358 



BIRDS OF THE REPUBLIC OF PANAMA PART 4 



cacique eggs were white with similar black marks. Eggs taken from 

 Psarocolius nests were longer, more elongated, with white ground color. 

 An egg from Costa Rica was "spotless white, rough in texture and 

 slightly glossed" and measured 36.1x26.0 mm (Crandall, Zoologica, 

 vol. 1, no. 18, 1914, pp. 338-339). 



In Panama, N. G. Smith (Nature, vol. 219, 1968, pp. 690-694) 

 found that eggs either mimicked in size and color those of their host or 

 were "generalized" and non-mimetic. Response of the host species 

 varied: at some colonies the hosts were more aggressive to visiting cow- 

 birds and removed from the nest any eggs they detected as different 

 from their own, while the entire population of other colonies was much 

 more tolerant, neither driving away the cowbirds nor ejecting their 

 eggs. Smith correlated tolerance with the absence of wasp or bee col- 

 onies in the nest tree, since these insects somehow repel parasitic bot- 

 flies (Philornis) that place their eggs or larvae on nestlings; the young 

 cowbirds — themselves covered with down at hatching and less vulner- 

 able to the botflies — preen their downless nestmates and eat any insect 

 parasites they find. In the colonies lacking protective insects, produc- 

 tivity of the host species was higher in nests with cowbirds than in 

 those without. Smith suggested that the reason some female cowbirds 

 continue to place their eggs in nests where they have a great chance of 

 being rejected may be that those sufficiently mimetic also gain the pro- 

 tection from other predators provided by the wasps or bees. 



The adult male Giant Cowbird is rarely seen at oropendola or cacique 

 colonies, but fledglings often associate with young oropendolas and 

 when begging for food may stimulate feeding by adult cowbirds if 

 present (N. G. Smith in litt. to Eisenmann). 



MOLOTHRUS AENEUS AENETJS (Wagler): Bronzed Cowbird, 

 Vaquero Bronceado 



Figure 30 



Ps. [arocolius] aeneus Wagler, 1829, Isis von Oken, 22, heft 7, col. 758. (Laguna, 

 Veracruz, Mexico.) 



Medium size; entirely black. 



Description. — Length, male 195-208 mm, female 175-184 mm. Adult 

 male, entirely shiny black, iridescent purple on wings and tail. 



Adult female, entirely black, but with less iridescence than male; 

 undersurface, especially at throat, often tinged dark brown. 



An immature male taken at Puerto Armuelles, Chiriqui, on February 



