xvii. 7 COURTSHIP 497 



certain game birds (blackcock) there is no pair formation : display and 

 copulation occur at communal display grounds. The wren {Troglodytes) 

 and a few other birds are polygamous, each male forming continuous 

 association with several females. The great majority of birds form pairs 

 throughout a single season, occasionally they change mates for the 

 second brood. The same pair may mate in successive years (crows, 

 swifts) and a few birds stay together through the year (ducks). 



7. Courtship and display 



The breeding of birds is nearly always seasonal, even in the tropics 

 where conditions are apparently almost uniform throughout the year. 

 In temperate latitudes breeding begins in spring as the gonads develop, 

 probably under the influence of increasing illumination. The changes 

 in behaviour with ripening of the gonads vary, of course, greatly with 

 the species. Birds that have been social through the winter, for instance 

 buntings, begin to leave their flocks, and the voice of the male changes 

 from the simple winter notes to the more complex breeding song. The 

 production of the elaborate secondary sexual characters of the plumage 

 and other features used in display is controlled partly by direct genetic 

 effects on the tissues, partly through hormones. Injections of male or 

 female sex hormones or anterior pituitary extracts influence the pro- 

 duction of some characters but not others, according to the species. 

 In. the majority of birds there is a breeding-season, initiated, at least 

 in many, by the effect of increasing length of day in the spring, acting 

 through the pituitary on the gonads. Other factors such as degree of 

 activity and food taken play their parts, especially near the equator, 

 where there is little seasonal variation. 



The song is one feature of the elaborate business of display and 

 courtship. It has somewhat different functions from species to species. 

 In the simplest case the display serves to bring the sexes together, to 

 enable recognition, and at a later stage as a stimulus to copulation. 

 Moreover, in some birds (doves, budgerigars, and canaries), the dis- 

 play serves as part of the stimulus to ovulation. Involved in the display 

 actions, however, is often a threat to other males. The aggressive 

 displays are usually different from the courtship displays, though in 

 many species the male's first response to a potential mate is an 

 aggressive one. When the female does not flee or fight back, as a male 

 would do, he gradually changes over to courtship display. Finally, 

 some forms of courtship, especially those that are mutual, seem to 

 serve to keep the partners together for the period of incubation and 

 feeding. 



