The excrement of the young of many birds who build their nests without any pretensions to concealment, such as the swallow, crow, &c., may at all times be observed about or under the nest; while that of some of those birds whose nests are more industriously concealed is conveyed away in the mouths of the parent birds, who generally drop it at a distance of twenty or thirty yards from the nest. Were it not for this precaution, the excrement itself, from its accumulation, and commonly from its very colour, would point out the place where the young were concealed. When the young birds are ready to fly, or nearly so, the old birds do not consider it any longer necessary to remove the excrement.
Sir H. Davy gives an account of a pair of eagles which he saw on Ben Nevis teaching their young ones to fly; and every one must have observed the same thing among commoner species of birds. The experiments of Spalding, however, have shown that flying is an instinctive faculty; so that when he reared swallows from the nest and liberated them only after they were fully fledged, they flew well immediately on being liberated. Therefore, the 'teaching to fly' by parent birds must be regarded as mere encouragement to develop instinctive powers, which in virtue of this encouragement are probably developed sooner than would otherwise be the case.
A few observations may here be offered on some habits which do not fall under any particular heading.
The habit which many small birds display of mobbing carnivorous ones is probably due to a desire to drive off the enemy, and perhaps also to warn friends by the hubbub. It may therefore perhaps be regarded as a display of concerted action, of which, however, we shall have better evidence further on. I have seen a flock of common terns mob a pirate tern, which shows that this combined action may be directed as much against robbery as against murder. Couch says he has seen blackbirds mobbing a cat which was concealed in a bush, and here the motive would seem to be that of warning friends rather than that of driving away the enemy.
I have observed among the sea-gulls at the Zoological Gardens a curious habit, or mode of challenge. This consists in ostentatiously picking up a small twig or piece of wood, and throwing it down before the bird challenged, in the way that a glove used to be thrown down by the old knights. I observed this action performed repeatedly by several individuals of the glaucous and black-back species in the early spring-time of the year, and so it probably has some remote connection with the instinct of nest-building.
Nidification.
In connection with the habits and instincts peculiar to certain species of birds, I may give a short account of the more remarkable kinds of nidification that are met with in this class of animals. As the account must necessarily be brief, I shall only mention the more interesting of the usual types.
Petrels and puffins make their nests in burrows which they excavate in the earth. The great sulphur mountain in Guadaloupe is described by Wasser as 'all bored like a rabbit warren with the holes that these imps (i.e. petrels) excavate.' In the case of the puffin it is the male that does the work of burrowing. He throws himself upon his back in the tunnel which he has made, and digs it longer and longer with his broad bill, while casting out the mould with his webbed feet. The burrow when finished has several twists and turns in it, and is about ten feet deep. If a rabbit burrow is available, the puffin saves himself the trouble of digging by taking possession of the one already made. The kingfisher and land-martin also make their nests in burrows.
Certain auks lay their single egg on the bare rock while the stone curlew and goatsucker deposit theirs on the bare soil, returning, however, year after year to the same spot. Ostriches scrape holes in the sand to serve as extemporised nests for their eggs promiscuously dropped, which are then buried by a light coating of sand, and incubated during the day by the sunbeams, and at night by the male bird. Sometimes a number of female ostriches deposit their eggs in a common nest, and then take the duty of incubation by turns. Similarly, gulls, sandpipers, plovers, &c., place their eggs in shallow pits hollowed out of the soil. The kingfisher makes a bed of undigested fish-bones ejected as pellets from her stomach, and 'some of the swifts secrete from their salivary glands a fluid which rapidly hardens as it dries on exposure to the air into a substance resembling isinglass, and thus furnish the "edible birds' nests" that are the delight of the Chinese epicures.'[166]
The house-martin builds its nest of clay, which it sticks upon the face of a wall, and renders more tenacious by working into it little bits of straw, splinters of wood, &c. According to Mr. Gilbert White:—