SNAKESKINS IN BIRDS' NESTS [Ref]
There are occasionally discovered behavior patterns of birds that are so unusual as to make one stop and wonder. They are unusual for birds generally, but in a species here and there they are the regular thing. Such is the placing of a shed snakeskin in their nests by some birds.
A bird like the English sparrow, or the road runner, which uses a variety of material coarse or fine, would be expected to use shed snakeskins occasionally, as it came across them. But there are a number of species that seem to use snakeskins regularly in their nests. It would seem that the birds deliberately sought out the skins for this purpose, as though they were as much a part of the nest as the mud in the bottom of a robin's nest or the fresh green grass heads ornamenting the entrance to some weaverbirds' nests.
SOME HABITS BAFFLING I have long since given up thinking that every aspect of a bird's life must serve a useful purpose. Indeed I have already pointed out some definite maladaptations. But usually every type of behavior has a logical origin. By considering its occurrence in various species and against the background of the bird's everyday life some correlations usually can be found.
The list of birds habitually using snakeskins in their nests is short, as follows:
1. Great-crested flycatcher—belonging to the New World flycatchers, breeding in Eastern North America and nesting in holes.
2. Arizona crested flycatcher—a relative of the great-crested variety, with similar habits.
3. Blue grosbeak—an American member of the sparrow family, making an open nest in bushes.
4. Black-crested titmouse—a member of the chickadee family, living in Western North America and nesting in holes.
5. Bank mynah—a starling, living in southern Asia and nesting in holes in banks.
6. Rifle bird—an Australian bird of paradise, making a cup-shaped nest in trees.
7. Madagascar bulbul—making a cup-shaped nest in trees.
LIKE A DECORATION Twenty or more other species of birds have been recorded as using snakeskins more or less commonly, or occasionally perhaps on the basis of availability or of chance. But with the above they're an essential part of the nest. In some of the species the snakeskins are arranged as a rim around the edge of the nest almost as a decoration; sometimes the snakeskins may make up most of the nest.
Now as to possible correlations. The species are not closely related. Except for the two flycatchers the other five represent five different families. The distribution over the world is wide, too: America, Asia, Madagascar, Australia. Various explanations for the behavior have been advanced. It has been suggested that it's correlated with hole nesting, but three of the seven do not nest in holes. The most common theory is that it's to frighten away possible predators by making them think there is a snake in the nest. However, this is not very likely, and, too, one wonders why the birds that use the snakeskins are not frightened themselves. Indeed, one writer, surely not seriously, has suggested that the fright in early life of crested flycatchers at finding a snakeskin in the nest accounts for the upstanding crest in this species!
"BURGLAR ALARM" THEORY Another suggestion is that the snakeskin, by the rustling noise it makes when touched, acts as an alarm bell or a burglar alarm to warn the rightful occupants of the nest when an intruder approaches. This also seems a rather weak explanation.
We are left, then, with the fact that this curious habit has been developed in a few birds, not closely related, that live in various parts of the world and that have very different habits. It is usual with them. A number of others occasionally have this habit.
My first clue as to the proper background against which to solve this habit came when, unpacking a bird collection made in Borneo by curator of anatomy D. Dwight Davis, I took out a bulbul's nest. In its outer edge were flat, weathered leaves that resembled snakeskins. Later, when we received a bird collection from Dr. D. S. Rabor of the Philippines there was a nest of another species of bulbul and this too had flat, dead, weathered leaves in it that looked like snakeskin. When I was in Madagascar, in 1929-31, I had found three nests of the Madagascar bulbul with a snakeskin used in each. Here was a clue. I decided to investigate the nests of the other species of bulbuls of southern Asia and Africa where the family is represented by many species. By considering the snakeskin-using species against the background of the nesting of the other species some correlation might appear.
BOOKWORK This became a library problem at once. I had to look up the earlier reviews of the problem in the ornithological journals, The Auk and the Ornithologische Monatsberichte, then in Strong's Bibliography of Birds, to make sure that no important papers were missing from my own subject file. Stuart Baker's Fauna of British India, Birds had a large part of one volume devoted to bulbuls, and gave excellent summaries of the nidification of each species occurring there. Bannerman's Birds of Tropical West Africa covered the western part of that continent, and Jackson's and Sclater's Birds of Kenya Colony did the same for the eastern part. For collateral material I looked in Mathews' Birds of Australia, Volume 12, Forbush's Birds of Massachusetts, and Mrs. F. M. Bailey's Birds of New Mexico, and a dozen minor publications.
But it was worth it.
Perhaps my earlier thinking was dominated by the thought that the shed snakeskins had been parts of animals toward which many birds show an antipathy. But it's extremely probable a bird does not recognize the snakeskin as such. Rather to it the shed snakeskin is a strip of thin, flexible material. Obviously it would be used, by chance, by many bird species, such as the house wren, which, in addition to using such natural materials as twigs, grass, and hair, has been recorded as using lead pencils, paper, nails, safety pins, and snakeskins in its nest.
As to the regular users of snakeskin, the snakeskin-using Madagascar bulbul did fit into a pattern. Bulbuls in general make characteristic simple cup nests. Some species use almost any available material. But quite a few species had specific choices of materials: one species' nest had tendrils of vines in its base; another a lining of grass heads of a certain color; another pine needles; another red dead leaves; and the Madagascar bulbul snakeskins.
A SOLUTION There seems to be a tendency for many species to make distinctive nests. They often accomplish this by a choice of material used by few or no other species. What more natural than that one species, being in a country where snakes are common, should hit on shed snakeskins!
To show that the choice of snakeskin as nesting material is an expression of a tendency for each species of bird to make a different kind of nest may not be much of an answer. But it is to an extent. No longer do we say, "Why are certain birds' nests characterized by snakeskins?" Rather we have the broader, more general question, "Why does each kind of bird tend to build a nest different from that of every other kind?" Thus, little by little, we clear away small, vexing questions and resolve them into larger, more general questions. For answers to these we sometimes plan extended work involving field studies, studies of specimens, and books. And sometimes, as we examine a specimen, read a paper, or unpack a shipment, an answer, or at least a clue, springs to our mind.