PHASES OF SOCIAL LIFE 137 



That the gregarious habit is formed originally not so much 

 by community of interests as by the common attraction of an 

 abundance of food is, as we have already hinted, indisputable. 

 Thus it is that birds of prey are rarely gregarious. An ex- 

 ception is found in the case of the little Red-footed Falcon 

 (Fa'co vespertinus) — a summer visitor to Europe — which just 

 before migration assembles in immense flocks, reuniting in 

 autumn previous to departure for its African winter quarters. 

 But it also forms small breeding communities, and further 

 assembles together in some number to roost. This is possible 

 because the food of this species consists almost entirely of 

 insects, such as dragon-flies, large moths, beetles and grass- 

 hoppers, varied by lizards, shrews and field-mice. The Osprey 

 again breeds in enormous colonies wherever the food supply 

 admits. As many as three hundred pairs are described as 

 nesting together in North America, and large colonies are 

 formed in parts of Europe, as in Pomerania. Thus we see, in 

 the case of the Red-footed Falcon, how a species, assembling at 

 first merely because drawn by a bountiful food supply, may later 

 develop more social instincts. 



While among some birds the gregarious instinct is deeply 

 rooted, in others it forms but the slenderest of ties. Some 

 species, for example, appear to associate during migration and 

 disperse immediately on their arrival at their destination. 



With most gregarious species the flocks are made up of 

 birds of both sexes, but in some cases the males and females, 

 during a part of the year at least, form separate companies. 

 This is true, for instance, of the Chaffinch, which, in some 

 localities at any rate, passes the winter after this fashion. 



Speaking generally, it would appear that gregarious species 

 hold their own best in the struggle for existence, and this is 

 most noticeable where the development of the social instinct 

 is highest. Such species are certainly numerically stronger 

 than solitary species. Thus Rooks and Jackdaws are far more 

 abundant than Ravens, though the last species has been 

 mercilessly persecuted by many and so further reduced. Swifts 

 are more numerous than Night-jars, Plovers than Rails. Nut- 

 hatches and Titmice are nearly allied, yet the latter are more 

 numerous than the former, which is a solitary bird. This 

 difference is certainly not due to the relative abundance of food 



