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, their 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 an 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.
Food.—Edward H. Forbush (1907) writes of the food of the eastern yellow warbler in Massachusetts:
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 canker-worms 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 canker-worms, 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 little 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 81⁄2. 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.