In conclusion, I must mention the characteristic flight of swifts just before sundown. The birds close the day in what has been called “a jubilant rout”; as if they had not already taken sufficient exercise, they fly at a breakneck pace round about the building in which their nests are placed, dodging in and out of the pillars of the verandah, and fill the air with their shivering screams. This seems to be a characteristic of swifts wherever they are found.
BIRDS AS AUTOMATA
The sudden change that comes over the nature of most birds at the nesting season is, perhaps, the most wonderful phenomenon in nature. Active, restless birds, which normally spend the whole day on the wing, are content to sit motionless in a cramped position upon the nest for hours together. Birds of prey, whose nature it is to devour every helpless creature that comes within their grasp, behave most tenderly towards their young, actually disgorging swallowed food in order to provide them with a meal. Timid birds become bold. Those which under ordinary circumstances will not permit a human being to approach near them, will sometimes, while brooding, actually allow themselves to be lifted off the nest.
At the breeding season intelligence, which counsels self-preservation, gives way before the parental instinct, which causes birds to expose themselves to danger, and, in some cases, even to sacrifice their lives for the sake of their offspring.
From the construction of the nest until the time when the young ones are fledged the actions of the parent birds are, at any rate in the neighbourhood of the nest, those of automata, rather than of creatures endowed with intelligence.
On this hypothesis alone are many of the actions of nesting birds comprehensible.
That the construction of the nest is in the main an instinctive habit and not the result of intelligence is proved by the fact that a bird which has been hatched out in an incubator will, at the appointed season, build a nest. If birds were not guided by instinct they would never take the trouble to do such a quixotic thing. What benefit can they derive from laboriously collecting a number of twigs and weaving them into a nest?
It is, of course, natural selection that has originated this instinct; for those species in which the parental instinct is not developed, or in which there is not some substitute for it, must inevitably perish. When once this instinct has taken root natural selection will tend to perpetuate it, since those species which take the best care of their young are those which are likely to survive in the struggle for existence.
Many instances can be adduced to show how automatic are the actions of birds at the nesting season.
It sometimes happens that a bird lays an egg and then proceeds to build a nest on top of it.