342 ANIMAL AGGREGATIONS 



are as integrating social factors, they do not form the sole outlet 

 for the expression of the fundamental social appetites, nor are they 

 the only foundation upon which social structures have arisen. 



Observations on bird behavior (Allee, 1923a; Sherman, 1924) 

 furnish interesting information concerning the problem of the ex- 

 tent to which social groups originate through individual, and to 

 what extent through family, behavior. The question at hand is, 

 Do these annually or semiannually recurring bird flocks form by 

 the coming together of individuals or by the collections of some 

 sort of family groups? The answer is that both methods occur. 

 There is much evidence that ducks and geese migrate in flocks in 

 which family units can be recognized (McAtee, 1924) and that in 

 the tropical rain-forests parrakeet flocks are made up of pairs rather 

 than of individuals. With the whistling swan and the Canada goose 

 supposed family-groups have been identified in mid-winter (Miner, 

 1923). The large flocks of bronze grackles make a conspicuous 

 feature of summer bird life in the Mississippi Vafley. On July 3, 

 M. Nice' reports seeing one of the birds of a flock beg from another 

 as young ones do from their parents, and interprets this as evidence 

 that a farnily joined the flock before the young were entirely in- 

 dependent. 



On the other hand, heterotypic flocks are frequent in which a 

 species may be represented by a single specimen. An extreme in- 

 stance of such a flock is furnished by Beebe (19 16), who records a 

 flock of 28 birds composed of 23 dift'erent species. He comments 

 upon the common occurrence of heterotypic flocks in British Guiana. 

 Such extremely heterotypic groups could scarcely have been formed 

 by family rather than by individual units. 



Sherman (1924), known, like Nice, to be a careful observer of bird 

 habits, gives much detailed information to show that not all flocking 

 of birds is on a family basis. Proof of this is easily given by calling 

 attention to the flocking habits of the cowbirds. The female cow- 

 bird deposits her egg in the nest of some other bird, usually smaller, 

 and leaves it there to be cared for by the latter. This socially 

 parasitized foster-mother frequently hatches and rears the young 



' Personal communication. 



