Ecology of avian brood parasitism at an early interfacing of host and parasite populations

The shiny cowbird (Molothrus bonariensis), a brood parasite, has recently spread into the Greater Antilles from South America via the Lesser Antilles. This species is a host generalist and upon reaching Puerto Rico exploited avian communities with no history of social parasitism. Forty-two percent of the resident non-raptorial land bird species were parasitized in mangrove habitat study areas. Cowbird parasitism affected hosts by (1) depressing nest success an average of 41 percent below non-parasitized nests, and (2) reducing host productivity. Parasitized hosts produced 12 percent fewer eggs and fledged 67 percent fewer of their own chicks than non-parasitized pairs. Growth rates of chicks of some host species were lower in parasitized nests compared with non-parasitized nests while growth of others was not affected by brood parasitism. Cowbird chick growth varied directly with host size; i.e., cowbird chicks grew faster and attained greater fledging weight and body size in nests of larger hosts. Factors important in shiny cowbird host selection were examined within the mangrove study community. Cowbirds did not parasitize avian species in proportion to their abundance. The cowbird breeding season coincided with that of its major hosts, which were high quality foster species, and did not extend into other periods even though nests of poor quality species were available. Food habits and egg size of cowbirds were similar to those of their hosts, suggesting that cowbirds choose hosts partly on the basis of this alignment. Cowbirds locate nests by cryptically watching activities of birds in likely habitat. Despite the recency of the cowbird's arrival in Puerto Rico, some nesting species have effective anti-parasite strategies, including alien egg rejection and nest guarding. Behavior effective in avoiding parasitism is similar to that used by certain birds in evading nest predators. It is suggested that anti-predator behavior is preadaptive to countering cowbird parasitism.