222 U.S. NATIONAL MUSEUM BULLETIN 233 



that of giant cowbirds. He also noted that there was no evidence to 

 even suggest that the adult Psomocolax attempted to destroy or to 

 remove eggs of its victims. He found Cacicus cela was parasitized to 

 a greater extent than Xanthornus decumanus, but, unfortunately, he 

 gave no actual figures for either. 



In Venezuela, Schafer (1957, pp. 135-138) found that Xanthornus 

 decumanus was parasitized by the giant cowbird, but he found no 

 evidence that another local oropendola, X. angiistifrons , was so 

 affected, even in mixed colonies. This may reflect a selectivity on 

 the part of the parasite, or a greater degree of nest protection on the 

 part of X. angustifrons, or it may be merel}^ that the total observational 

 data is incomplete. 



Although the giant cowbird is intermediate in size between these 

 two frequent victims, it lays an egg considerably smaller than either 

 of them. Psomocolax eggs average 28.6 x 19.2 mm.; those of Cacicus 

 cela, 32.5 x 24.5 mm. ; and those of Xanthornus decumanus, 33.8 x 24.1 

 mm. In this respect, Psomocolax is unusual since, in the smaller 

 cowbirds, Molothrus and Tangavius, the parasites' eggs are ordinarily 

 larger than, or at least as large as, those with which they are placed. 

 The problem of small egg size in proportion to body size becomes 

 acute in the case of the European cuckoo, Cuculus canorus. In this 

 species the egg weight is one-thirtieth of the body weight, whereas in 

 164 species of birds, as tabulated by Romanoff and Romanoff (1949, 

 p, 63), the egg weight averages one-ninth of that of the body. It has 

 been suggested by many wi'iters that the small egg size here is a direct 

 adaptation to parasitism on small fosterers, but it seems safer to say 

 that the small egg size made it possible for the cuckoo to use many 

 small hosts. There is no reason why Cuculus, had its egg been corre- 

 spondingly large, could not have been parasitic on larger birds, as are 

 the species of Clamator. 



