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.
We may next notice the weaver (Ploceus textor) and tailor (Prinia, Orthotomus, and Sylvia). The former intertwines slender leaves of grass so as to produce a web sufficiently substantial for the protection of its young. The tailor-birds sew together leaves wherewith to make their nests, using for the purpose cotton and thread where they can find it, and natural vegetable fibres where they cannot obtain artificial. Colonel Sykes says that he has found the threads thus used for sewing knotted at the ends.[167]
Forbes saw the tailor-bird of the East Indies constructing its nest, and observed it to choose a plant with large leaves, gather cotton which it regularly spun into a thread by means of its bill and claws, and then sew the leaves together, using its beak as a needle, or rather awl.
This instinct is rendered particularly interesting to evolutionists from the fact that it is exhibited by three distinct genera. For, as the instinct is so peculiar and unique, it is not likely to have originated independently in the three genera, but must be regarded as almost certainly derived from a common ancestral type—thus showing that an instinct may be perpetuated unaltered after the differentiation of structure has proceeded beyond a specific distinction. The genus Sylvia inhabits Italy, the other two inhabit India. Sylvia uses for thread spiders' web collected from the egg-pouches, which is stitched through holes made in the edges of leaves, presumably with the beak.
The baya bird of India 'hangs its pendulous dwelling from a projecting bough, twisting it with grass into a form somewhat resembling a bottle with a prolonged neck, the entrance being inverted, so as to baffle the approaches of its enemies, the tree snakes and other reptiles.'
Sir E. Tennent, from whom this account is taken, adds:—
The natives assert that the male bird carries fire-flies to the nest, and fastens them to its sides by particles of soft mud. Mr. Layard assures me that although he has never succeeded in finding the fire-fly, the nest of the male bird (for the female occupies another during incubation) invariably contains a patch of mud on each side of the perch.
Dr. Buchanan confirms the report of the natives here alluded to, and says:—