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.