Screaming Cowbird 



Molothrus rufo -axillaris Cassin 



The host-parasite relations of this, the most primitive of the para- 

 sitic cowbirds, are quite simple. The species is parasitic only on its 

 ancestral and very closely related non-parasitic congener, the bay- 

 winged cowbird, M. badius. This relationship was first discovered by 

 Hudson (1874, pp. 161-166) and was further elucidated and described 

 in greater detail in my 1929 book (pp. 46-53). Unfortunately, except 

 for a still unpublished study by Hoy and Ottow, almost nothing of 

 importance has been added to our knowledge since then and there is 

 no need to repeat here the details available in my earlier publication. 

 Suffice it to say that the eggs and the nestlings of the parasite and of 

 the host are very similar and evince no signs of interspecific competi- 

 tion in any way different from what would normally transpire between 

 eggs or young of the host alone. The young of the two species grow 

 up together in apparent amity. 



Only one recent observation should be mentioned here. In the 

 province of Salta, northwestern Ai'gentina, Hoy (mss., 1961) found 

 evidence that a certain amount of mutual egg destruction may take 

 place between this species and its host. However, in view of my 

 feeling of uncertainty as to the identification of the eggs in each of 

 the cases he noted, it is still not clear to me which species did the egg 

 pecking and to which species the pecked eggs belonged. Hoy has 

 found a situation that differed from what I found in Tucuman and 

 Entre Rios provinces, where no such frequent egg destruction was 

 noted. It is only fair to add that Hoy and Ottow have far less of a 

 feeling of uncertainty about this than I do. Eventual publication of 

 their data should be of much interest and may be expected to clarify 

 the situation. 



There have been reports to the effect that the screaming cowbird 

 was parasitic on other birds at least occasionally, but none of these 

 cases has been authenticated, and their status is highly doubtful. 

 Grant (1911, p. 104) wrote that he had found eggs of the screaming 

 cowbird in nests of the yellow-breasted marsh bird, Pseudoleistes 

 virescens. However, the bay-winged cowbird has been known to 

 breed in old nests of this species, and Grant admitted that he had 

 found Pseudoleistes nests with nothing but cowbird eggs in them. 

 Since the eggs of the bay- wing and of the screaming cowbird are 

 distinguishable only with difficulty, the identification of these eggs as 

 the latter species is not certain. In this connection, it may be recalled 

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