THE BREEDING SEASON 25 



mined by the relation of daylight to darkness, having been 

 brought into being through the agency of natural selection, in 

 consequence of the necessity to most birds of dayhght for the 

 procuring of food. This hypothesis explains both the northerly 

 migration in spring and the southerly migration in autumn, 

 since at both times the birds are travelling in the direction of 

 increased light (or, if they start before the equinox, towards 

 regions where they will enjoy longer dayUght later in the season). 

 The suggestion that the time of the spring migration is deter- 

 mined in each individual by a stimulus set up by the growing 

 genital organs is in no way opposed to Schafer's theory, which 

 provides an explanation of the general fact of migration. 



It has been noted that the northerly spring migration is far 

 more hurried than the somewhat leisurely autumn migration in 

 the reverse direction. Furthermore, although the north-south 

 migratory movements are as a rule extraordinarily regular, it 

 has been observed that the birds do not all set out together, 

 and that the times of departure and arrival for each species 

 may vary in any one year by several weeks. Moreover, golden 

 plover are found migrating across Britain on their way north- 

 ward (perhaps to Iceland) at a time when other individuals of 

 the same species are rearing young in Britain. (The breeding 

 season in Iceland is about a month or six weeks later than in 

 Britain.) In view of these facts it is evident that the occurrence 

 of the migratory movement is dependent not merely upon 

 external or environmental influences, but also upon internal 

 or individual ones, and, as already stated, it is not improbable 

 that one of the factors involved is the state of development of 

 the organs of generation. 



Many birds are double-brooded, having young ones not only 

 in spring, but also in autumn before the close of the mild weather 

 (in temperate climates). Swifts are stated to have a second 

 brood in Southern Europe after leaving Britain in August, and 

 the same is said to be the case with nightingales. Wiltshire ^ 

 mentions that a pair of swifts that stayed behind the others, 

 had a brood in September, which migrated with the parent 

 birds in October. Whether birds are single- or double-brooded 

 probably depends to a large extent upon the duration of the 

 ' Wiltshire, loe. cit. 



