Swifts over the possession of the nesting-holes will account for many an egg being knocked out of the nests. I have also noticed that the Swifts themselves, when disturbed, have a habit of fluttering and scrambling about in their nesting-holes, so that eggs may sometimes be dislodged by the parent birds.
The question also occurs—Are birds which nest in deep holes or crevices known to lay in each other's nests? This takes place most usually in the case of birds which make open nests on the ground, especially those which breed in colonies, as Gulls or Terns; also in the case of Game-Birds, as Partridges or Pheasants; and Water-Birds of various species, as Ducks, Coots, Moor-hens, &c. It certainly occurs less frequently with those which build their nests in trees or bushes, as do most Passerine birds; and, I should say, very rarely, if at all, in the case of birds which nest in holes. This, however, is a subject upon which further investigation is desirable.
The nesting materials which Mr. Jourdain mentions as used by the Swift agree entirely with my own experience. Feathers, small straws, and pieces of rubbish are always found—just such materials as would be blown into the air on windy days; but I have also generally found a quantity of the blossoms, catkins, or bud-scales of various trees, especially those of the oak and beech, which are blossoming just at the time when the Swifts are building, and whose blossoms are frequently blown about by the strong breezes of the end of May. These materials are always cemented together by a glutinous substance secreted by the bird. Indeed, without this curious provision of nature, the scanty materials could hardly be woven together into a nest at all; and it is no doubt intended to keep the substance of the nest from being dispersed, as the Swift generally builds in a large and irregular crevice, and is not able to lay down a large bed of materials filling the entire bottom of the hole, after the manner of the Tits or the Creeper; nor, as in the case of the Sand-Martin, would the loose feathers and straws be held together by being placed in a small and comfortable space.
There is no doubt a good deal yet to be learned about the nesting habits of these interesting birds, for the difficulty of observing them is very great, owing to their breeding in dark holes and crevices out of sight, and often in rather inaccessible situations.
