2 U.S. NATIONAL MUSEUM BULLETIN 233 



with them, it is not unduly rapid in its growth or especially aggressive 

 towards its food rivals in the nest. As Nice (1932, p. 47) has put it 

 in the case of the brown-headed cowbhd, the nestling cowbird "does 

 not grow faster than its nest-mates purely through greed; it has to 

 make nearly twice as big a bu'd in the same short space of nine or 

 ten days." While the young cowbird is frequently found to become 

 the sole occupant of the nest, this is not because it evicts its nest 

 mates the way some cuckoos do. The expired, unsuccessful competi- 

 tors for food are removed by the adult hosts, not by the young parasite. 



The range of host selection by the parasitic cowbirds reveals two 

 trends. The most primitive of the parasitic species, the screaming cow- 

 bird, Molothrus rufo-axillaris, is parasitic wholly on the very closely 

 related and ancestral form, the non-parasitic bay-winged cowbird, 

 M. badius. Both are very late breeders in the Ai'gentine summer, 

 chiefly from January to March, and the mere circumstance that other 

 small passerine bkds have finished nesting by then may be the factor 

 which helps to maintain such a stringent, seemingly obligate, host 

 specificity. From this basic, exclusive selection two trends branch out. 



One, developing through the shiny cowbird, M. bonariensis, and 

 the brown-headed cowbird, M. ater, is characterized by extremely 

 wide host selection, the known victims including small passerine birds 

 of all sorts and families, ecologically and geographically sympatric 

 with the parasite. As might be expected in parasites with broad 

 host tolerance, not a few unsuitable and improbable species of birds 

 have been utilized by them, but such instances are no more meaningful 

 than casual or accidental occurrences are in plotting the normal inclu- 

 sions of a local fauna. 



The second trend is toward a more restricted host selection, although 

 not nearly as much as that in the screaming cowbird. Here again, 

 two species of parasites are included, the bronzed cowbird, Tangavius 

 aneus, and the giant cowbird, Psomocolax oryzivorus, both of which 

 tend largely to utilize nests of related icterine species. In the case 

 of the bronzed cowbird, about half of all the recorded instances of 

 its parasitism involve hangnests of the genus Icterus, but the other 

 half shows that the species has extended its range of hosts to include 

 numerous other birds as well, some of them frequently. Its host 

 catalog includes 52 species in all, of which 1 1 are species of Icterus. 

 The giant cowbird, less weU observed and less perfectly known than 

 the bronzed, is known to restrict its choice of brood victims to larger 

 birds of the oropendola-cacique portion of the Icteridae, but there is a 

 record on one occassion of its parasitizing a jay. 



On the whole, the survey of the host relations of all the cowbirds 

 reveals that the hosts have exerted a surprising lack of selective effect 

 on the development of the cowbird's brood parasitism. It is true 



