MEASURES OF BREEDING ACTIVITY
Age at first breeding presumably 1 yr and breeds annually thereafter. Three second-year males collected in St. Lucia had enlarged testes, which were the same size as those of adult (after-second-year) males collected at same time (WP).
Clutch and clutch size, in usual definition, may not be applicable to cowbirds. High numbers of cowbird eggs found in host nests suggest potentially high capacity for egg-laying by female Shiny Cowbirds. No information exists specifically for Shiny Cowbird on potential egg-laying capabilities for whole breeding season, nor is there any specific information on annual and lifetime reproductive success. Success of individual eggs of any brood parasite depends much on which host species are selected. Because females often parasitize >1 nest, measures of reproductive success for individuals are particularly difficult to obtain. With brood parasites, field studies record cowbird success in relation to host breeding activity - e. g., cowbird eggs/host nest, cowbird young/successful host nest. The success of individual females is likely to vary widely, as proportions of eggs producing fledglings change with host suitability. Egg success rates may be as high as 77% (in Black-cowled Oriole [Icterus dominicensis] nests) and as low as 7% (in Rufous-collared Sparrow [Zonotrichia capensis] nests). Number of young/female cowbird and proportion of all females successful in their reproductive effort not known.
Fate of 59 cowbird eggs in 29 nests of Rufous-collared Sparrow as follows: 20 abandoned, 27 taken by predators, 2 did not hatch, 10 hatched; of the 10 that hatched, 3 starved, 1 died of unknown cause, 2 were taken by predators and 4 left the nest successfully (Fraga 1978).