Giant Cowbird 



Psomocolax oryzivorus (Gmelin) 



The giant cowbird is related to the bronzed cowbird stock, from 

 which it differs chiefly by its much greater size and by the more 

 developed "cape" feathers. Apart from one recent observation 

 (Lehman, 1960), it is known to be parasitic only on other icterine 

 birds of the oropendola and cacique group, and, while the situation 

 needs further investigation, enough is known to make it apparent 

 that the range of host choice is usually restricted to these birds. 

 All of these fosterers have in common the habit of colonial nesting: 

 many of their long, woven, pouch-like nests are suspended from the 

 branches of a single tree. In its restricted range of hosts the giant 

 cowbird is more like the screaming cowbird, M. rufo-axillaris, than 

 it is like the bronzed cowbird. The latter, as stated earlier, is pre- 

 dominantly parasitic on icterine species, chiefly orioles, but has 

 extended its parasitism to include many other birds as well (see 

 pp. 173-188). 



We may recall that the bronzed cowbird, Tangavius, is considered 

 an evolutionary offshoot of the stock represented by the screaming 

 cowbird, which is parasitic entirely on one species, the closely related 

 bay-winged cowbird. I have long considered it probable that the 

 phjdetic arrangement between these three cowbirds would rank them 

 thus: M. rufo-axillaris as the most primitive, then Tangavius, and then 

 Psomocolax. However, the fact that there is greater similarity in 

 restricted host dependence between the large Psomocolax and the 

 smaller M. rufo-axillaris than there is between either of them and 

 Tangavius raises the possibility that the giant cowbu"d may be a very 

 large derivative from the screaming cowbird line, and that it, in turn, 

 gave rise to the more "normal" sized bronzed cowbird stock. The 

 present geographical distribution of the three would support such a 

 possible interpretation. The screaming cowbird occurs from central 

 Argentina north to extreme southern Brazil and to Paraguaj^; 

 the giant cowbird ranges from northeastern Argentina, Paraguay, 

 eastern Bolivia, and southern Brazil to Mexico; while the bronzed 

 cowbird is found in Colombia, and again from western Panama, 

 north through Costa Rica, Nicaragua, El Salvador, Guatemala, 

 and Mexico to Texas and Arizona. The chief difficulty in this con- 

 ception of relationships is that we have to accept a giant form 

 between two smaller ones. The evidence, if it may be dignified by 

 such a term, is not at all conclusive, but merely suggestive. If one 

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