THE PHILOSOPHY OF MIGRATION. 73 



influences more or less important, the chief of which 

 is undoubtedly the supply of food. It is also 

 worthy of remark that the birds migrating earliest 

 generally stay the longest ; whilst those that are 

 the latest to arrive are the earliest to depart. 



The amount of sociability in birds wdiilst on 

 migration is not only very interesting, but very 

 variable. Some species are solitary pilgrims indeed, 

 coming and going in a very exclusive manner ; if 

 crowded together from some unusual cause en route, 

 taking the first opportunity of separating again, 

 even from the companionship of their own kindred. 

 The Woodcock {Scolopax rusticola) may be in- 

 stanced as a recluse; the Nightjar {Caprimidgus 

 europceus) and the Cuckoo {Cuculus canorus) as 

 others. Who has ever witnessed any gregarious 

 or even social tendency in the migrations of these 

 birds, always excepting the occasional crowds or 

 "rushes" that are due entirely to accident, as we 

 shall shortly learn ? Other birds, however, are 

 remarkably gregarious during migration, many 

 species only becoming so at such a time, and even 

 in species that are alw^ays gregarious the habit is 

 more intensified during Flight. Other birds are not 

 only gregarious but social also, and it is not unusual 

 to find migrating flocks composed of parties of 

 several species : witness the great congregations 

 of the Sw^allow tribe, and of various Waders and 

 Ducks at these periods. Many odd birds will also, 

 from time to time, join a flock of some other and 

 often very distantly related species, apparently for 



