This continuous calling is one of the most characteristic things about our American nightjar, the whippoorwill, whose loud, musical cry is heard in summer from almost every hillside in the land, during the dusk of evening or morning or when the woods are whitened with moonlight; and sometimes two or three birds will sing against one another, as if in jealous rivalry, repeating the call several hundred times without a pause. In the Western United States, and in tropical America, are several kinds of whippoorwills; and in the Southern States a bigger cousin which calls its name loudly through the darkness--_chuck-will's-widow_. More nearly deserving the name nightjar, however, is our night-hawk, or bullbat, which is often seen flying swiftly about, high in the air, even before sunset, uttering a hoarse scream, or a queer-booming note, as it rushes, open-mouth, after unlucky insects. All the birds of this group are insect catchers and eaters, and their mouths, which have only a tiny pretence of a beak, open exceedingly wide, so that they may scoop in a dozen little flies at once, or seize and swallow a great moth. Then the tongue is exceedingly sticky, like that of an ant-eater; besides this, the sides of the beak are fringed with long, stiff bristles. So, when the bird catches an insect, its victim nearly always sticks firmly to its tongue, while, if it should break away from that, the bristles act just like a cage, and prevent it from escaping. The nightjars make no nest at all, but lay their eggs in a small hollow in the ground, generally under the shelter of a fern, or a tuft of bramble or heather. These eggs are never more than two in number, and are grayish white in color, mottled and marbled with gray and buff. SWIFTS In these arrangements and habits the nightjars show how nearly they are related to the very differently appearing chimney-swifts, which look so much like swallows that we often call them chimney-swallows, but this is wrong. Before this country was inhabited by white men, the swifts dwelt in companies in hollow trees, but as fast as the settlers built houses and chimneys the swifts left the trees and made their homes in the chimneys, where they fasten to the bricks little shelf-shaped nests composed of their glue-like saliva and bits of twigs. In the East Indies a kind of swift makes such a nest wholly out of its saliva, which hardens into a whitish material like isinglass. This is fastened against the wall or roof of some cave by the sea, and the Malays and Chinese gather these nests at the peril of their lives, where they are built in hundreds in dark caverns, and sell them as delicacies to be made into bird's-nest soup.