The plumages and moults correspond to those of the male. In juvenal plumage the wing edgings are usually duller, the first winter plumage being similar to that of the male but browner, the yellow tints nearly lost and the streakings obscure and grayish. The first nuptial plumage, assumed by a more or less limited prenuptial moult, is grayer above and paler below, except on the chin and throat where new pale orange feathers contrast with the worn and faded ones of the breast. The adult winter plumage is practically the same as the male first winter, the auriculars and transocular stripe usually duller. The adult nuptial plumage is brighter below than the first nuptial and with more spotting on the crown, but the black head and bright orange throat of the male are never acquired.
Food.—The Blackburnian warbler is mainly insectivorous like other wood warblers, feeding almost entirely on the forest pests that are so injurious to the trees. F. H. King (1883), writing of its food in Wisconsin, says: “Of nine specimens examined, four had eaten nine small beetles; five, nineteen caterpillars; one, ants; and one, small winged insect. In the stomachs of three examined collectively, were found four caterpillars, four ants, one dipterous insect .09 of an inch long, one medium sized heteropterous insect, four large crane-flies, and one ichneumon-fly (?). Another bird had in its stomach one heteropterous insect (Tingis), nine small caterpillars, two leaf-beetles, and two large crane-flies.”
Ora W. Knight (1908) writes: “In general I have found large quantities of the wing cases and harder body portions of beetles in the stomachs of such Blackburnian Warblers as I have dissected, also unidentifiable grubs, worms, larvae of various lepidopterous insects and similar material. As a rule they feed by passing from limb to limb and examining the foliage and limbs of trees, more seldom catching anything in the air.”
R. W. Sheppard (1939), of Niagara Falls, Ontario, observed a male Blackburnian warbler in his garden for several days, November 5 to 11, 1938, that appeared to be traveling with two chickadees, among some willow trees. “An examination of the row of low willow trees which appeared to be so attractive to this particular warbler, revealed the presence of numbers of active aphids and innumerable newly laid aphis eggs, and it is probable that these insects and their eggs provided the major incentive for the repeated and prolonged visits of this very late migrant.”
Henry D. Minot (1877) observed “a pair feeding upon ivy berries” on April 21, when insects were not yet common in Massachusetts.
Behavior.—William Brewster (1938) describes what he thought was the unique behavior of a female Blackburnian warbler at its nest, although a similar habit has been observed in other wood warblers. Even though the eggs “were perfectly fresh the female sat so closely that thumping and shaking the tree (a slender one) failed to start her, and when Watrous climbed it he nearly touched her before she slipped off. She then dropped like a stone to the ground over which she crawled and tumbled and fluttered with widespread tail and quivering wings much like a Water Thrush or Oven Bird and evidently with the hope of leading us away from the nest.”
The Blackburnian is pre-eminently a forest warbler and a tree-top bird. On migrations it frequents the tops of the trees in the deciduous forests, often in company with other wood warblers; and on its breeding grounds in the coniferous forests the male loves to perch on the topmost tip of some tall spruce and sing for long periods, his fiery breast gleaming in the sunlight. As his mate is probably sitting on her nest not far away, his serenity may be disturbed by the appearance of a rival; but the intruder in his territory is promptly driven away and he resumes his singing.
Voice.—Aretas A. Saunders has sent me the following study: “The song of the Blackburnian warbler is one that is usually of two distinct parts, the first a series of notes or 2-note phrases all on one pitch, and the second a faster series, or a trill, on a different pitch. It is very high in pitch, with a thin, wiry quality, rather unmusical, and not loud but penetrating.
“Of my 34 records, 25 have the second part higher in pitch than the first, while in the other 9 it is lower. I do not think, however, that this means that the higher ending is commoner, for there is reason to think that the difference is geographical. Of 11 records of migrating birds in Connecticut, 10 end in the higher pitch. Of 15 records from breeding birds in the Adirondacks, 13 end in the higher pitch; but of 8 records of breeding birds in Allegany State Park in western New York, only 2 end in the higher pitch, and 6 in the lower.
“In 20 of the records the first part of the song is of 2-note phrases, but the remainder is of single repeated notes. In 6 records, ending in a higher pitch, the final trill slurs upward in pitch, suggesting the ending of a typical parula song in form. In 10 of the records the second part is much shorter than the first.