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 [Ref]
The importance of co-operation, contrasted with competition, has assumed increased importance in discussions of evolution, as it has in discussions of human social progress. Co-operation in nature is of various kinds; from the manner in which a forest shelters the squirrel to the manner in which two or more individuals of one species work together for a common object. The working together of two birds to rear a family is so well known an affair that one forgets that it is an example of co-operation, not only in building the nest and brooding and feeding the young, but also in defending the nest and the young.
Sometimes more than one species will join in ousting an enemy. For example, when a cat caught a young robin, recently out of the nest, the parents, in their frantic effort to make the cat release the bird, attracted the attention of another robin and a pair of cardinals nesting nearby in a honeysuckle. All five birds dived at the cat, screaming and pecking it so vigorously that it released the young robin and fled.
EAGLES JOIN EFFORTS More spectacular are some of the co-operative activities of birds in food getting. Bald eagles sometimes feed on ducks. Frequently two eagles may combine their efforts. The two birds may work together to force a black duck from the air onto the water, and when they are trying to catch a diving duck, they much more quickly exhaust their prey by swooping at it in turn. Bald eagles sometimes take water birds too large for them to carry, and then they must flap along dragging their prey on the surface of the water to the nearest shore. On one occasion an eagle dragging a large cormorant ashore was joined by two other birds, and all three took turns in dragging it. When they got it ashore, all three shared it.
Several fish-eating birds co-operate in capturing their prey. "The merganser is primarily a fishing duck ... very skillful and a voracious feeder. It pursues underwater and catches successfully the swiftest fish. Often a party of sheldrakes may be seen fishing together, driving the panic-stricken fish into the shallows or into some small pool where they may be more easily caught," according to A. C. Bent.
When a school of fish approached a flock of white pelicans, the birds suddenly assumed a circular position, surrounding the school. All the pelicans moved slowly but cautiously toward the center of the circle, their heads near the surface of the water or partly submerged and their necks slightly extended. The birds moved in perfect unison, making the circle progressively smaller, ready to engulf their helpless victims at the first opportunity. When all the pelicans were close to the fish, the birds made rapid jabs at the fish and apparently consumed a large number of them. It appeared that every bird got from one to several fish.
13,000 BAND TOGETHER Avocets and, to a lesser extent, the black-necked stilts also band together for co-operative drives on small fry and aquatic insects. Such drives are made in water of wading depth. Instead of forming circles the birds present compact spearhead and wedge formations and sweep the bottom muck with the characteristic back-and-forth side movements of their long bills. As many as 13,000 avocets have been observed taking part in such co-operative feeding projects.