SOCIAL PARASITISM IN BIRDS 



555 



mere handful out of the thousands of kinds 

 of birds known to science. 



As the number of birds known to be 

 parasitic increased, interest in the subject 

 increased accordingly, and an enormous 

 literature grew up around the problem. 

 The number of theories brought forth to 

 account for the origin of the habit became 

 almost as great as the number of writers 

 on the subject. Before adding still 

 another to the long list of theories, we 

 may at this point examine the evidence 

 and material available. In order to make 

 the problem more approachable we may 

 limit it for the present to one group of 

 birds, — the cowbirds. 



THE COWBIRDS 



The term cowbirds as used in this paper 

 includes the true cowbirds (genera 

 Agelaioides, Molothrus, and Tangavius) and 

 the rice grackle (Cassidix). The latter is 

 in reality nothing but a large edition of 

 Tangavius, although it is not generally 

 called a cowbird. The genus Agelaioides, 

 restricted to Argentina, Paraguay, Uruguay, 

 and Brazil, is the oldest and most primitive 

 of the cowbirds. It contains two closely 

 related species, A. badius of Argentina, 

 Paraguay, and Uruguay, and A. fringil- 

 larius, a pale representative form in eastern 

 Brazil. The former is the one that is now 

 well known and will be called the Bay- 

 winged Cowbird in this paper. The genus 

 Molothrus contains the most typical cow- 

 birds; — three species with many races; 

 — M. rufo-axillaris of Argentina, Uruguay 

 and Paraguay; M. bonariensis of South 

 America from Patagonia to Panama; and 

 M. ater of North America, from the 

 highlands of central Mexico to the region 

 of Lake Athabaska, and from the Atlantic 

 to the Pacific. M. rufo-axillaris will be 

 referred to as the Screaming Cowbird, M. 

 bonariensis as the Shiny Cowbird, and M. 

 ater as the North American Cowbird. 



The genus Tangavius contains one species 

 known in life and one known only from 

 four skins preserved in the American 

 Museum of Natural History in New York 

 and the Konigliche Museum fur Natur- 

 kunde in Berlin. The former species is 

 T. aeneus, the Red-eyed Cowbird, the latter 

 is T. armenti, Arment's Cowbird. 



Before taking up the reproductive 

 habits of the different species it is neces- 

 sary to form a mental picture of their 

 phylogenetic relationships, so that we 

 shall be able to fit the habits into the 

 genealogical tree of the group. There is 

 not space available here to present all the 

 evidence, of which there are several 

 independent lines; a mere diagrammatic 

 outline will have to suffice. The evidence 

 is given fully in my book The Cowbirds 

 now being published by Charles C. 

 Thomas of Springfield, Illinois, by whose 

 permission some quotations are made in 

 this article. 



3. Tangavius 



1. A. badius — 1. M. rufo-axillaris 



3. M. bonariensis — M. ater. 



This scheme of relationship is supported 

 by geographical (distributional) data, as 

 well as by various lines of biological data 

 such as coloration, song, migration, and 

 courtship display. 



BREEDING HABITS OF THE COWBIRDS 



The Bay-winged Cowbird is in everyway 

 the most primitive species of the group and 

 probably represents the original condition 

 of the ancestral cowbird stock. It is non- 

 migratory, and is strictly monogamous. 

 It winters in flocks, and early in spring the 

 individuals leave the flock in pairs . There 

 is no courtship display of any sort. The 

 pairs then wander about looking for old 

 or empty nests, but frequently fight with 

 other birds for possession of occupied 

 nests, usually with the result that the 



QUAR. REV. BIOL., VOL. Ill, NO. 4 



