Young.—Nothing seems to have been recorded on the period of incubation, which is performed by the female alone. Nor do we know anything about the care of the young or their development.
Plumages.—Dr. Dwight (1900) says that the young Tennessee warbler in juvenal plumage, in which the sexes are alike, is similar to the young Nashville warbler in similar plumage but lacks the brownish cast and has a faint transocular stripe. He describes it as “above dull grayish olive-green, the rump brighter. Wings and tail clove-brown, the primaries whitish edged, the secondaries, tertiaries and wing coverts greenish edged with two yellowish white wing bands. Below grayish buff rapidly fading when older to a greenish gray; abdomen and crissum pale straw-yellow. Trace of ducky transocular streak.”
The incomplete postjuvenal molt, involving the contour plumage and the wing coverts, but not the rest of the wings or the tail, begins about the middle of July. This produces the first winter plumage, in which the young male is “above, bright olive-green, gray tinged on the pileum. Below, olive-yellow darker on the flanks, the abdomen and crissum white. Superciliary line and orbital ring buff. Transocular streak dull black.” The young female “differs from the male in having the lower parts more washed with olive-green.” Young and old birds are now practically indistinguishable.
Dickey and van Rossem (1938) say that the prenuptial molt “begins in late February and is not finished before about the middle of March. The molt involves most of the anterior body plumage, but progresses so slowly that this species never has the ragged ‘pin-feathered' appearance so often seen in Dendroica aestiva at the spring molt.” Dr. Dwight noticed the beginning of this molt as early as January 14. He says it “involves chiefly the head, chin and throat. The ashy gray cap is acquired, the chin, throat, and superciliary line become white, the throat is tinged with cream-buff and the transocular streak black. The yellow tints of the feathers retained below are lost by wear.” In the female, this molt is less extensive than in the male, and “the crown never becomes, even in later plumages, as gray as that of the male, but always has a brown or greenish tinge.”
Subsequent molts consist of a complete postnuptial in July and a partial prenuptial molt in late winter and early spring as in the young bird.
Food.—Bowdish and Philipp (1916) sent four stomachs of birds collected in June to the U. S. Biological Survey for analysis. One of these was empty. Of the other three, one contained 8 small caterpillars (Tortricidae), 35 percent; dipterous fragments, 23 percent; a small spider, 2 percent; and scalelike fragments (perhaps of some catkin), 40 percent. Another held a camponotid ant, 16 percent; at least 78 small caterpillars (Tortricidae), 75 percent; a snail (Vitrea hammoides), 4 percent; and unidentified vegetable fragments, 5 percent. The other contained 3 lampyrids (near Podabrus), 8 percent; a small coleopterous (?) larva, 3 percent; about 15 small caterpillars (as above), 25 percent; a neuropterous insect (apparently a caddis fly), 50 percent; 2 small spiders, 14 percent; and a trace of unidentified vegetable matter.
Several observers have complained that Tennessee warblers do considerable damage to grapes, and this is undoubtedly true. W. L. McAtee (1904), while investigating the damage done by this and the Cape May warbler, found that—
in the arbor under observation, which was a small one, scarcely a grape and not a cluster was missed. The damage, however, was inconsiderable as the birds did not commence to use their appropriated share of the crop until the owner had taken all he desired. * * * Both species were constantly busy catching insects on the vines, and on a walnut and some appletrees near by. Frequently, however, they dashed into the vines and thrust their bills quickly into a grape. Sometimes they withdrew them quickly; again they poked around in the interior of the grape a little, and always after these attacks, they lifted their heads as in drinking. This action suggested a reason for piercing the grapes, that I am satisfied is the true one, that is, the obtaining of liquid refreshment.
A supply of available drinking water for the birds, might help to protect the grapes. And, as the warblers feed on insects that seriously damage the grapevines, the good work they do may compensate for the grapes that they damage. The stomach of one Tennessee warbler examined by Mr. McAtee contained a Typhlocyba comes, an especial pest of the grape, a destructive jassid or leafhopper, 6 caterpillars which were doing all in their power to eat up the leaves remaining on the vines, 2 spiders, a bug (Corizus), a weevil, and one parasitic hymenopteron (the only insect that was not harmful).
S. A. Forbes (1883) found that a stomach taken from an orchard infested by canker-worms contained about 80 percent of these destructive larvae and about 20 per cent beetles. Professor Aughey (1878) observed these warblers catching young locusts in Nebraska. Clarence F. Smith adds, in some notes sent to me, that “in the fall, during migration time, Tennessee warblers often glean their food from dense patches of such weeds as sunflower, goldenrod, and ragweed,” and that “sumac, poison ivy, and other berries are sometimes eaten in small quantity.”