Another nest before me, from the Huachucas, has a foundation of moss and lichens, dry leaves, and strips of cedar bark, over which are finer strips of the bark and shreds of dry weed stalks and grasses, with a lining of still finer fibers; it is a shallow nest, its diameter being 3 by 31⁄2 inches outside and 2 inches inside.
Eggs.—While 4 eggs seem to constitute the usual set for Virginia’s warbler, as few as 3 and as many as 5 have been reported. These are ovate to short ovate and only slightly lustrous. They are white, finely speckled or spotted with shades of reddish brown, such as “chestnut” and “auburn,” intermingled with faint specks of “pale vinaceous-drab.” Some eggs are profusely spotted over the entire surface, while others have the markings concentrated at the large end. The measurements of 40 eggs average 15.9 by 12.4 millimeters; the eggs showing the four extremes measure 17.0 by 12.4, 16.0 by 13.0, 14.2 by 12.2, and 16.3 by 11.2 millimeters (Harris).
Young.—On the period of incubation and on the development and care of the young we have no information except the following observations of Bailey and Niedrach (1938): “The hatching time of many species of Colorado birds seems to coincide with an abundance of larvae feeding upon plants among which the birds are nesting. We have noticed time and again, that pests are numerous upon the vegetation when the fledglings are in the nest, but a few weeks later, after the little fellows have taken wing and are able to move to other parts, the caterpillars have gone into the pupa stage.” At a nest they were watching, they observed that both parents shared the work of feeding the young, averaging a trip every 6 minutes.
A. J. van Rossem (1936) took young birds that were not fully grown on July 10, and others on July 13 that had nearly completed the postjuvenal molt, from which he inferred that two broods might be raised in a season. H. S. Swarth (1904) noted that the young birds began to appear in the Huachuca Mountains about the middle of July, after which both old and young birds moved down into the foothills.
Plumages.—The young Virginia’s warbler in juvenal plumage is plain grayish brown above; the throat, chest, and sides are paler brownish gray; the abdomen and center of the breast white; the upper and under tail coverts are dull greenish yellow; there is no chestnut crown patch; and the greater and median wing coverts are tipped with dull buffy. The sexes are alike.
The postjuvenal molt begins early in July and is often complete before the end of that month. The first winter plumage is similar to that of the adult female at that season. In this plumage the sexes are not very different, and the crown patch is not much in evidence or is altogether lacking in the young female; both sexes are browner and with less yellow than in the adult plumage, and the female is duller than the male.
A partial prenuptial molt occurs between February and May, mainly about the head, during which the chestnut crown patch is at least partially assumed and the young birds become almost indistinguishable from adults. There is, however, considerable individual variation in the advance toward maturity.
Subsequent molts consist of a complete postnuptial molt in July and August, and a partial prenuptial molt in early spring. The adult male in the fall is browner above and on the flanks, and the yellow on the chest is duller than in the spring, while the chestnut crown patch is concealed by brownish gray tips. The female, also, is browner than in the spring, with little if any yellow on the chest and with the crown patch similarly concealed. In spring birds there is much individual variation, perhaps owing to age, in the amount of yellow on the breast, throat, and chin. Some females are nearly as brightly colored as are the duller males, some have very little yellow on the chest and some lack the chestnut crown patch.
Food.—Our information on the food of Virginia’s warbler is limited to the observation of Bailey and Niedrach (1938) who saw a pair of these warblers feeding their young on the caterpillars that eat the foliage of the trees and shrubs on their nesting grounds. It is significant that after these caterpillars are no longer available the warbler leaves its breeding haunts and moves down into the foothills, perhaps in search of other food; and it would be interesting to learn what that food is. It has been seen foraging on the ground, as well as in the foliage, and flying up into the air to capture insects on the wing.