BIRDS IN THEIR NESTS
JUST as every Englishman is of opinion that his house is his castle, so does every little bird resent all attempts at prying into its private affairs in the nest. For this reason we really know very little of the home-life of birds. It is not that there are no seekers after such knowledge. Practical ornithology is a science that can boast of a very large number of devotees.
Many men spend the greater part of their life in endeavouring to wrest from birds some of their secrets, and such must admit that the results they obtain are as a rule totally disproportionate to the magnitude of the efforts. At present we know only the vague generalities of bird life.
We know that the hen lays eggs; that she, with or without the help of the cock, as the case may be, incubates these eggs; that the young, which are at first naked, are fed and brooded until they are ready to leave the nest, when they are coaxed forth by the parents, who hold out tempting morsels of food to them. But these are mere generalities. Our ignorance of details is very great.
The nests of most passerine birds are scrupulously clean. Young birds have enormous appetites, and much of the food which they eat is indigestible and must pass out as droppings, yet in the case of many species no sign of these droppings is visible, either in the nest, or on the leaves, branches, or the ground near the nest. What becomes of these droppings? Ornithological treatises are silent upon this subject.
Again, young birds are born naked, and in India are frequently exposed to very high temperatures, so that much liquid must pass from their bodies by evaporation. How is this liquid made good ? Do the parents water the birds, if so, how? I have never seen any mention of this in an ornithological treatise.
Let us to-day consider these two subjects : the sanitation of the nest and the method of assuaging the thirst of young nestlings.
As regards the first we have some knowledge, thanks to the patient labours of Mr. F. H. Herrick, an American naturalist, whose book, The Home Life of Birds, I commend to every lover of the feathered folk. Unfortunately, Mr. Herrick's book is to some extent spoiled for Englishmen, because it deals with birds with which they are unfamiliar ; nevertheless, its general results apply to all passerine birds.
Mr. Herrick is a very keen bird photographer. As every one knows, he who wishes to obtain good photographs of birds has two great difficulties to overcome. The first is to get near to his subjects, and the second is to find them and their nests in situations suitable for photography.
The former is usually overcome by the photographer concealing himself and his camera in a tent or other structure. At first the birds are afraid of the concealing object, but soon maternal affection overcomes their fear.
Mr. Herrick's method of overcoming the second of these two difficulties is to remove the nest to be photographed from the concealed situation in which it is usually built, and place it in a more open place. If the nest be thus moved when the young are some seven or eight days old, the parents will almost invariably continue to feed their young in the new situation, for at that particular period the parental instinct is at its zenith. In addition to obtaining a splendid series of photographs, Mr. Herrick has observed, from a distance of a few inches, the nesting habits of several American birds. As the result of these observations he is able to declare that nest-cleaning follows each feeding with clock-like regularity. " The excreta of the young," he writes, "leave the cloaca in the form of white opaque or transparent mucous sacs. The sac is probably secreted at the lower end of the alimentary canal, and is sufficiently consistent to admit of being picked up without soiling bill or fingers. The parent birds often leave the nest hurriedly bearing one of these small white packages in bill, an action full of significance to every member of the family. . . . Removing the excreta piecemeal and dropping it at a safe distance is the common instinctive method, not only of insuring the sanitary condition of the nest itself, but, what is even more important, of keeping the grass and leaves below free from any sign which might betray them to an enemy." These packets of excrement are quite odourless, and they are often devoured by the parent bird instead of being carried away. The digestion of very young birds must be feeble, and doubtless much of the food given them passes undigested through the aliment¬ary canal, so that it is capable of affording nourishment to the parents. Birds are nothing if not economical.
Of course, all birds are not so careful of the sanitation of the nest. Every one knows what a filthy spectacle a heronry is. According to Mr. Herrick, the instinct of inspecting and cleaning the nest is mainly confined to the great passerine and picarian orders. It is obviously not necessary in the case of those birds, such as fowls, of which the young are able to run about when born; nor is it needful in the case of birds of prey, who take no pains to conceal the whereabouts of the nest. Young raptores eject their semi-fluid excreta over the edge of the nursery; thus the nest is kept clean, but the drop¬pings on the ground betray its presence to all the world.
Coming now to our other question: How do young birds obtain the water which they require ? we have no help from Mr. Herrick. He makes no mention of this in his most interesting book. It is possible that nest¬lings are not given anything to drink, that the juicy, succulent insects or fruits with which they are supplied contain sufficient moisture for their requirements. We must remember that the skin of birds is very different from that of man. It contains no sweat glands, so that a bird, like a dog, can only perspire through its mouth.
The breath of mammals is so surcharged with moisture that when it is suddenly cooled the water vapour in it condenses; the result is we can " see the breath " of a mammal on a cold day. I have never succeeded in seeing a bird's breath, so am of opinion that the fowls of the air do not exhale so much moisture as mammals do. But even allowing for this, a considerable amount of moisture must be given out in expiration, so that it seems probable that young birds require more moisture than they obtain in their food. Drops of water have to be administered to hand-reared birds. Many birds fill up the crop with food and then discharge the contents into the gaping mouths of their young. In this condition the food must be mixed with a considerable quantity of saliva and possibly with water. The crop of a bird is a receptacle into which the food passes and remains until actually utilised. There seems no reason why water should not be stored for a short time in this receptacle just as food is. Perhaps birds " bring up " water as they do solid food, and thus assuage the thirst of their young. Such a process would be very difficult to detect; it would be indistinguishable from ordinary feeding to the casual observer. I hope that some physiologist will take up the matter. A quantitative analysis of the air exhaled by a bird should not be very difficult to make.