MIGRATION 167 



we can say how many migratory birds conform to this 

 rule, and how much elasticity there is in their observance. 



The same remark applies to the autumnal flight south- 

 wards. The order is in general reversed, for the young 

 birds are the first to leave and the older birds follow later. 

 It is impossible to tell whether the flocks of young birds 

 who seem to set off alone towards an unknown goal are or 

 are not accompanied by a few old experienced hands who 

 have made the journey several times before. We have 

 mentioned the exceptional case of the cuckoo, where the 

 callous parent birds leave Britain some six weeks or so 

 before the young birds, who are left in the care of the foster- 

 parents. Some of these foster-parents are non-migratory, 

 so that these at least will not be in a position to help the 

 young cuckoos on their southern journey. 



(e) Contrast o£ Spring and Autumn Flight, — In addition 

 to the diff"erences in the order of their coming and 

 going, there seems often to be a contrast in the manner of 

 flying. In many cases the departure in autumn is gradual 

 and hesitating — as if there was no particular hurry. The 

 birds make false starts ; they fly for some distance and 

 settle down to feed again ; they bid us a lingering farewell. 

 In spring, when the impulse in the mature birds is partly 

 sexual, there is more impetuosity. The flight is said to 

 be in some cases continuous — the dotterel which sups in 

 North Africa is said to breakfast next morning on the 

 Scandinavian Tundra. The spring flight is said to be 

 swifter than the autumnal flight and often shorter. It will be 

 borne in mind, however, that all these statements come under 

 the general heading " Problems in Process of Solution." 

 In regard to each and all it is necessary to have more facts. 



§ 3. Nature of the Migratory Custom 



It seems useful to keep the word " habit " for routine 

 established in the individual lifetime, but migrating is a 

 racial custom, rooted in the hereditary make-up or organisa- 

 tion. It is the expression of a constitutional impulse, an 



