Host-Parasite Interactions between Chalk-Browed Mockingbirds and Shiny Cowbirds

• Published in: Ornithological monographs 1985-01 (36), p.829-844
• Main Author: Fraga, Rosendo M.
• Format: Article
• Language: English
• Subjects: Animal nesting; Bird nesting; Breeding; Breeding seasons; Brood parasitism; Eggs; Evolutionary and Behavioral Ecology; Female animals; Ornithology; Reproductive success; Sparrows
• ISSN: 0078-6594; 1941-2282
• DOI: 10.2307/40168319

The host-parasite interactions between a generalist brood parasite, the Shiny Cowbird (Molothrus bonariensis), and one of its hosts, the Chalk-browed Mockingbird (Mimus saturninus), were studied from 1972 to 1979 at a study site in Buenos Aires Province, Argentina. Particular attention was paid to the possible co-evolution of host defenses and parasite counterdefenses in the system. Field observations and experiments were used to estimate selective pressures and to detect correlative responses in both species. The relationship between host and parasite is characterized by a high incidence of parasitism, as cowbird eggs were observed in 78.1 percent (50 of 65) of the mockingbird nests. Parasitism reduced the mean number of mockingbirds reared per nesting attempt by 69.9 percent (0.93 in non-parasitized nests, 0.28 in parasitized ones). Reproductive success of the parasite was low, as only six young cowbirds were reared from 102 eggs (5.9%); still, the lower success of the cowbird in nests of other local passerines makes mockingbirds a reasonably good host choice. Possible mockingbird defenses against brood parasitism include early nesting (before the cowbird laying season) and nest guarding. Cowbird eggs in the study site are of two morphs, immaculate and spotted, occurring in roughly equal frequencies in nests of most host species. Mockingbirds ejected nearly all immaculate cowbird eggs, but few spotted ones. Cowbird nestlings in mixed broods have a significantly lower chance of fledging than host nestlings: 37 percent starved in one to seven days. The smaller size and (presumably) the lower competitive ability of the parasitic nestling may explain this. This differential pattern of mortality resembles brood reduction; it may not have evolved as an antiparasitic defense of mockingbirds. Shiny Cowbirds do not seem to have evolved any specific, successful adaptation to deal with this particular host, except perhaps gregariousness during nest searching; this may counteract nest guarding by mockingbirds. There was no evidence that cowbirds avoided laying immaculate eggs in nests of this host, although the success for this egg morph was almost zero. Female cowbirds that lay immaculate eggs have alternative, suitable hosts. No evidence exists of host fidelity among female Shiny Cowbirds, and probably most individuals that parasitize mockingbirds are reared by more abundant host species with fewer antiparasite defenses. /// Entre 1972 y 1979 en un sitio de estudi