200 THE MIGRATION OF BIRDS 



are not gregarious at any other time of the year become 

 so during the periods of migration, in order to avail 

 themselves of the experience of the veteran travellers of 

 their own or of other species. The desire to migrate is 

 a hereditary instinct originally formed and continually 

 kept up by the necessity to do so, in order to maintain a 

 struggle for existence against the changes of temperature, 

 but the direction in which to migrate must be learned 

 afresh by each individual. The theory that migration 

 ordinarily takes place at high elevations is supported by 

 the fact that it is only in dark or cloudy weather that 

 migration on a large scale is observed. It is supposed 

 that the landmarks being obscured by clouds, the birds 

 are obliged to descend to see their way, for it is observed 

 that as soon as the clouds begin to break, the migration 

 apparently comes to an end. On dark nights the stream 

 of migration suddenly stops when the moon rises. Each 

 bird has its time of migration. Weather has, apparently, 

 nothing to do with this date. Good weather does not 

 seem to hasten the birds to their breeding haunts, nor 

 bad weather retard their starting. If the suitable con- 

 junction of circumstances occur during the season of a 

 certain bird's migration, that bird visits the island. If 

 the season goes by without such conjunction, the bird 

 does not visit the island. The period of its migration is 

 over. The migration of this species has taken place at 

 high altitudes, it may be, or by other routes ; and it is in 

 vain to look for it until the next season of migration 

 comes round, when, given the necessary wind and 

 weather, the appearance of the bird may confidently be 

 expected. 



The period of migration of each species lasts about a 

 month. In spring, during the first week, the flocks con- 

 sist principally of adult males ; during the second week, 



