Certain auks lay their single egg on the bare rock while the stone curlew and goatsucker deposit theirs on the bare soil, returning, however, year after year to the same spot. Ostriches scrape holes in the sand to serve as extemporised nests for their eggs promiscuously dropped, which are then buried by a light coating of sand, and incubated during the day by the sunbeams, and at night by the male bird. Sometimes a number of female ostriches deposit their eggs in a common nest, and then take the duty of incubation by turns. Similarly, gulls, sandpipers, plovers, &c., place their eggs in shallow pits hollowed out of the soil. The kingfisher makes a bed of undigested fish-bones ejected as pellets from her stomach, and 'some of the swifts secrete from their salivary glands a fluid which rapidly hardens as it dries on exposure to the air into a substance resembling isinglass, and thus furnish the "edible birds' nests" that are the delight of the Chinese epicures.'[166] The house-martin builds its nest of clay, which it sticks upon the face of a wall, and renders more tenacious by working into it little bits of straw, splinters of wood, &c. According to Mr. Gilbert White:-- That this work may not, while it is soft and green, pull itself down by its own weight, the provident architect has prudence and forbearance enough not to advance her work too fast; but by building only in the morning, and by dedicating the rest of the day to food and amusement, gives it sufficient time to dry and harden. About half an inch seems a sufficient layer for a day. Thus careful workmen, when they build mud walls (informed at first perhaps by these little birds), raise but a moderate layer at a time, and then desist, lest the work should become top-heavy, and ruined by its own weight. By this method, in about ten or twelve days is formed a hemispheric nest, with a small aperture towards the top, strong, compact, and warm, and perfectly fitted for all the purposes for which it was intended. Other birds build in wood. The tomtit and the woodpecker excavate a hole in a tree, and carefully carry away the chips, so as not to give any indication of the whereabouts of their nests. Wilson says that the American woodpecker makes an excavation five feet in depth, of a tortuous form, to keep out wind and rain. The orchard starling suspends its nest from the branches of a tree, and uses for its material tough kinds of grass, the blades of which it weaves together. Wilson found one of these blades to be thirteen inches long, and to be woven in and out thirty-four times.