The warbling vireo, if it were not for its song, would not be a notable species, for it is a little bird in leaf-green plumage, inconspicuous as it moves about among the foliage on the highest branches of its favorite elms and poplars where it spends the summer days, surrounded by green leaves and almost hidden by them.

High up in the trees, one of its nearest neighbors is the wood pewee, another leafy-green little bird. But unlike the pewee that sits motionless on its perch, flying out from it now and then into the air to catch its prey, the vireo rambles about among the leafy branchlets, finding its food there.

Spring.--When the warbling vireos arrive in New England early in May, we of their human friends hope that a pair will settle in the roadside trees near our homes, for if they do, although we may rarely see them, we know that the male will entertain us with his delightful song, filling the days with charming, simple melody all through the summer, even on the hottest days of July and August.

The song, as it goes on hour after hour, suggests a spirit of quiet happiness, a contrast to the flaunting, martial bugling of the Baltimore oriole, another of the vireos' neighbors, and to the slow, sweet notes of the wood pewee with their hint of pathos. In the vireo's song there is an air of unhurried calm, a leisureliness we seldom hear in the voice of a bird. Spring brings us greater artists, more proficient technicians, birds of more exuberant joyousness, but no such comfortable and welcome "guest of summer" as the warbling vireo.

Courtship.--We know little in detail of the nesting activities of the warbling vireo, for the bird stays so high above the ground at this season that we rarely see him at short range. Audubon (1842), however, by a fortunate chance, was able to watch the building of a nest under favorable circumstances, and noted a bit of courtship behavior of which he remarks: "During the love days of the pair mentioned above [see below under nesting], the male would spread its little wings and tail, and strut in short circles round the female, pouring out a low warble so sweet and mellow that I can compare it only to the sounds of a good musical box. The female received these attentions without coyness, and I have often thought that these birds had been attached to each other before that season." Audubon also mentions the odd, swaying motion which is characteristic of the red-eyed vireo (q.v.) both in the season of courtship and after the young are fledged. He says: "I observed that they now and then stood in a stiffened attitude, balancing their body from side to side on the joint of the tarsus and toes, as on a hinge, but could not discover the import of this singular action."

Nesting.--Dr. Thomas M. Brewer (Baird, Brewer, and Ridgway, 1874), speaking of the nests, says:

The Warbling Vireo builds its nest usually in more elevated positions than any others of this family. For the most part in the vicinity of dwellings, often over frequented streets, they suspend their elaborately woven and beautiful little basket-like nest, secure from intrusion from their human neighbors, and protected by the near presence of man from all their more dreaded enemies. . . .

The nests of the Warbling Vireo, while they resemble closely those of the other species in all the characteristics of this well-marked family, are yet, as a rule, more carefully, neatly, and closely built. They are usually suspended at the height of from thirty to fifty feet, in the fork of twigs, under and near the extremity of the tree-top, often an elm, protected from the sun and storm by a canopy of leaves, and just out of reach of most enemies. They vary little in size, being about two inches in height and three and a half in their greatest diameter, narrowing, toward their junction with the twigs, to two inches. They are all secured in a very firm manner to the twigs from which they are suspended by a felting of various materials, chiefly soft, flexible, flax-like strips of vegetable fibers, leaves, stems of plants, and strips of bark. With these are interwoven, and carried out around the outer portions of the nest, long strips of soft flexible bark of deciduous trees. They are softly and compactly filled in and lined with fine stems of plants.

William Brewster (1906) writes: "The nest of the Warbling Vireo is ordinarily built at least thirty or forty feet above the ground, at the end of a long, slender branch. Silver-leaved poplars are preferred to all other trees, but where these are not available the birds content themselves with large, spreading white ash trees, or with elms, lindens or maples, while they occasionally choose apple or even pear trees."

A. C. Bent writes of a nest which he collected "25 feet from the ground in the top of a pear tree, attached to some small, leafy twigs close to an outer, topmost branch. The nest was deeply hollowed and well made of strips of inner bark of shrubs, various soft fibers, leaves, feathers, spiders' nests and cobwebs; it was lined with fine grasses and horse hair." And Coues (1878) points out that "the nest is quite deeply cupped, with a somewhat contracted brim, for the still greater safety of its precious freight."

M. G. Vaiden, of Rosedale, Miss., sends Mr. Bent the following data on nests of the warbling vireo: "A nest 60 feet from the ground, out on a limb, in a crotch of a small limb branching from a larger one. However, the nest was only 14 feet from the trunk of the tree. The nest was very similar to that of the red-eyed vireo, but a little heavier material had been used, and there was less workmanship on the outer side, not so much inner bark strips or moss, although there was a dab here and there." Another nest: "At the very top outer branches of a pecan, 90 feet high." A third nest: "In young sycamore tree, out on limb and semipensile, not over 15 feet from the ground on a branch over a little-used dirt road. This was the tallest tree (20 feet) in the vicinity." And A. Dawes DuBois sends the following: "About 40 feet from the ground in top of willow tree on bank of river; about 40 feet up in red oak tree 30 yards from our house; 10 1/2 feet from the ground in apple tree in orchard." Of the second nest he says: "While I was watching the singing bird on the nest, his mate came and replaced him. The change was made as quick as a flash; as he slipped off the nest, his mate instantly slipped into it. A rather stiff wind was blowing, so that the eggs would not have been safe for half a second if left uncovered. However, I found later that, even when there was no wind, the birds changed places rather quickly."

Audubon (1842) gives an account of the building of a nest in a Lombardy poplar which almost touched his window. He says:

Never before had I seen it placed so low, and never before had I an opportunity of examining it, or of observing the particular habits of the species with so much advantage. The nest, although formed nearly in the same manner as several others, which I have since obtained by cutting them down with rifle balls, from the top twigs of the tall trees to which they were attached, instead of being fastened in the fork of a twig, was fixed to the body of the tree, and that of a branch coming off at a very acute angle. The birds were engaged in constructing it during eight days, working chiefly in the morning and evening. . . . One morning I observed both of them at work; they had already attached some slender blades of grass to the knots on the branch and the bark of the trunk, and had given them a circular disposition. They continued working downwards and outwards, until the structure exhibited the form of their delicate tenement. Before the end of the second day, bits of hornets' nests and particles of corn husks had been attached to it by pushing them between the rows of grass, and fixing them with silky substances. On the third day, the birds were absent, nor could I hear them anywhere in the neighborhood, and thinking that a cat might have caught them from the edge of the roof, I despaired of seeing them again. On the fourth morning, however, their notes attracted my attention before I rose, and I had the pleasure of finding them at their labours. The materials which they now used consisted chiefly of extremely slender grasses, which the birds worked in a circular form within the frame which they had previously made. The little creatures were absent nearly an hour at a time, and returned together bringing the grass, which I concluded they found at a considerable distance. Going into the street to see in what direction they went, I watched them for some time, and followed them as they flew from tree to tree toward the river. There they stopped, and looked as if carefully watching me, on which I retired to a small distance, when they resumed their journey, and led me quite out of the village, to a large meadow, where stood an old hay-stack. They alighted on it, and in a few minutes each had selected a blade of grass. Returning by the same route, they moved so slowly from one tree to another, that my patience was severely tried. Two other days were consumed in travelling for the same kind of grass. On the seventh I saw only the female at work, using wool and horse-hair. The eighth was almost entirely spent by both in smoothing the inside. They would enter the nest, sit in it, turn round, and press the lining, I should suppose a hundred times or more in the course of an hour. . . .

In the course of five days, an equal number of eggs was laid.

Eggs.--[AUTHOR'S NOTE: The warbling vireo lays three to five eggs to a set, usually four. These are practically ovate and without any appreciable gloss. They are pure white, with only a few scattered spots of various shades of reddish or darker browns, or blackish, the darker spots being commonest.

The measurements of 50 eggs average 19.1 by 14.2 millimeters; the eggs showing the four extremes measure 20.3 by 14.7, 18.7 by 15.1, 17.8 by 13.6, 18.8 by 13.2 millimeters.]

Young.--We meet the young warbling vireos at close range when they come down from their lofty nest and follow their parents about in the shrubbery. They are odd, pale little birds when we first see them in July, not long from the nest--light brown on the back, with a wash of yellow on the breast and flanks, and hoary about the head, almost white, although they soon lose this latter mark. A. Dawes DuBois remarks: "The plumage of the fledglings is so pale that they look like little white birds." The old birds feed them with larvae (often a long, green worm); large moths (after pulling off the wings), and later, when the shrubs have fruited, with cornel berries.

The young birds at this time, as well as the adults, give a curious note which attracts our attention to these family gatherings. It strongly suggests the distant clipping of garden shears--a sort of sneeze.

Audubon (1842) gives the incubation period as 12 days, and says of the young birds: "On the sixteenth day after their exclusion from the egg, they took wing, and ascended the branches of the tree, with surprising ease and firmness."

Plumages.--[AUTHOR'S NOTE: Dr. Dwight (1900) calls the natal down "pale wood-brown" and describes the juvenal plumage as "above wood brown, very pale on pileum and nape, darker and faintly tinged with olive on the back. . . . Below, white, the crissum tinged with pale primrose-yellow. Auriculars, orbital ring and superciliary line white."

There is a partial postjuvenal molt, beginning early in August, which involves the contour plumage, and the wing coverts, but not the rest of the wings nor the tail. This produces a first winter plumage which is practically indistinguishable from the winter plumage of the adult, greener above and more buffy white below than the previous plumage.

Dr. Dwight says that the nuptial plumage is acquired by wear, but Ned Dearborn (1907) found March and April specimens of the western race undergoing a scattered molt on the head and breast. This may also be true of other vireos, though we have not the proper specimens to show it.]

Food.--In his study of food habits of the vireos, Dr. Edward A. Chapin (1925), summarizing his findings, says:

The economic status of the warbling vireo is in some ways more distinctly unfavorable than that of the other species of this family of birds, especially in its consumption of ladybirds. In more than a third of the stomachs examined the remains of these beneficial beetles were found. . . .

On the other hand, the injurious insects taken by the warbling vireo make up the greater part of the food. Lepidopterous remains, including adult moths and butterflies, caterpillars, pupae, and eggs, were taken from about 77 percent of those examined. This alone should  atone for the bird's injurious proclivities along other lines. . . . Little if any of the vegetable food taken was obviously cultivated, in most cases being from plants not used for their fruits. It seems reasonable, then, to class the bird as neither beneficial nor injurious.

Elliott Coues (1878) adds an interesting food item. He says in a footnote: "Prof. Samuel Aughey gives the Warbling Vireo among the birds of Nebraska which destroy the scourge of that country--the grasshopper," quoting him as follows: "I frequently saw it light down within a rod of me where locusts abounded and feed on them. This species seemed to eat them in all stages of their growth, and brought them constantly to their nests for their young."

Tilford Moore writes to Mr. Bent that he has several times seen one hang upside down from a twig to get food out of an apple blossom.

Behavior.--William Brewster (1906), writing of the bird in eastern Massachusetts, says: "The warbling vireo is a bird of somewhat peculiar and restricted distribution. It shuns extensive tracts of woodland and, indeed, most wild and primitive places, although it nests sparingly in orchard or shade trees near secluded farmhouses, and rather frequently along country roads bordered by rows of large elms or maples. We find it most commonly and regularly, however, in or near village centers such as those of Lexington, Arlington, Belmont and Watertown."

Mr. Brewster is referring here to the early years of this century. I remember that in those days I used to hear warbling vireos about half a mile apart along the main street through Lexington, but before many years, about 1912, we noted a diminution in their numbers; every year fewer and fewer breeding pairs returned, until, early in the 20's, the species became practically unknown in the town, and was rare throughout eastern Massachusetts. However, since about 1938, there has been a decided increase in its numbers.

The warbling vireo is so partial to the lines of trees along our village streets and to isolated trees in open country that, thinking back to the time when this land was covered chiefly by unbroken forest, we wonder where the bird could have found in those days a habitat to its liking. It is thought that the well-watered trees on the border of the broad lanes opened by rivers through the forest were the former habitat of the bird, for these would afford a situation not unlike the vireos' present breeding ground. Aretas A. Saunders (1942) expresses this conjecture: "I believe that the warbling vireo originally inhabited trees along stream borders. With the coming of civilization, shade trees along city streets formed a rather similar habitat, and it adopted such places. This will explain its preference for elms and silver maples, trees that originally were found along stream borders."

In former times, apparently, the warbling vireo was a resident in large cities. Dr. Brewer (Baird, Brewer, and Ridgway, 1874) says: "It is especially abundant among the elms on Boston Common, where at almost any hour of the day, from early in the month of May until long after summer has gone, may be heard the prolonged notes of this, one of the sweetest and most constant of our singers." Henry D. Minot (1895), speaking of the 1870's, also mentions the birds' occurrence "among the elms of Boston Common."

Many observers have noted the warbling vireo's habit of singing while he is incubating. William Brewster (1937) speaks of it thus: "Soon after leaving the Yellow-throat's nest, I heard our Warbling Vireo singing in the orchard. Thinking that he might be on the nest, I followed up the sound and directly saw the nest in the very top of a rather tall tree attached to the horizontal twigs of a long, upright leafy branch. I could see the bird's head distinctly. He raised it high when he sang and his white throat swelled and flashed in the sunlight."

Francis H. Allen describes an unusual observation: "I once saw a pair perched in bushes in low trees on a river bank and flying frequently down to the surface of the stream, striking it forcibly, and then returning to their perches, where they preened their feathers. Both birds participated, but not simultaneously. Whether the purpose was for bathing or to take insects from the surface of the water I could not make out, but, intentionally or not, they got their baths. In all cases it was a straight dash to the water at an angle of perhaps 25 or 30 degrees."

Voice.--Wherever we turn in the literature of the warbling vireo we find that the author, after commenting on the bird's inconspicuousness, speaks enthusiastically of its song, pointing out the difference from the songs of the other vireos, the length of the song period, and the charm of the smoothly flowing warble.

The song of the warbling vireo is not broken up into short, exclamatory phrases like those of the other common New England vireos, the red-eyed, the solitary, and the yellow-throated, but continues on in a long series of slow, quietly delivered musical notes increasing in force to the end. The pitch undulates gently to the final note, which is generally the highest and the most strongly accented. Some writers find a resemblance in the song to that of the purple finch, but the finch's notes are very rapid and energetic and have none of the calm deliberateness of the vireo's melody. The most suggestive rendering of the vireo's song, perhaps, is Wilson Flagg's (1890): "Brig-a-dier, Brig-a-dier, Brigate," which, pronounced slowly, brings out the rhythm admirably.

Aretas A. Saunders sends to Mr. Bent this summary of the song: "The song of the warbling vireo consists of a series of connected notes, with no two consecutive notes on the same pitch, and is therefore a true warble. Individuals often sing several different songs, and in a number of cases I have recorded from three to seven different songs from one individual. The pitch varies from D'''' to C#''', half a tone more than an octave. The average song ranges about 3 1/2 tones in pitch. Songs consist of 7 to 25 notes each and vary in length from 1 to 3 seconds. The notes are not all the same length. A common form is made up of one long note followed by two short ones, and when this is repeated several times it is like dactylic feet in poetry. It is common for the song to end on a high note."

In the summer of 1912 a bird that was breeding on Lexington Common, within hearing from my windows, showed a marked departure from the normal song. My notes say: "He often utters a part of his song in a squeaky voice with no whistled quality whatever, the tone becoming so high that it contains a sibilant sound. Sometimes he changes to the squeak in the middle of the song, returning to the whistle before the end; sometimes he ends with the squeak." Strange to say, later in the same year I heard a similar song from another warbling vireo breeding 5 miles from Lexington. This variation, however, must be rare, for I have not heard it since, although I have heard the red-eyed vireo sing in this manner.

The bird often sings until well into September: Mr. Bent has heard it singing daily from August 31 to September 13, inclusive, and my records for 10 years average August 27, the latest being September 18, 1910.

The warbling vireo has two common minor notes; one the sound that resembles the sharp clipping of garden shears, mentioned under "Young," and a complaint note, corresponding, apparently, to the quee of the redeye, but with no downward inflection. It is a hard, tense snarl, with sometimes a slight upward inflection, easily recognized as a diagnostic note of the species.

Dr. Jonathan Dwight, Jr. (1897), makes an interesting comment on this latter note. Speaking of a similar note of the Philadelphia vireo, he says: "It does not resemble the corresponding complaint note of olivaceus, but is almost exactly like the aggressive 'mya' of gilvus, which has a suggestion of the katydid about it."

Tilford Moore says in his notes: "Today I saw one singing in flight; he finished his song just after alighting but sang three-quarters of it in flight."

Field marks.--The warbling vireo has no mark in its plumage that enables us to identify it at a glance as a species. It has no wing bars, no eye ring, no distinctive lines on the head, like some of the other vireos; it is merely a gray-green little bird, but, from the shape of its bill and its manner of moving about, clearly a vireo. So we have to come to an identification by elimination, by the process of reductio ad absurdum.

Yet, before long, when we have seen the bird time and time again, it begins to take on an individuality of its own, as all birds do when we learn to know them well, and we recognize it, not, as we recognize many birds, by some peculiarity of plumage, not even because it lacks any distinctive marks, but because it suggests the definite personality we have attributed to the warbling vireo. The side of the head, marked only by a slight paleness above the eye, has an expression of bland innocence; the delicate coloring of the plumage, with no spot of ornament to set it off, gives an air of quiet refinement, like the bird's song; and the diminutive bill gives the bird a youthful appearance.

Enemies.--Herbert Friedmann (1929) says of the relation of the cowbird to the warbling vireo: "A very common victim. . . . Eaton lists the Warbling Vireo as one of the commonest molothrine victims in New York State, and I have numerous records from other parts of the country. . . . All together over forty records have come to my notice. In common with the other species of its family, this Vireo normally makes no attempt to rid herself of the parasitic eggs."  In recent years the warbling vireo has probably suffered more from the spraying of the shade trees with poison than from the natural enemies that commonly beset small arboreal birds. Their nests have been imperiled by the high-pressure spraying that rocks the elm branches at the vital points of the birds' summer distribution, the roadside trees of our country towns.

Winter.--Donald R. Dickey and A. J. van Rossem (1938) speak of the bird on its winter quarters:

The winter home of the eastern warbling vireo can now be stated to be in the foothills of El Salvador and adjacent parts of Central America. . . . On Mt. Cacaguatique in late November and in December, 1925, warbling vireos were abundant at 2,500 feet elevation, all through the berry-bearing trees which provided shade for the coffee groves. From there up to the oak- and pine-covered summit of the mountain (about 4,000 feet) they were also very numerous. In February and March, 1926, both on Volcan de Conchagua and Volcan de San Miguel, numbers were observed in similar environments at from 2,500 to 3,500 feet, but much less commonly than in the interior. At Chilata in April, 1927, warbling vireos were migrating and were usually in pairs.

Ludlow Griscom (1932) writes: "It is apparently quite common and generally distributed in Guatemala in winter, arriving principally in October, the earliest date being September 28, 1926."