Bronzed Cowbird 



Tangavius aeneus (Wagler) 



Discussion 



The following catalog of the known hosts of the bronzed cowbird 

 includes 52 species, or, with subspecies, 64 forms of birds. They 

 comprise 12 families, one of which, the pigeons, can only be looked upon 

 as accidental, as these bu'ds are quite unsuited for the role of potential 

 fosterers. One other family, the cotingas, is known as a host from 

 a single instance; the thrushes are represented by a single species, 

 with two records of parasitism; the jays are known from three instances 

 involving a single species; the warblers, with three instances involving 

 2 species; and the vireos are represented by 2 species, one with a single 

 record of parasitism and the other with five such records. The 

 remaining 6 families are represented as follows : flycatchers — 4 species 

 with 7 records; wi-ens — 5 species with 1 record apiece; thrashers — 

 4 species with 7+ records; tanagers — 3 species with 5 records; orioles 

 and blackbii'ds — 11 species with 84 records; finches — 16 species with 

 51 records. 



Since the bulk of all the cases recorded are in the last two families, 

 it becomes increasingl}^ clear that these two groups comprise the 

 primary hosts of the bronzed cowbh-d. This parasite is, thus, more 

 selective, or more restricted, in its choice of fosterers than are the 

 brown -headed and the shiny cowbirds. The infrequent use of vu'eos 

 and wood warblers and the relative scarcity of records of tyrant 

 flycatchers are striking and significant differences from the situation 

 revealed in a survey of the frequent hosts of the brown-headed 

 cowbird. 



As I described in my first account (1929, pp. 328-334) of the 

 bronzed cowbird's fosterers, its relatively restricted range of host 

 choice becomes meaningful when we remember that its nearest relative 

 (and possible ancestral stock) is the screaming cowbu-d, Molothrus 

 rujo-axillaris. T'hat species is parasitic entirely on its very close 

 ancestral relative, the bay-winged cowbird, M. hadius. From M. 

 rvfo-axUlaris, or the stock of which it is the extant representative, 

 two lines have diverged, one leading to a wide host-tolerant group 

 of parasites, AI. bonariensis and M. ater, and one that retained more 

 of the restricted host relationships of its ancestor, the present species 

 T. aeneus. Not having any self-breeding immediate relatives to 

 parasitize, as does M. rujo-axillaris, the bronzed cowbird apparently 



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