Apparently, the nuptial plumage is produced mainly by wear, or by a limited prenuptial molt. The postnuptial molt is evidently complete in late summer.
The adult winter plumages of both sexes differ but little from the spring plumages; in the male, the feathers of the upper parts and cheeks are margined with brownish gray and the throat with white, the sides are washed with brown and the black streaks are obscured; in the female, the plumage is tinged with brownish in the same way and the black streaks are obscured.
Food.—No extensive study of the food of the black-throated gray warbler seems to have been made. It is evidently mainly, if not wholly, insectivorous, for several observers have mentioned its zeal in foraging among the foliage of trees and bushes for insects, with a special fondness shown for oak worms and other green caterpillars. Bowles (1902) says that “it seems to prefer oak trees in the spring because of the small green caterpillars that are very numerous on them and which are devoured on all occasions. One female must have eaten nearly half its weight of them (from three-fourths to one and one-half inches long) while its nest was being taken.” Mrs. Wheelock (1904) writes in the same vein: “In the spring these oaks are particularly infested with the green caterpillars, and the Warblers never seem to tire of devouring the pests. They lean way over to peer under every leaf, or reach up to the twigs overhead, never missing one. Twenty of these worms is an average meal for a Black-throated Gray Warbler, and the total for a day must reach into the hundreds.”
Behavior.—The black-throated gray warbler is not one of the most active wood warblers except when it is busy feeding; even then it goes about it in a quiet, business-like manner, without much concern over the presence of humans. At other times, it is rather shy and retiring, difficult to follow, as it slips away silently in the thick underbrush, where it spends so much of its time. Its nest is difficult to find, for it is not only well concealed, but the bird is careful not to betray it; our usual method of following a bird to its nest was not very successful, as it was soon lost to sight while we were watching it.
Mr. Bowles (1902) writes of its behavior that an incubating female “passed the time eating caterpillars while the nest was being examined. She did not go over five feet from it this time, till I left when she followed for about twenty feet, and kept almost within reach, watching me very closely. * * * Black-throated gray warblers do not object to human association at all; one nest was fifteen feet up on an oak branch, directly over a trail that was used at least six times a day by people going for mail, and generally much oftener.”
William L. Finley (1904a) describes quite different behavior at a nest containing young: “The moment the mother returned and found me at the nest she was scared almost out of her senses. She fell from the top of the tree in a fluttering fit. She caught quivering on the limb a foot from my hand. But unable to hold on, she slipped through the branches and clutched my shoe. I never saw such an exaggerated case of the chills. I stooped to see what ailed her. She wavered like an autumn leaf to the ground. I leaped down, but she had limped under a bush and suddenly got well. Of course I knew she was tricking me! But I never saw higher skill in a feathered artist.”
Voice.—The simple, but pleasing song of the black-throated gray warbler is described in Rathbun’s notes as follows: “The song as ordinarily sung consists of three rather quickly given notes, of a somewhat lisping quality, that rise and fall but are alike in construction and a closing fourth note that may slur upward with a decided accent, or may fall. The real construction of this song is lost unless the singer is close by, for then it will be found that each of the first three notes is a double one. It is a clear and pleasing song, of good carrying quality, and somewhat smooth when heard at a distance. During the nesting season the males will be heard in song much of the time during the day. The habit of the bird is to perch on or near the top of a young evergreen tree and sing repeatedly without shifting its perch, then to fly to another tree of similar character and repeat its actions.”
As I heard it in Washington, I wrote it swee, swee, ker-swee, sick, or swee, swee, swee, per-swee-ee, sic. Dr. Walter P. Taylor writes it in his notes zee zeegle, zeegle, zeegle, zort, tseeee. Grinnell and Storer (1924) describe it as "a rather lazy, drawling utterance, deep-toned rather than shrill. Wēē-zy, wēē-zy, wēē-zy, wēē-zy-weet; tsewey, tsewey, tsewey, tsewey-tsew; zuēē, zuēē, zuēē, soop; sĭĭ-wēēzy, wēēzy we-tsú; owēzē-wēzē-wēzē-wēzē-chŭr, are syllabifications written by us at different times when individual birds were singing close at hand. There are modifications in the song; sometimes the terminal syllable is omitted and again only three of the two-syllabled notes are given. The ordinary call is a rather low, one-syllabled chit.”