84 THE BIOLOGY OF THE SEASONS 



the data, we have to face the fact that, whether the 

 young birds that alight on Heligoland started alone or 

 not, they have to leave the island again without adult 

 guidance. 



In the presence of considerable information in regard 

 to a number of birds, we adhere to the view that in many 

 cases the young are the first to start on their southward 

 journey in Autumn and the last to arrive on their return 

 journey in Spring. Even among the adults there is 

 sometimes a curious separation of the sexes possibly 

 because they fly at unequal rates. 



That the old birds should delay longer than their 

 children is not in itself surprising, for many of them require 

 rest after the arduous labours of nest-making and feeding 

 the young. Moreover, many of them have to undergo 

 a moult before they start, and from this the young are 

 exempt. 



Another matter, in regard to which generalisation is 

 rash because data are few, is the influence of weather on 

 migration. It seems likely that the weather conditions 

 that obtain when and where a mass-movement begins are 

 of more moment than those into which the birds pass in 

 the course of their flight. Puffins, for instance, arrive 

 on our coasts with extraordinary punctuality, no matter 

 what the weather is like. It is certain, however, that 

 unfavourable weather may involve enormous increase of 

 mortality, which is probably always very high, and may 

 even put a stop to the flight altogether. Very dense fog 

 and a head-on gale may make flight impossible. Mr. 

 Eagle Clarke has supplied definite facts as to the delay 

 of some migrations by stormy weather, but he admits the 

 general statement that birds in their movements seem to 

 be very indifferent to meteorological conditions. As to 

 the fact often referred to, that many birds are seen 



