498 BIRD BEHAVIOUR xvn. 7 



We may recognize in courtship, therefore, three elements, first 

 sexual stimulation, secondly threat to other males, and thirdly mutual 

 stimulation while rearing a family, but the various types of display 

 are combined in so many different ways that any classification or 

 analysis is bound to be arbitrary and only a few examples that have 

 been thoroughly studied can be given. There are, of course, also 

 begging displays, given by young to parents, various displays given 

 to potential predators, and others. 



Song and displays that bring the sexes together are responsible in 

 part at least for many of the pronounced secondary sexual characters 

 in which the male and female differ. The beautiful plumage of the 

 cock pheasant or peacock and the more bizarre combs and wattles of 

 turkeys are displayed before the female in a manner that is clearly an 

 excitant to copulation. The secondary sexual characters by which the 

 sexes are differentiated may affect features as different as the colour, 

 length, and structure of the feathers, the colour of the iris and size of 

 the pupil, the shape and size of the body, the voice, the ornamentation 

 of the head, and the spurs on the feet. The importance of species 

 recognition in leading to differentiation of male plumage is shown by 

 Darwin's finches (p. 524) which, in the isolation of the Galapagos 

 Islands, where there are few other passerine birds, have abandoned 

 the highly coloured male plumage found in other finches. The recogni- 

 tion of individuals of the same species in this case is based on the 

 characteristics of the beak; a male will begin to attack an intruder only 

 when the face is seen. 



Display takes place either by one bird to the other or mutually, and 

 has the effect of bringing the animals together and keeping them 

 together for periods varying from a few minutes to several years. Long 

 unions are common in the large birds of prey and are often a result as 

 much of mutual association with the nest as with display by the other 

 bird. Some birds pair in the winter long before the gonads are ripe 

 (ducks), but more usually after two birds pair off the display produces 

 a gradual heightening of tension, leading to nest-building, copulation, 

 and ovulation within a few days. Probably the process of bringing the 

 birds together and ensuring coition requires even more elaborate 

 stimulation in birds than other animals because of their great mobility 

 and the fact that the male cannot grasp the female. Before he can 

 tread her in such a way as to ensure coition she must be brought into 

 a suitably receptive state. The function of the display is certainly 

 largely to induce this state in the female, though much more is 

 involved in addition. The processes of sexual stimulation, nest- 



