i62 THE BIOLOGY OF BIRDS 



southwards. The spring movement is on the whole north- 

 wards, with curves to the east and west. Besides the 

 difference in direction, there seem to be differences in 

 mood, for the autumnal flight is more dallying and the 

 spring flight more impetuous. Audubon notes that the 

 American rice-bird flies in spring by night and in autumn 

 by day. In some cases the spring route is shorter than the 

 autumn route, as if the birds drew the S — N.E hypotenuse 

 of a triangle whose perpendicular was N — S and base 

 W — E. In many cases the first representatives of a species 

 to arrive in a North Temperate country in spring are the 

 mature males ; then follow the mature females ; the im- 

 mature birds bring up the rear. In the autumn the order 

 tends to be reversed, the young birds leaving first. In the 

 case of the aberrant cuckoo the adults leave Britain a month 

 or more before the young birds, but this is an exception 

 that proves the rule. The normal delay of the mature 

 birds in autumn may be associated with the labours of 

 nest-making and feeding the young and by the incidence of 

 a late summer moult. 



(D) In spite of minor perturbations, often due to the 

 weather, there is no doubt as to general regularity of the 

 migration movements. In different parts of a country the 

 dates of the arrival and the departure of migrants vary 

 within comparatively narrow limits. Like the dates of the 

 appearance of wild flowers, they are determined by old- 

 established internal rhythms correlated with external 

 periodicities, though they may be swayed to one side or 

 the other by the temperature and other physical conditions 

 of a particular place or a particular year. Fog and head- 

 winds may delay arrival, abundance of insects and mild 

 weather may delay departure, but on the whole there is 

 notable regularity in the comings and goings. It seems 

 that the weather conditions that obtain when a migratory 

 movement starts are of much more importance than those 

 into which the birds pass in the course of their flight, for 

 birds are not easily baulked when they get fully agoing. 



Another side to the regularity is seen in the return of 



