618 BIRDS. 



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 this power of utterance and the 

 possession of language, which implies the expression of a judgment, 

 e.g. food is good. 



The vocal organ of birds is not situated in the larynx, as it is in 

 Mammals, but in the syrinx — a song-box at the base of the windpipe. 

 In this syrinx there are vocal membranes or folds of skin ; their vibra- 

 tion as the air passes over them causes sound ; the note varies with the 

 muscular tension of the folds, with the muscular state of the complex 

 associated parts, and with the column of air in the windpipe. 



Courtship. — Birds usually pair in the springtime, but there are many 

 exceptions. Some, such as eagles, 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 polyandrous. 



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 natural and necessary expressions of the fundamental constitutional 

 differences involved in maleness and femaleness. 



Nests. — After pairing, the work of nest-building is begun. Almost 

 all birds build nests ; the well-known habit is a characteristic expression 

 of their parental care. Other creatures, indeed, such as sticklebacks 

 among Fishes, and squirrels among Mammals, besides numerous Insects, 



