lyo THE BIOLOGY OF BIRDS 



the music. We must avoid the other error of thinking that 

 the stimuH act on the intelUgence of the individual ; they 

 set into operation a predisposition enregistered in the race. 



Of the constitutional change that follows the end of the 

 reproductive period for the year we know almost nothing. 

 The reproductive organs often dwindle into insignificance. 

 A moult frequently occurs at this time. The growing 

 scarcity of food, especially of insects, slugs, fruits and seeds, 

 will imply hunger and its discomfort. The shortening of 

 the daylight also leaves less time in which to secure the 

 minimum rations. The increasing cold and the onset of 

 stormy weather may also serve to awaken constitutional 

 memories. 



In some cases migration is a social affair, and it is likely 

 enough that migrants of the same kin excite one another. 

 Some young Black-Headed Gulls which we hatched and 

 reared in the laboratory and afterwards set free in a garden, 

 became interested at the time of migration in their kindred 

 who flew overhead, and they eventually joined them. 

 Matthew Arnold gives a beautiful suggestion of the social 

 stimulus : — 



" And as a stork which idle boys have trapped, 

 And tied him in a yard, at autumn sees 

 Flocks of his kind pass flying o'er his head 

 To warmer lands and coasts that keep the sun, 

 He strains to join their flight and from his shed 

 Follows them with a long complaining cry." 



§ 5. The Problem of Way-Finding 



There is often great mortality during migration, and 

 probably a discriminate elimination of those that wander. 

 Some lose their way on the pathless sea and perish of hunger, 

 others stray into areas which are too cold for them, others 

 are fatally attracted to the lighthouses and dash themselves 

 against the windows. Yet it must be admitted that the 

 migration is often very successful : the birds find their way 

 to their winter-quarters and they find their way back again. 

 Can any light be thrown on their way-finding ? 



