SONG—COURTSHIP—WNESTS. 677 
2. By active strokes of the wings, in which the wings move down- 
ward and forward, backward and upward, in a complex curve. This 
is of course the commonest mode of flight, 
3. By sailing or soaring with motionless spread wings, in which the 
bird does. not necessarily lose in velocity, or in vertical position, as is 
the case in gliding. It is illustrated by such birds as crow, falcon, 
stork, albatross, and has been observed only when there was wind. 
Song of birds.—Singing is a natural expression of emotional 
intensity. It is richest at the breeding season, and is always best and 
often solely developed in the males. But song in any excellence is the 
gift of comparatively few birds, though nearly all have a voice of some 
sort, often so characteristic that the species may be recognised by its 
call. The parrot and the jackdaw, and others, can be taught to 
pronounce articulate words; and the power of imitation‘is widespread 
among birds, which are notorious plagiarists. This power of imitation 
is important in relation to the general theory of instinct, for the song of 
all birds is probably in great part imitative, though to a limited extent 
inherited. Young birds taken away from their nests when very young, 
so that they have hardly heard the voices of their kind, may utter the 
characteristic note of the species, but they sing the song imperfectly. 
Many birds, apart from those who have been educated, have ‘‘ words,” 
expressing pleasure, pain, sense of danger, presence of food, and the 
like. But there is a difference between uttering words and having a 
language, which implies the expression of a judgment. 
Courtship.—Birds usually pair in the springtime, but there are many 
exceptions. A few, ¢.g. some of the birds of prey, live alone except at 
the pairing time; others, notably the doves, always live together in 
pairs ; many, such as rooks, parrots, and cranes, are sociable, gregarious 
birds. A few, like the fowls, are polygamous; the cuckoo is poly- 
androus. 
In most cases, however, birds pair, and the mates are true to one 
another for a season, The pairing is often preceded by a courtship, in 
which the more decorative, more vocal males win their desired mates, 
being, according to Darwin, chosen by them. Darwin attributed the 
captivating characteristics of the males, well seen in peacocks and birds 
of paradise, or as regards musical powers in most of our own British 
songsters, to the sexual selection exercised by the females ; for if the 
more decorative or the more melodious males always got the preference 
in courtship, the qualities which contributed to their success would tend 
to predominate in the race. He believed, moreover, that characteristics 
of male parents were entailed on male offspring. Wallace regarded the 
differences between males and females in another way, arguing that in 
the course of natural selection the more conspicuous females had been 
eliminated, brightness being disadvantageous during incubation. It 
seems likely enough that both conclusions are to some extent true, 
while there is much to be said in favour of a deeper explanation, to 
which Wallace inclines, that the secondary differences between the sexes 
are correlated with the fundamental constitutional differences involved 
in maleness and femaleness. 
Nests.— Alter pairing, the work of nest-building is begun. Almost 
all birds build nests ; the well-known habit is a characteristic expression 
