Eider ducks may nest in dense colonies, but each bird has its own nest in which it lays its own eggs, and in which the female alone incubates. But after the young hatch and the mother leads them to the water, the young may band into larger flocks, accompanied by a number of females, and the young seem to be independent of their particular parent, but attach themselves to and are tended by the nearest duck.
PENGUIN SOCIAL GROUPS A much more elaborate system for caring for the young has been evolved by certain penguins. The sexes alternate in their care of the young in the early stages. But when the young are partly grown the family unity breaks up for a communistic type of social organization. The young are now grouped into bands of up to twenty or more birds and are left under the care of a few old birds, while the rest of the adults go to the water, which may be some distance away. Periodically they return with food for the young. Apparently the individual young is not recognized by the parent, which goes to the particular group of which its young is a part, and there may feed any one of the "child groups."
Here we have two definite cases of a social organization that has resulted in division of labor: in the incubation of the ani, and in the care of young penguins. In addition we have two less specialized cases of the same thing, showing the sort of raw material on which evolution can operate to produce new behavior patterns.
BIRDS' NESTS AND THEIR SOUP [Ref]
In caves near the ocean in the Far East nest myriads of tiny swiftlets whose chief impact on the civilized world is that their nests provide an edible article of commerce. "Birds' nest soup" at once comes to the mind of the Occidental, few of whom have ever eaten of the nests, or even seen the birds to know them. For those who would like to see the nests, some museums have them on exhibition, such as in the Chicago Natural History Museum, where two nests are placed in their natural setting, and beside them is a quantity of the material of commerce in its raw state.
The birds themselves are dusky-colored swifts only a few inches long, and belong to a group of swifts that represents perhaps the most puzzling problems of species identification in the bird world. As yet we do not know even how many species there are. The genus is called Collocalia. Only some of its members make the edible nests; others mix so much moss into the nest that it is useless for soup. One species has the scientific name of esculenta, given in reference to the supposed edible nature of the nest, but through error the name was applied to a species whose nests are not edible. In habits all these swiftlets seem very similar, flying about with a rather weak flight for a swift, catching their insect food on the wing.
A number of swifts, including our chimney swift, use the secretion of their salivary glands as a glue to stick together their nest, and to stick it to the wall of a cave, the inside of a hollow tree, or the inside of a chimney. But some of the edible-nest swifts go further and make their nest entirely of this secretion from their enormously enlarged salivary glands. This material, as it comes from the mouth of the bird, resembles a saturated solution of gum arabic and is very viscid. If one draws out a strand from the mouth of the bird and sticks it on a rod, by rotating the rod and winding up on it the thread of saliva one can empty the salivary glands of the bird. This material dries quickly, and is the material of which the nest is made. When the bird makes its nest, which it does in large colonies in caves, it flies up to the rock wall, applies the saliva to the rock in a semicircle or horseshoe. Gradually a little shelf is built out, and in the finished nest one can see the many little strands that have gone into the structure. It may take the birds as long as three months to make this nest, even if undisturbed. The birds lay their two eggs in the nest, and raise their naked, helpless hatchlings into facsimiles of themselves in it.
But in the Orient, especially in China, the nests are highly prized by epicures as a delicacy. As the supply is limited the price is high. A note with some material we saw stated that the price was $12 to $36 a pound in Siam.
The climbing for and collecting of these nests requires daring, skill and is not without danger. The nests may be far back and high up in the cave. Ropes and poles may have to be fixed in place to aid the climber, who has a flaming torch in one hand and carries a sack or basket for the nests. In Siam, at least, the collecting of these nests was hereditary, father training son. The rights to collect nests are valuable. In Siam, where the rights to collecting the nests were vested in the state, revenue of as high as £20,000 has been received from the rights for this collection.