Because swallows are much lighter than water, and could not sink in clusters, as they are represented to do. If their feathers are previously wetted, to destroy their buoyant power, in what manner can they resist the decomposing effect of six months' maceration in water, and appear in spring as fresh and glossy as those of other birds.
Swallows do not moult while they remain with us in an active state ; so that, if they submerge, they either do not moult at all, or perform the process under water. In the case of other torpid animals, some vital actions are performed, and a portion of oxygen is consumed ; but in the submersed swallows, respiration, and consequently, circulation, must cease. Other torpid animals, too, in retiring to their winter slumbers, consult safety: while the swallow, in sinking under the water, rushes to the place where the otter and the pike commit their depredations. It is now ascertained that migration is in ordinary cases practised by the swallow; yet their submersion has been believed by many naturalists ; - such as, Klein, Linnaeus, and others. - Fleming.
Because flies are not so plentiful in London as in the open country, and most of the chimneys have conical tops to them ; which, if they do not preclude, are certainly no inducement for their building in such places; the top of a chimney being its favourite site for its nest. - Jennings.
Because they by no means build altogether in chimneys, but often within barns and out-houses, against the rafters. In Sweden, the swallow builds in barns, and is called lada swala, the barn-swallow. - G. White.
Because swallows pursue the flies and gnats, and flies and gnats usually delight in warm strata of air; and as warm air is lighter, and usually moister, than cold air, when the warm strata of our air are high, there is less chance of moisture being thrown down from them by the mixture with cold air; but when the warm and moist air is close to the surface, it is almost certain that, as the cold air flows down into it, a deposition of water (or rain) will take place. - Sir H. Davy.
Because, probably, they withdraw, and slumber in some hiding-place during the interval; for it cannot be supposed that they had migrated, and so returned again for one day: more probably, they are awakened from sleep, and, like the bats, are come forth to collect a little food. - G. White.
Because its nests are eaten as great delicacies. They are found in the Indian Archipelago, and form an article of trade to the China market, where those of the first quality fetch their weight in gold! They are used to make soup, to which are ascribed powerfully restorative qualities. The substance of which these nests consist, resembles isinglass, and is disposed in irregular, transverse threads, with a few feathers interposed. Neither the analytical experiments of Dobereiner, nor those of Brande demonstrate it to be of animal origin. The relatively small portion of ammonia, indeed, which it yields, and its facility of incineration, rather lead to the conclusion that it is a vegetable gum. It was once supposed to be procured from the scum of the sea. Those individuals, however, residing fifty miles from the sea, employ the same materials as those which dwell on the shore. The other species in those districts likewise employ a portion of the same substance in the fabrication of their nests. - Fleming.