The New Student's Reference Work/Nighthawk

Night′hawk, an American insect-catching bird related to the whip-poor-will, a member of the goatsucker family.  It is quiet all day but flies at dusk and is often called the bull-bat.  It is common in many parts of the United States from May to October, and may be seen at nightfall, high in the air, sailing back and forth in search of flying insects.  It is about the size of the robin, of a dark color mottled with gray, and can always be recognized from its wide wing-spread, making it seem longer than the robin, and its white wing spots, conspicuous in flight, distinguishing it from the whip-poor-will, for which it is often mistaken.  It is sometimes called night-jar, and also goes by the name of mosquito-hawk; names more apt than nighthawk, for it is far removed from being a hawk save in keenness of vision.  As a rule this bird hunts in small companies of his fellows; one never tires of watching it in its hunting.  In the evening, high, high overhead the bird sai1s along, from height and ease making suddenest drop down to lower atmosphere, where its wonderful vision has discovered a fly, mosquito, beetle or moth.  During the heat of the day it rests, sitting motionless on limb, wall or lichen-covered rock, — any place where it will be inconspicuous.  The nest is made in hollow rock or on bare ground, and there are two speckled gray eggs.  These eggs are sometimes found on a house-top in the city.  Frequently after nesting-season is over, night-hawks gather in towns, hunt the myriad insects about street-lights, resting on roofs by day.  They are widely distributed in North America.  When they migrate, they travel in large flocks.  The sound made by them is another way in which to distinguish them from the whip-poor-will; as they fly their call is a sharp “pee-ent! pee-ent!” and when they make a drop through the air and then turn suddenly upward, there is heard a peculiar “boo-oom, boo-oom” — thought to be caused by the action of the air on the outstretched wings and tail.  In localities where they are numerous the evening air resounds with the nighthawk’s boom, which, heard at a distance, betrays the unseen bird.  See Chapman: Bird Life.