Food.—We have only scattering reports on the food of the black-throated green warbler. S. A. Forbes (1883) examined the stomach of one taken in an orchard infested with canker-worms in Illinois, and found it to contain 70 percent of these destructive caterpillars, 15 percent beetles, 5 percent Hemiptera, and the remaining 10 percent Hymenoptera, gnats, coleopterous larvae and mites. Five stomachs of Nebraska birds, collected by Professor Aughey (1878), contained an average of 23 locusts and 21 other insects. Of twelve specimens examined by F. H. King (1883) in Wisconsin, “one had eaten a moth; three, seven caterpillars; one, two diptera; one, six larvae—probably caterpillars; three, eleven beetles; and one, a heteroptera.”
Knight (1908) from Maine writes: “The food consists almost entirely of insects, including beetles, flies, moths, spiders, grubs, larvae and in general the sorts of insects found on the limbs and foliage of the various evergreen trees and especially on the pines. Only rarely do they take their prey in the air, preferring to diligently seek it out among the branches and foliage.”
Probably all the items mentioned in the food of the young are also eaten by the adults. Forbush (1929) adds to the list leaf rollers, leaf-eating caterpillars of various kinds, and plant-lice. Evidently these warblers are among the best protectors of our forest trees. W. B. Barrows (1912) says that they are “particularly fond of the berries of the poison-ivy, and to a less extent of those of the junipers.” J. K. Terres (1940) saw them tearing open the nests of tent caterpillars, devouring large quantities of the larvae, which were about three-quarters of an inch long.
Behavior.—Although the black-throated green warbler is one of our tamest and most confiding wood warblers, as shown by the intimate studies of its home life made by several observers, it is much more often heard than seen, for it is a tiny mite and spends most of its time in the tree-tops, gleaning in the foliage of both coniferous and deciduous trees. As Miss Stanwood (1910b) says: “The bird is quick in its movements, but often spends periods of some length on one tree, frequently coming down low to peep inquisitively at an observer, once in a while flying toward a person as if to alight on his hand or head.” Forbush (1929) draws a picture of its confidence: “Like all the wood warblers it is fond of bathing, its bath tub often some pool in a mountain trout brook. One day as I stood beside such a brook, a very lovely male, disregarding my presence, alighted on a stone at my feet, and at once hopped into the clear spring water and performed his ablutions, dipping into the stream and throwing off the sparkling drops in little showers. As he stood there in the sunlight which streamed through an opening in the tree-tops, he left an enduring picture in my memory.”
Those who have studied the home life of the black-throated green warbler have noted its intolerance of some avian intruders in the vicinity of its nest, and its tolerance of others. Pitelka (1940b) writes:
On the eighth day after the hatching, a red squirrel (Sciurus hudsonicus) was observed to approach the blind, coming to within seven feet of the nest. At this time, the female simply left the vicinity of the nest at once and gave no alarm notes. Later the same day, when a young Black and White Warbler approached the nest to a distance of five feet, the female pounced upon it and struck with considerable force. When the intruder returned a second time the female flew at it and drove it away. The indifference to red squirrels and at the same time the offensive reaction toward small passerine intruders (Vireo olivaceus and Penthestes atricapillus) has also been noted by the Nices (1932: 160).
Reading and Hayes (1933) write: “While at the nest, we noticed an inquisitive Chestnut-sided Warbler in a maple a short distance away. He hung around for several minutes, peering at us, until suddenly the male, ably seconded by his mate, attacked him and drove him off. A male Blackburnian met the same fate a little while later, while peacefully hunting insects in the big spruce and, about an hour after that a Red-eyed Vireo changed his intended route at the first warning note and promptly withdrew. Curiously enough, a small family of Black-capped Chickadees travelling slowly through the spruce was totally disregarded.”
The black-throated green warbler is seldom bothered by the cowbird, although mentioned by several writers as imposed upon.
Voice.—Aretas A. Saunders has sent me the following full account of the two songs of the black-throated green warbler: “The quality of the songs is sweet and musical and exceedingly pleasing. With the possible exception of the yellow warbler, this species has the most attractive of the Dendroica songs. The quality has something indescribable that is all its own and enables those familiar with it to recognize the song, however variable the form.
“The black-throated green warbler has two distinct forms of song. Both may be sung by the same individual, and both are equally common in the migration and through the nesting season, so that they cannot be considered as territory and nesting songs. I distinguish them as first and second, but my choice is purely arbitrary. Both are delivered in the same quality. The first is a little longer than the second, for it contains more notes; but it is not proportionately longer, for the notes are shorter.