In the last weeks of August the evening sky fills with birds most people never look up to see. Common nighthawks are not hawks but nightjars, and they are on their way to South America, feeding as they travel and catching insects in a gape that opens far wider than the tiny bill suggests. Look for the flight before you look for the bird: buoyant and erratic, almost bat-like, the wings crooked back to a point, with a white bar flashing partway out along each one. Plenty of them were raised a few blocks from downtown, because a nighthawk builds no nest at all and lays its two eggs straight onto open gravel, including the gravel on a flat city roof. By the middle of September the dusk sky is quiet again.