Part 21

_Breeding range._--The northern olive warbler breeds =north= to central Arizona (Baker's Butte and White Mountains) and central western New Mexico (Reserve). =East= to western New Mexico (Reserve and McKnight's Canyon); Chihuahua (Colonia García); southeastern Coahuila (Diamante Pass); and southwestern Tamaulipas (Miquihuana). =South= to southwestern Tamaulipas (Miquihuana) and southern Durango (Durango). =West= to Durango (Durango); Sonora (Sierra Saguaribo); and southeastern and central Arizona (Huachuca Mountains, Santa Catalina Mountains, and Baker's Butte).

Other races occur in southern Mexico and Central America.

_Winter range._--The northern olive warbler is probably migratory to some extent, individuals withdrawing to the southern part of the range, but it is found in winter occasionally or in small numbers as far north as southern Arizona.

_Egg dates._--Arizona: 14 records, May 23 to July 1; 7 records, June 2 to 18, indicating the height of the season.

DENDROICA PETECHIA AESTIVA (Gmelin)

EASTERN YELLOW WARBLER

PLATES 24, 25

HABITS

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, gleaming in 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 Perú--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 black-poll.

_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 Yucatán to Cuba and Florida, but the main flight is probably directly north from Yucatán 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 65° 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 47°. 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 then 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 rôle 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. Du Bois 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._--Mr. Smith (1943) says on this subject:

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. Du Bois 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 boxelder 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 coalbin, 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 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 has seen nests placed at elevations of 40 to 60 feet. Others have also recorded nests at heights of 40 and 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 in 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 with 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 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 of a few feathers. Du Bois 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 the enemies of the yellow warbler.

_Eggs._--Four or five eggs made 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 gull 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 the 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: