ANIMAL AGGREGATIONS AND SOCIAL LIFE 343 



cowbird. When able to fly, these young cowbirds, reared separately 

 and by foster-parents of different species, join in the well-known 

 cowbird flocks. Such flocks can form only by the collection of in- 

 dividuals. 



Bobolinks and goldfinches begin the formation of their autumn 

 flocks by the congregating of old males. Chimney swifts, a pre- 

 eminently flocking species, leave their nests by one's and two's to 

 join the immense late summer flock. Many birds rear more than one 

 brood a season; and Nice writes that the only young of the first 

 brood that she ever knew to stay with their parents throughout the 

 raising of the second brood were one set of bluebirds. 



Family ties apparently sit more lightly with birds than some 

 would have us believe. Passerine birds may change mates for the 

 second brood. Nice reviews the literature on banded birds and finds 

 • that in 7 pairs of 3 species there was a known shifting of mates in one 

 season, as contrasted with 20 pairs of 1 1 species in which there was 

 none.^ 



Nice writes further that she does not believe that song spar- 

 rows retain the family unit when flocking or for flocking, and she 

 cites trapping experience with a robin to show that the female 

 outstayed her mate and her three sets of offspring, and so evidently 

 could not have joined in a postbreeding-season flock. Much similar 

 evidence exists that with such relatively highly social individuals as 

 birds, which are capable of forming well-integrated flocks that ex- 

 hibit definite physiological division of labor and concerted group 

 action, the flocking may occur by the congregation of individual 

 birds, just as has been observed for the sleeping aggregations, where 

 the male birds may come together to sleep even during the breeding 

 season. 



It has just been stated that these bird groups form well-integrated 

 social units. Something of the intricacy of the social organization 

 possible among them has been revealed by the work of Schjelderup- 



' In this connection Miss Sherman contributes the following pertinent observation in 

 a personal letter: "My chimney swifts led a perfectly upright life for ten seasons, but 

 on the eleventh a shocking scandal occurred. An unprincipled female supplanted the 

 mate that had helped build the nest and had begun to lay her eggs, which were thrown 

 from the nest in which the interloper raised her brood." 



