partial prenuptial molt, as in the young bird, before they arrive in the spring. In El Salvador, according to Dickey and van Rossem (1938), "both adults and young of the year were in complete fall (postnuptial) plumage by the time they arrived. . . . An adult male taken April 10 is in the midst of the spring (prenuptial) molt and presents and extremely ragged appearance. Another, collected on April 24, has entirely finished this molt."
In both adult male and female plumages the colors are richer and the streakings below heavier than in the young bird, but the female is always duller in color and the streaking is less prominent or entirely missing.
It would be hard to find a summer bird more useful among the shade trees or in the orchard and small-fruit garden than this species. Almost entirely insectivorous, it feeds on many of the greatest pests that attack our fruit trees, vines and berry bushes. Whenever the caterpillars of which it is fond are plentiful, they form about two-thirds of its food. It is destructive to the small caterpillars of the gipsy moth and the brown-tail moth, and is ordinately fond of cankerworms and other measuring worms. Tent caterpillars are commonly eaten. Small bark beetles and boring beetle are eaten, among them the imago of the currant borer. Weevils are greedily taken. A few useful beetles are sacrificed; among them ground beetles, soldier beetles, and small scavenger beetles. The Yellow Warbler has some expertness as a flycatcher among the branches, and seizes small moths, like the coddling moth, with ease, but apparently does not take many parasitic hymenoptera, although some flies are taken. Plant lice sometimes form a considerable portion of its food. No part of the tree where it can find insect food is exempt from its visits, and it even takes grasshoppers, spiders, and myriapods from the ground, grass, or low-growing herbage.
He (1929) says elsewhere: "It attacks none of the products of man's industry, so far as our records go, except the raspberry, of which it has been known to eat a few occasionally." S. A. Forbes (1883) reports that 5 stomachs from a canker-infested orchard contained 94 percent insects; of which 66 percent were cankerworms, Coleoptera 23 percent, spiders 6 percent, Hymenoptera 2 percent, and Hemiptera 1 percent. A. H. Howell (1907) found a cotton-boll weevil in one stomach from Texas; E. R. Kalmbach (1914) reports that of seven Utah stomachs, two contained alfalfa weevils, forming 25 percent of the food in one; and Prof. Aughey (1878) found an average of 11 locusts in 7 Nebraska birds.
Behavior.--The gentle yellow warbler is not only one of the prettiest but one of the tamest and calmest of our bird neighbors. It comes to us in the most friendly and confiding manner to build its cozy nest and rear its little golden family in the lilac bush under our window or in the climbing rambler over our porch. Nor does it mind our company in the least as we watch its home life almost within arm's reach. I have sat for an hour within a few feet of a pair of these lovely birds and watched them building their nest. The many fine photographs that I have received show that it is an easy subject for close-up pictures; the near presence of the camera does not seem to disturb them in their feeding routine. Many intimate home-life studies have been very successful, for they are brave and devoted parents. Robie W. Tufts (1927) has had a male yellow warbler come at least twice to feed a brood of young that he was holding in his hand, and once he even wiped his bill on his thumb. It is such displays of confidence that endear us to the little golden gem.
Voice.--Aretas A. Saunders contributes the following study of the song of this warbler: "The song of the yellow warbler is a bright, sweet and musical refrain of about 8 notes. My records show that the number varies from 5 to 15 and averages 8 1/2. The songs are quite variable in form, so much so that it is the quality, rather than the form, that makes the song recognizable. This quality is difficult to describe, yet that quality, after a little familiarity, is easily recognized; the tones though musical and pleasing, are not quite clear, but slightly sibilant.
"Two forms of the song are fairly typical, but there are a number of others that vary so much that they are quite unlike either of these. The most common form begins with four or five notes of even time, and all on the same pitch. These are followed by two or three more rapid notes on a different pitch, usually lower; and the song is ended by one or two notes back on the original pitch and time. Such a song, in its simplest form, might be written see see see see tititi see. Of my 87 records, 45 may be classed as this form.
"The second form begins in the same manner, but has all the notes of equal time, the last three or four successively lower in pitch. I have records of 24 such songs. There remain in my records 18 songs so variable that they belong to neither of these forms, and yet no two of them are similar in form. A number of songs of the different forms begin with slurred notes, the slurs being about equally up or down in pitch.
"Songs vary from 1 1/5 to 2 seconds in length, averaging about 1 2/5 seconds. The pitch varies from A'''' to D''''', only three and a half tones altogether. Single songs vary from one to two and a half tones in range of pitch, averaging about one and a half tones. Individual birds may sing as many as three different songs, and sometimes sing two different songs in regular alternation.
"Singing continues from the first arrival in migration until the third week of July, ceases for a short time, but is usually revived in August, and is to be heard irregularly until the birds depart for the south."
Francis H. Allen gives me his impressions of the two common songs as follows: "One of these I have been accustomed to render as wee see wee see wiss wiss'-u. Occasionally the final wiss'-u is doubled. The other of these two songs goes something like wee wee wee witita weet, without the drop in the pitch that the first song has at the final note. I have also heard a song of five single notes with no variation in pitch or tempo--weet weet weet weet weet. Besides a rather sharp chip, which is the ordinary call-note, I have heard a dzee from a yellow warbler."
The yellow warbler is an early riser. Mr. Smith (1943) heard one begin singing at 4:56 a.m., "daylight time," and another at 4:05, "but with only one song until 4:08 when seven were given during the space of one minute. During the song period of fifty minutes 197 songs were given." Dr. Charles W. Townsend told Mr. Allen that he heard one at Ipswich, Mass., on June 13, 1908, that began singing at 3:10 a.m., but this was standard time.
Dr. Winsor M. Tyler (1937) mentions a peculiar note, heard during the migration in August, which had puzzled him for nearly 30 years until he finally traced it to an eastern yellow warbler. "As we walk under the trees, listening, we hear a long, wild, high, sharp bird-note, abrupt, and very slightly vibratory, lasting perhaps half a second. It is a characteristic sound of this time of year, and we hear it best on these quiet, silent days. It comes from a bird moving restlessly up in the trees, and before we can see the bird, it is gone. . . . In pitch, it suggests the call of a migrating Ovenbird, but it is too long-drawn-out; it suggests the chip of a Northern Water-Thrush in its sharp abruptness, but again it is too long."
According to Albert R. Brand (1938) there is considerable variation in the pitch of the song of the eastern yellow warbler, from 8,775 vibrations per second in the highest note to 3,475 in the lowest note, and with an approximate mean of 5,900 vibrations per second. This is far below the approximate mean of 8,900 for the black-poll warbler, but well above the average of 4,000 for all passerine birds.
Field marks.--One hardly needs field marks to recognize a yellow warbler; it is the yellowest of all our warblers at all seasons, even the wing and tail feathers are edged with yellow, and there is no white in either wings or tail. The youngest birds likewise show some yellow on the under parts and in the flight feathers. See the descriptions for plumages for details.
Enemies.--The arch-enemy of the yellow warbler is undoubtedly the cowbird. This warbler is one of the very commonest victims of this parasite, and comparatively few of its nests are not visited at least once by a cowbird in regions where the latter is very common. Dr. Friedmann (1919) has about 500 records of such imposition on the eastern yellow warbler. Everyone who has examined nests of this warbler in any number has found one or more eggs of the cowbird in some of the nests. This parasitic habit has cost this species of warbler many extra hours of unexpected labor and the loss of many eggs and young. But the most interesting fact about it is that the warbler has found a way to combat the evil and, in many cases, to defeat the plans of the cowbird, by either deserting the nest in which the strange egg is deposited or by building a second floor over it and leaving the alien egg to cool off in the "cellar."
The yellow warbler is not the only bird that has learned to do this occasionally, but it is the only one that does it regularly and persistently in spite of repeated contributions from the cowbird. Even if the warbler has one of its own eggs in the nest when the cowbird's egg is deposited it may bury both the eggs by building a story above them, but if there are two or three warbler's eggs in the nest before the alien egg appears, the warbler may feel obliged to incubate and hatch out the stranger, with the usual results of her own young being crowded out and lost. Two or more cowbird's eggs are almost sure to be deserted or buried. But the cowbird is very persistent and keeps on laying, as successive stories are added to the nest by the energetic and persevering warblers. Two-story nests are very common, and as many as three, four, five, and six stories have been recorded. Mr. Forbush (1929) was told by Dr. H. F. Perkins "of one case where a six-storied nest was built, with a cowbird's egg in every one." Mr. DuBois tells me of a new nest he found in a low bush, with another nest, about half completed and only about a foot below it, containing a fresh, cold cowbird's egg. Out of 43 nests found by Dr. George M. Sutton (1928) in Pymatuning Swamp, Pa., "a Cowbird egg was found in only one nest. This is most unusual, but is due, as elsewhere stated, to the protection against these parasites afforded by the Red-winged Blackbirds which would not tolerate a Cowbird anywhere about the marshes."
Snakes sometimes destroy the young, as related above; squirrels, blue jays, and other predatory mammals and birds rob the nests; and the adults must always be on the alert to escape the many enemies that prey on all small birds.
Harold S. Peters (1936) records only one louse, Philopterus subflavescens (Geof.), as an external parasite on the eastern yellow warbler.
Fall.--The striking feature of the fall migration of the eastern yellow warbler is its earliness. The birds begin to move away from their nesting haunts as soon as the young are able to take care of themselves, and the southward migration is well under way before midsummer. Smith (1943) says that, in Vermont, "during many seasons, the species is not seen later than July. Departure dates for local summer residents range from July 15 to the 30th. Later records occur between August 18 and September 9th." These later records are probably for birds from farther north. There seems to be a wide spread between the times that the earliest and latest birds leave.
Dr. L. H. Walkinshaw writes to me: "To me it is interesting how soon after nesting has been completed these warblers disappear. After July 10, it is very hard to find one of the species here in Michigan, and after August 10, almost impossible. It does stay some in certain good feeding areas, but the majority have left long before August." According to Milton B. Trautman (1940), the migration in Ohio begins early in July, reaches its height during the first half of August, and only stragglers are seen after September 10.
Arthur T. Wayne (1910) says that, in South Carolina--
the Yellow Warbler is positively uncommon during the spring migrations, but exceedingly abundant in summer and autumn. . . . By July 4, the return migration takes place and a few young birds arrive, but it is not until the 10th or 15th that they are common. . . . The habits of the birds are entirely changed, however, in summer and autumn, for then they frequent the cotton fields, as well as lands which have been planted with peas for forage. It is not unusual in autumn to see as many as twenty or more of these little birds far out in the salt marshes, where they find food in abundance. The species is so very abundant in late summer and autumn that it is not unusual to encounter hundreds of individuals in a few hours on plantations or in close proximity to salt water.
Prof. W. W. Cooke (1904) writes: "Though in migration the yellow warbler occurs in Florida as far south as Key West and is sometimes fairly common in northern Florida, the numbers that migrate through the southern part of the State must be very small, for not a bird passing north or south has been reported from any of the Florida lighthouses. The migration route of the yellow warblers that breed near the Atlantic coast is evidently southwest to northern Georgia and Alabama, and then across the Gulf of Mexico."
Perhaps the main flight from Florida and the other Gulf States is across the Gulf to Yucatan and then down through Central to South America, for there seem to be no records for Cuba for the eastern yellow warbler. There is a regular migration along the coast of Texas. Dickey and van Rossem (1938) say that "the eastern yellow warbler migrates through El Salvador in fair numbers, but no specimens were taken at any time during the winter. In the fall, particularly, great numbers are in evidence. The first arrivals reached Lake Olomega on August 1, but the main body did not begin to drift through until about the middle of that month."
Frederick C. Lincoln (1939) remarks: Redstarts and Yellow Warblers, doubtless the more southern breeders in each case, have been seen returning southward on the northern coast of South America just about the time that the earliest of those breeding in the North have reached Florida on their way to winter quarters."
Winter.--Dr. Alexander F. Skutch contributes the following winter notes: "This morning as I sat at breakfast a yellow warbler flitted among the shrubbery outside the window. Here in Central America, through 8 or 9 months out of the 12, this well-known bird occupies the same place in dooryard, garden, hedgerow and scrubby pasture as during its briefer sojourn in the more northerly regions where it nests. None of the resident warblers of Central America is quite so abundant and familiar about human dwellings. Everywhere it avoids the heavy forest and prefers the sunlight that floods the clearings made by man.
"It is one of the first of the visitants from the North to arrive in Central America, appearing in Guatemala as early as August 9, reaching Honduras by at least the fourteenth, Costa Rica by the seventeenth, and Panama by the twenty-second of the month. These early dates are for the Caribbean lowlands, along which it appears to migrate. It arrives later on the Pacific side of the Isthmus, especially in Costa Rica, where it has not been recorded before August 24, at San Jose, and not until September 11 in the Terraba Valley, still more isolated from the Caribbean flyway by lofty, forested mountains. But by the end of September, it is well distributed as a winter resident over both coasts of Central America, and in the interior up to at least 5,000 feet, becoming rarer at the upper limit of its altitudinal range. Much above 5,000 feet it apparently does not winter; but it is occasionally seen in September in the high mountains as a bird of passage. A heat-loving warbler, it is most common in the lowlands where, in the plantation districts of northern Central America during the winter months, it is among the most abundant birds, whether resident or migratory.
"Although a number of wood warblers which winter in the Central American highlands are gregarious, those that center in the lowlands are typically solitary. In this, the yellow warbler is no exception. Each wintering bird appears to have its own territory, from which it attempts to drive others of its kind. Trespassers are scolded with insistent chips; or more rarely, soon after his arrival, a male will sing while defending his claim. Near San Miguel de Desamparados, Costa Rica (4,600 feet), on October 1, 1935, I made the following note: "This morning, which for a change was bright and calm, I heard a yellow warbler singing in the low fig trees near the house. Upon going out to look, I found that there were two yellow warblers in the trees. One was trying to drive the other away; but the pursued always circled around and returned. I watched them for a long time; but this indecisive action continued without any change in the situation. In the intervals of the pursuit, the warblers (or at least one of them) would sing, but in a low and imperfect fashion, far inferior to the yellow warbler's summer song.' Again, on October 31: 'After the Wilson warbler, the most abundant winter visitor is the yellow warbler. The bird who on October 1 drove its competitor out of the fig trees beside the house still retains these trees and the surrounding Inga trees as its domain.'
"The yellow warbler sings far less while in Central America than many other wintering species. Exceptionally, one will be found singing profusely. In early October, 1934, I came upon such a bird among the coffee groves of a great plantation on the lower Pacific slope of Guatemala. His behavior was so far out of the ordinary that I am tempted to copy in full the notes I made upon it at the time: October 5--On the afternoon of my arrival at 'Dolores,' I went out for a walk through the coffee groves. From among the 'chalum' (Inga) trees which shaded the coffee bushes, I heard a bird's song which seemed to belong to a warbler; but I did not recognize it as the utterance of any species I knew. After searching for a time among the tree-tops, I spotted the singer, and was surprised to find him a yellow warbler. He was apparently a young bird, for he lacked the chestnut splashes along the sides which distinguish the mature males. He repeated over and over again his little song of four or five notes, which was so unlike the familiar song of the yellow warbler in the eastern United States that I did not at first recognize it; but once I had identified the singer, I realized that I was listening to a shortened and modified form of the typical song.
"As I stood watching and listening to this eccentric
warbler, the rain clouds which had been gathering darkly in the
west began to surrender their pent-up waters; and the sudden
shower approached across the plantation with the roar of a myriad,
fat drops striking against the large leaves of the Ingas and the
far larger ones of the bananas which shaded the plantation. I took
refuge from the rain beneath the broad expanse of a banana leaf,
which completely shielded me from the beating downpour. Soon the
heavy shower exhausted itself; and I emerged from beneath my green
roof. The warbler, who had taken shelter from the shower somewhere
in the foliage above me, resumed his cheerful singing." ***
Yellow Warbler* Dendroica petechia [Eastern Yellow Warbler]
*Original Source: Bent, Arthur Cleveland. 1953. Smithsonian Institution United States National Museum Bulletin 203: 160-182. United States Government Printing Office