74 THE MIGRATION OF BIRDS. 



the sake of company on the road. Other birds, 

 especially those that pair for Hfe, invariably migrate 

 each in the company of its mate ; and in such 

 species as pair at their winter quarters, the vernal 

 migration is always performed together, whether the 

 individuals gather into flocks or not. 



It very often happens during the course of the 

 migration period, which, roughly speaking, lasts 

 about three or four months in spring, and again in 

 autumn, that a spell of bad weather, or the long 

 persistence of unfavourable winds, will arrest the 

 flight of migrants. Throughout the area of 

 meteorological disturbance, Migration practically 

 ceases for the time being. During the prevalence 

 of unfavourable weather, however, great numbers 

 of birds, sometimes of many different species, 

 gradually accumulate, like so many wind- and 

 tempest-bound vessels in a quiet bay, all delayed 

 and waiting the first favourable moment to push on 

 again. As soon as the weather changes the hosts 

 of birds pass on ; and it is to this cause we may 

 chiefly attribute the spasmodic rushes of a species, 

 or many species together, which so often occur, 

 especially during the stormier and most unsettled 

 months of the migration season. Owing to this, 

 many eminently solitary birds are sometimes ob- 

 served to arrive at a gi\en point in company, but 

 they invariably separate as the great influx spreads. 

 These rushes are most frequent in autumn, not only 

 because storms are more prevalent then, but because 

 the number of migrants is greater than in spring. 



