Coming now to those nurseries in which the building materials are cemented together, we must first consider the nests of the swallows and swifts. These birds secrete a very sticky saliva, which quickly hardens when it is exposed to the air. This constitutes an excellent cement. Watch a swift working at its nest under the eaves of a house. It flies to it with a feather or piece of straw carried far back in the angle of its mouth, hangs itself by means of its four forwardly directed toes on to the half-completed nest, which is stuck on to the wall of the house, and, having carefully placed the feather or straw in the required position, holds it there until the sticky saliva it has poured over it has had time to harden and thus firmly glue the added piece of material to the nest. The bits of straw, feathers, etc., may be said to constitute the bricks, and the saliva the cement of the swift’s nest. Some swifts build their nests exclusively of their saliva. These constitute the “edible birds’ nests” of commerce, and may be likened to houses built entirely of cement. The martin (Chelidon urbica), the common swallow (Hirundo rustica), and the wire-tailed swallow (H. smithii) construct their nests of clay and saliva. They repair to some puddle and there gather moist clay, which they stick on to some building, so as to form a projecting saucer-shaped shelf. In this the eggs are laid. But nature has not vouchsafed sticky saliva to all birds, so that many of them have to find their cement just as they have to seek out the other building materials they use.
The chestnut-bellied nuthatch (Sitta castaneiventris), which nestles in holes in trees, fills up all but a small part of the entrance with mud “consolidated with some viscid seed of a parasitical plant.”
The hornbills close up the greater part of the orifice of the hole in which they nest with their droppings mixed with a little earth.
Hume informs us the rufous-fronted wren-warbler (Franklinia buchanani) utilises a fungus as its cement. “In all the nests that I have seen,” he writes, “the egg-cavity has been lined with something very soft. In many of the nests the lining is composed of soft, felt-like pieces of some dull salmon-coloured fungus, with which the whole interior is closely plastered.”
The cement which is most commonly used is cobweb. I do not think that I am exaggerating when I say that cobweb enters extensively into the structure of the nests of more than one hundred species of Indian birds. What birds would do without our friend the spider I cannot imagine.
The nest of some birds is literally a house of cobwebs. The beautiful white-browed fan-tail fly-catcher (Rhipidura albifrontata) is a case in point. Its nursery is so thickly plastered with cobweb as to sometimes look quite white. It is a tiny cup that rests on a branch of a bush or small tree, and is composed of fine twigs and roots, which are cemented to the supporting branch and to one another by cobweb. This the bird takes from the webs of those trap-door spiders which weave large nets on the ground.
Utterly regardless of the feelings of the possessor of the web, the fly-catcher takes beakful after beakful of it, and smears it over the part of the branch on which the nest will rest. It then sticks to this some dried grass stems or other fine material, next adds more cobweb, and continues in this manner until the neat little cup-shaped nest is completed. This, as I have already said, is thickly coated exteriorly with cobweb to give it additional strength.
The sunbirds or honeysuckers make nearly as extensive use of cobweb in nest construction as do the fan-tailed fly-catchers. Loten’s honeysucker (Arachnechthra lotenia) seeks until it finds a large spider’s web stretched horizontally across some bush; it then proceeds to build its nursery in the middle of this. As the material is added the nest grows heavier, and thus depresses the middle of the web until it at last assumes the shape of a V, in the angle of which the mango-shaped nest is situated. The nursery is thus suspended from the bush by the four corners of the cobweb.
A spider’s web looks such a flimsy affair that it does not seem possible that it could support a nest peopled by a number of birds. Sometimes the nest derives additional support by being attached to other branches. Moreover, a tiny creature such as a sunbird is almost as light as the proverbial feather. Then cobweb is exceedingly elastic, and, considering its attenuity, is able to support a surprising amount of weight. It occasionally happens that the common garden spider (Epeira diadema) is not able to find a point d’appui to which it can attach the lower part of its web; it then utilises a stone (which may be as much as a quarter-inch in each dimension) as a plummet to make the nest taut. This comparatively heavy stone hangs by a single thread.
I have sometimes amused myself by testing the strength of a strand of cobweb stretched across a path, by hanging bits of match or other light material on it. In one experiment a gossamer thread, thirty feet in length, stretched across a road, bore the weight of five blades of grass which were hung upon it. The sixth blade proved to be the last straw that broke the camel’s back.