On June 18, at 7 p. m., the heads and bodies were no longer flesh-colored but were well enough covered to appear dark. The eyes were open. At a cluck from me their mouths flew open. Both parents fed them with green-colored larvae. When the male rested a moment on a brier above the nest, the female flew down and drove him away, fed the young, re-appearing with excrement in her beak, which was carried in an opposite direction from the regular approach via maple bough and poplar sapling. The male fed the young from a mouthful of very minute larvae or eggs, which were gathered from the silken nests in the unfolding leaves of a nearby poplar; after this (7.30 p. m.) the female covered the young for the night.
On June 20, at from 6.50 to 7.35 p. m., the young had been seven days in the nest. They were well feathered and of a yellowish-green cast, the short tails being tipped with yellow. The parents were more suspicious. The female came to the maple bough with something in her beak and flew down to the briers and back again several times before she dropped to the edge of the nest and fed her young. The male appeared immediately but swallowed a green grub himself upon discovery of me twenty-five feet away. The female came again in five minutes with a brownish object in her bill, but appeared more timid and refused to drop to the nest until the male set her an example of courage.
On June 21, at 6.12 p. m., the young were fully fledged in green plumage above and dirty yellow beneath. They showed fear of me for the first time, eyeing me in the same manner as the parent bird when on the nest. They were evidently ready to vacate at a moment’s notice or hasty movement on my part. The parents appeared, scolding rapidly. The female fed the young as soon as I retired to my old stand under a bush, with a rather large green grub (6.20 p. m.) and flew out to the top of a blackberry bush, followed immediately by the topmost fledgling. It could do little more than run. The adults flew to within a yard of my head, making a great outcry, and in the midst of the excitement the remainder of the young vacated the nest with feeble chips. The male gave his attention to them, while the female followed me as I beat a hasty retreat to enable them to collect their little family before dark. Eight days had elapsed since incubation was completed, and it is not at all unusual for the young of this species to leave the nest while so tiny and ragged.
Plumages.—Dr. Dwight (1900) calls the natal down “mouse-gray,” and describes the juvenal plumage, in which the sexes are alike, as, “entire body plumage olive-yellow darkest on the back and throat. Wings and tail slate-gray largely edged with plumbeous gray, the tertiaries and coverts with olive-yellow; the greater and median coverts tipped with white, yellow tinged. Rectrices largely white. Lores dusky.”
A partial postjuvenal molt begins early in July, involving the contour plumage and the wing coverts, but not the rest of the wings or the tail. This molt produces the first winter plumage in which the sexes are very much alike, the female being duller in color, especially the streak through the eye, and having less yellow on the crown. Dr. Dwight (1900) describes the first winter male as “above, bright olive-green, lemon-yellow on the crown veiled by greenish tips. Below, bright lemon-yellow, the crissum white or merely tinged with yellow. Transocular streak black. Wing coverts plumbeous gray, edged with olive-green, the greater and median tipped with white, yellow tinged, forming two broad wing-bands.”
The birds are now practically adult in plumage. The first and subsequent nuptial plumages are acquired by wear, which produces little change beyond removal of the greenish tips. Subsequent winter plumages are acquired by a complete postnuptial molt each July.
The interesting hybrids between this and the golden-winged warbler are discussed on pages 3 and 4. Kumlien and Hollister (1903) mention a probable mating of this with the Nashville warbler.
Food.—Nothing seems to have been published on the food of the blue-winged warbler beyond that mentioned above as food given to the young, which is doubtless eaten by the adults as well. It is apparently wholly insectivorous, seeking its food near the ground in the weed patches and underbrush where it lives and among the lower branches of the trees in its haunts. Probably any small insects that it can find in such places, as well as their larvae and eggs, including many small caterpillars, are eaten. Small grasshoppers and spiders are probably included. Prof. Aughey (1878) observed it catching small locusts in Nebraska. It is evidently a harmless and a very useful bird in destroying insects that are injurious to foliage.
Behavior.—Dr. Chapman (1907) writes: “It is rather deliberate in movements for a Warbler, and is less of a flutterer than the average member of the genus Dendroica. Some of its motions suggest those of the tree-inhabiting Vireos, while at times, as the bird hangs downward from some cocoon it is investigating, one is reminded of a Chickadee.” And he quotes Burns as follows:
Perched inconspicuously near the top and well out on the branchlets of a tree or sapling, preferably facing an opening, if in a thicket; it is in itself so minute an object as to be passed unseen by many, more especially as it is much less active than most of our Warblers. With body feathers puffed out to a delightful plumpness, except for the backward sweep of the head while in the act of singing, it remains motionless for quite a while. When it moves it is with a combination of nervous haste and deliberation, and its song may be heard from quite another part of the landscape with no apparent reason for the change. While it has its favorite song perches, it is quite a wanderer and not infrequently sings beyond possible hearing of its brooding mate, but oftener within fifty to two hundred feet of the nest.