496 BIRD BEHAVIOUR xvn. 5- 



conditions being then appropriate if anything for that direction. On 

 the other hand, control birds, which had been kept in Alberta but 

 without extra light, showed no tendency to move away if released in 

 mid- winter. Since the gonads of these controls had, of course, by then 

 undergone reduction, the stimulus to migrate south was not felt. This 

 would agree with the fact that only some individuals of species such 

 as the thrush and blackbird migrate. These experiments suggest that 

 seasonal changes in illumination may be an important factor deter- 

 mining migration. However, they leave many points unsettled. For 

 instance, is there a stimulus that starts the bird migrating north again 

 if it has 'wintered' near the equator, where there is little or no seasonal 

 change ? It may be that once alteration of the gonads has been started, 

 say, by reduction in light, a cycle will be set up, the gonads developing 

 again with the longer days in the south and thus driving the bird north 

 again, and so on. 



6. The breeding-habits of birds 



The complexity and variety of bird behaviour show especially in 

 their breeding; perhaps in no other creatures except men is such 

 elaborate behaviour involved in bringing the birds together and caring 

 for the young. In order to ensure adequate provision for the develop- 

 ment of a warm-blooded animal and its nourishment until it can fend 

 for itself it is necessary either to keep it within the mother or to provide 

 a means of incubating the eggs. In either case a long period of care is 

 necessary after birth; the young animals, having a large surface area, 

 require a great amount of food to keep warm. Thus young starlings 

 and crows may eat as much as their own weight of food each day. 



Mammals and birds set about providing for this warmth and food 

 in different ways. Since a female mammal can move about and get 

 food while pregnant the father can desert her altogether, though fre- 

 quently he does not do so. In birds the eggs and young cannot be left 

 cold for long and it is therefore especially desirable that the father 

 should help. In birds, therefore, perhaps even more than in mammals, 

 the breeding-habits involve the development of elaborate systems of 

 mutual relations, serving not only to bring the parents together but also 

 to keep them together throughout the period of incubation and while 

 feeding the young. The actual building of the nest may be an intricate 

 business in which both birds collaborate and a further factor is that 

 the pair occupies a territory around the nest, which they defend against 

 others of the same species. 



The type of association of the sexes varies greatly. In the ruff and 



