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. CO-OPERATION BY BIRDS [Illustration]