Part 10

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

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.

Another striking example is furnished by black vultures observed by E. A. McIlhenny. A three-quarters-grown skunk was wandering across a field. A vulture alighted near the skunk which immediately stopped and raised its tail. Other nearby vultures joined the one nearby the skunk, and when six or eight of them had gathered one suddenly attacked it. The skunk immediately discharged its defensive scent, but without effect, for the vultures attacked in a mass and other vultures circling above joined in until there were probably twenty-five or more around the skunk. With much flapping and croaking, the vultures pulled it about until it was dead, and then devoured it.

On another occasion a black vulture came from high in the air to alight near two full-grown opossums following a narrow cattle trail. The first vulture was almost at once joined by many others until there were probably between seventy-five and one hundred black vultures following the opossums. Suddenly three or four of the vultures attacked and the others joined in. Quickly both opossums were covered with a swarm of hissing, flapping birds, and within fifteen minutes there was nothing left of them but the larger bones and the hides, and these were stripped of every vestige of flesh.

WATCHDOGS AT THE NEST

A savage watchdog outside his master's house helps to protect it. If an intruder comes, the watchdog, if it's the right kind, simply bites him without preliminaries. There's a parallel to this in the bird world. Some birds often have their nests close to wasps' or bees' nests, or in trees inhabited by biting ants. The birds and the ants, wasps, or bees get along without disturbing each other. But when intruders come along the insects swarm out, biting or stinging and driving the intruder away. The insects are protecting their own homes, but one of the results, the protecting of the birds' homes, is just as satisfactory to the birds as if they did it on purpose. This building of birds' nests close to wasps' nests is a common practice with certain sunbirds and weaverbirds, especially in Africa. It occurs too often to be chance. The question naturally arises as to how much the birds understand of it all--do they actually seek out the association? That's difficult to say, but the facts of the association are still there.

Though some of these associations are evidently fairly common and chosen deliberately by the birds--and one can easily visualize how the protection works--field observations as to the natural enemies against which they are effective, and how effective they are, are largely lacking. Usually the records are something like those of Van Rossem for the Giraud's flycatcher in El Salvador, in which he points out that this bird usually nests in certain mimosa trees armed with numerous heavy, curved thorns. These thorns are hollowed out and inhabited by swarms of small but extremely hostile antlike insects, so that the nest is well protected. However, the effectiveness of ant and bee protection against human predation can be seen in the following.

Take the case of Mr. M. E. W. North, who arranged a rope to climb to a fish eagle's nest in East Africa. He had gotten about fifty feet up and was considering going out on the big limb on which the nest was, when he noticed a wild bee on his sleeve. Realizing that he was disturbing a wild-bee hive, and knowing that the sting of these vicious bees can be dangerous, fatalities having been reported, he came down his rope at express speed, crashing through projecting branches and brambles. Reaching the ground, he freed himself from the rope and fled to a safe distance, considering himself lucky to have received only three stings.

On another occasion, again in East Africa, Mrs. R. E. Moreau attempted to reach a hawk's nest to measure the eggs, but when she was up in the tree, savage, biting red ants drove her out.

BIRD GUIDES TO HONEY

In Africa there are birds which lead men to honey. They are called honey-guides and their family name, Indicatoridae, has the same idea incorporated into it. Though there are several species of these small, dull-colored birds, which are related on the one hand to woodpeckers and on the other to barbets, it is only one species, the common or black-throated honey-guide that is well known as a guide to honey.

The traveler in the country may find one of these birds chattering and flying ahead of him. The natives, who know this bird well and favorably will tell the traveler to follow; it will lead to a bee tree. The native, as he follows this guide, gives occasional whistles, as if to encourage the bird. The bird continues, flying from perch to perch, ahead, and chattering noisily. Sometimes it may return to see if the men are following; sometimes it remains chattering on its perch until the followers catch up. Finally the bird will go no farther. It flies about aimlessly and allows one to approach closely. This is the spot. In a hole in the tree trunk, or in the ground nearby the bees' nest is to be found.

When the beehive is opened, and the honey taken, the honey-guide will eat the comb that is left, and apparently it is for this that the complicated behavior of leading of man to the beehive is developed.

Wax of the honeycomb is a usual food of this species, judging by stomach examinations, and one wonders how they get it when man is not about to open the bee trees for them. The birds have no special adaptation for getting into the hives; indeed their only apparent adaptation for this habit is a thick skin, perhaps a protection against bee stings. Perhaps, as has been suggested, other animals, squirrels, monkeys, or honey badgers may unwittingly aid them by opening up bee trees for their own purposes and allow the honey-guides to snatch food for themselves.

An amusing side of the picture is that sometimes the honey-guides may lead the honey hunter to a beehive owned by a native.

There are also records of the honey-guide leading men to big game: leopard or lions. That this occurs is amply documented, but one wonders whether or not this was accidental; the honey-guide leading the way to honey perhaps by accident leads the way past the resting place of one of these big cats so that the man stumbles over the big game and perhaps gets the impression he was led to the animal.

OXPECKERS

The lives of oxpeckers are so linked to those of large, hoofed game or domestic cattle that in West Africa where game is scarce the birds depend on cattle, and their range is restricted accordingly. There the cattle are confined to the higher and more northern areas, free of tsetse flies, from Senegal to Northern Cameroon. Thus tsetse flies help to determine the limits of the oxpeckers' range.

Except for their nesting, which is in holes in trees, and their sleeping, most of their time is spent on the bodies of the larger herbivores. There they run about over the hides and legs of the beasts, like woodpeckers on a tree. They stay remarkably close to the animals, and even ride on them as they travel. The oxpeckers' food is largely ticks, which it gets from the hide of the animal by working over it with the side of its bill, shearing off the ticks with a scissorlike action of its mandibles. But when an animal has sores or cuts or scratches the oxpecker may peck into them, and eat flesh and blood of its host.

Correlated with this unusual and close relationship, a modification in the oxpeckers has taken place. There are only two species, both African, and they are dull-colored, modified starlings. The legs are stout, with curved, very sharp claws for clinging to the hides of animals, and the bill, very sharp at the tip, with the cutting edge of the mandible very sharp to aid in scissoring off ticks.