Because it is almost continually on the wing. It eats, drinks, collects materials for its nest, and even propagates on the wing ; thus appearing to live in the air more than any other bird, and performing all functions there, save those of sleeping and incubation. In general they feed in a higher district than the other species ; a proof that gnats and other insects do also abound to a considerable height in the air; they also range to vast distances ; since locomotion is no labour to them, who are endowed with such wonderful powers of wing. Their powers seem to be in proportion to their levers, and their wings are longer in proportion than those of almost any other bird. - G. White.
Because many insects abide high in the air, even in rain, and the feathers of these birds are well preened to resist the wet. - G. White.
Because, when down, they can hardly rise, on account of the shortness of their legs and the length of their wings; neither can they walk, but only crawl. Their bodies being flat, they can enter a very narrow crevice; and where they cannot pass on their bellies, they will turn up edgewise. - G. White.