26 U.S. NATIONAL MUSEUM BULLETIN 233 



bird has not been known to give any attention to parasitized nests 

 after laying in them, other than to remove eggs from them. Ordinarily, 

 eggs are not removed from nonparasitized nests. 



However, whether the matter of continuing interest may be a 

 general or an unusual situation, the concept of such durable attention 

 has interesting phylogenetic aspects. We may recall that the non- 

 parasitic bay-winged cowbird, M. badius, the most primitive of the 

 existing cowbirds, usurps or adopts a nest of another bird, from which 

 it then proceeds to evict any eggs or other contents, after which it goes 

 on to use it as its own — a clear case of proprietary interest. It lays 

 its eggs there and incubates, hatches, and rears its young in the ac- 

 quired nest. The most primitive of the parasitic species, the scream- 

 ing cowbird, M. rufo-axillaris , is parasitic on its close relative, the 

 bay-mng, and in the parasitized nests of the latter, one finds multiple 

 eggs of the former more often than single ones. Of 51 nests of the 

 bay-wing found in Tucuman in northwestern Argentina by Pablo 

 Girard, 46 were parasitized by the screaming cowbird. Out of these 

 46 cases, 6 nests contained 1 egg each of the screaming cowbird, 

 19 contained 2, 10 held 3, 5 had 4, 4 had 5, and 1 nest held 6 scream- 

 ing cowbird eggs. While these multiple eggs suggest repetitive visits 

 by their depositore, it must be admitted that no evidence was pre- 

 sented as to whether one or several M. ruJo-axiUaris hens were involved 

 in the various cases. However, my own observations in the same 

 general area indicated that the screaming cowbird was less numerous 

 than the bay-wing, which would suggest (but only suggest) that 

 these multiple eggs may well have been the result of successive visits 

 of the same hen, at least in many cases. Successive visits is essentially 

 what Mayfield referred to as continuing or even proprietary interest. 



In my account (1929, pp. 48-49) of the screaming cowbird, which, 

 unfortunately, no one has enlarged or emended in the decades since it 

 was wi'itten, I suggested that, while I never found more than 2 eggs of 

 this parasite in any single nest and while I knew of one hen that laid 

 2 eggs in each of two nests, the fact that the species appeared to be 

 strictly monogamous and territorial implied that normally only one 

 pair of screaming cowbirds (and, hence, only one female), would be apt 

 to parasitize any one nest. 



The matter of egg removal by the brown-headed cowbird merits 

 further comment. In my experience, the primitive parasite, M. 

 rufo-axillaris, does not remove eggs from its hosts' nests (although the 

 bay-wing does so if any are present when it firet takes possession of a 

 nest), but the more advanced parasites, M. bonariensis and M. ater, 

 frequently, if not regidarly, do remove eggs of their hosts. In the 

 case of M. bonariensis, the year-old birds tend to remain in flocks to a 

 fair degree and to come into breeding condition late in the Argentine 



