[Illustration: FIG. 139.--The yellow-hammer, _Colaptes auratus_. (Photograph by W. E. Carlin; permission of G. O. Shields.)] The largest and one of the most interesting woodpeckers is the ivory-billed (_Campephilus principalis_), twenty inches long, glossy blue-black, with a high head-crest which is scarlet in the male. This bird lives in the heavily wooded swamps of the Southern States. Among the more abundant and widespread, and hence better known, woodpeckers are the yellow-hammers (fig. 139) or flickers (_Colaptes auratus_ in the East, _C. cafer_ in the West), the red-headed woodpecker (_Melanerpes erythrocephalus_), with its crimson head and neck and pure-white "vest"; and the black-and-white downy (_Dryobates pubescens_) and hairy (_D. villosus_) woodpeckers or "sap-suckers." The California woodpecker (_M. formicivorus_), a near relative of the red-headed woodpecker, has the curious habit of boring small holes in the bark of oak- or pine-trees and sticking acorns into these holes. Sometimes thousands of acorns are put into the bark of one tree, to which the birds come occasionally to break open some acorns and feed on the grubs inside. =The whippoorwills, chimney-swifts and humming-birds (Macrochires).=--All the birds of this order are remarkable for their power of flight. They have long and pointed wings; their feet are small and weak and used only for perching or clinging. All feed on insects, which are caught on the wing by the short-beaked, wide-mouthed swifts and whippoorwills and extracted from flower-cups by the humming-birds with their long and slender bills. The whippoorwill (_Antrostomus vociferus_) is common in the woods of the East and is readily known by its call. Its two brown-blotched white eggs are laid loose on the ground or on a log or stump. The night-hawk (_Chordeiles virginianus_), common over the whole country, is seen at twilight flying vigorously about in its search for insects. Its nesting habits are like those of the whippoorwill. The sooty-brown chimney-swifts (_Chætura pelagica_), popularly confused with the swallows, are the common inhabitants of old chimneys, in which they build their curious saucer-shaped open-work nests. Their eggs are pure white and number four or five. Of the humming-birds but one species, the ruby-throat (_Trochilus colubris_), is to be found in the Eastern States, but in the western and especially southwestern parts of the country several other species occur. In all seventeen species have been found in the United States. The nests (fig. 140) of the hummers are very dainty little cups lined with hair or wool or plant-down. The ruby-throat lays two tiny pure-white eggs. [Illustration: FIG. 140.--Nest and eggs of ruby-throat humming-bird, _Trochilus colubris_, seen from above, in apple-tree. (Photograph by E. G. Tabor; permission of Macmillan Co.)]