An Experimental and Teleonomic Investigation of Avian Brood Parasitism

The term "teleonomy" has been suggested by Williams (1966:258) to describe the scientific study of adaptations.Williams indicated that relatively few evolutionary studies deal primarily with teleonomy despite the fact that adaptation is the most basic feature of evolution and of all biology.Avian brood parasitism, the phenomenon in which certain birds, the parasites, deposit their eggs in the nests of other birds, their hosts, is especially well suited to teleonomic studies since it provides a system in which the presence or absence of relatively obvious adaptations can be examined in two interacting genetic lineages.Parasitism is typically detrimental to the host' s reproductive efforts and selection favors defenses that reduce the impact of the parasite.These host defenses are in turn damaging to the parasite' s reproductive efforts and selection favors counter-adaptations by the parasite.cause the death of all of the host' s own young through competition for food (Friedmann 1963 ) .In certain cases, brood parasites seem to have extirpated or caused declines of local host populations (Schiermann in Southern 1954:221, Bond in Friedmann 1971:250, Mayfield 1961a).Thus, the adaptive value of host defenses is clearly very great.Adaptations for and against brood parasitism have relatively high selective values.In other forms of parasitism, selection favors a benign effect (Croll 1966:8) because an individual parasite is dependent upon its host for its immediate survival and continuing reproductive output.However, brood parasites depend on their hosts only for their immediate reproductive output and the maximum possible harm to the host (zero reproductive output for the host) usually increases the individual parasite' s reproductive output.In the absence of host young, a parasite' s offspring is assured a maximum amount of food.To suggest that selection adjusts brood parasitism so that it inflicts as little harm to the host as possible necessitates strong teleological or group selection processes because when a brood parasite inflicts maximum harm to the host, any detriment to the parasite, such as scarcity of a host species, occurs in future breeding seasons and is dissipated over all members of the parasitic population that utilize the host species.In many instances brood parasitism is maximally harmful.Some cuckoo and honeyguide nestlings are known to kill all of the host' s own young (Friedmann 1955(Friedmann , 1968)).Cowbird ( Molothrus) nestlings often Adaptations for and against brood parasitism usually occur when eggs are in the nest.Host defenses occurring at other stages of the nesting cycle would seem to be evolved with greater difficulty.Avoiding parasitism would be the best defense, but a mechanism for achieving it is not easily available.Refusing to feed the parasitic nestlings would remove some of the deleterious effects of being parasitized, but as Hamilton and Orians (1965) pointed out, the adaptive value of a positive response to nestlings begging for food is apparently so great that host adaptations based on insufficient care of the parasitic nestlings are largely precluded.Lack' s (1968:327) listing of parasites whose nestlings mimic the host' s young suggests that some hosts have evolved adaptations which operate during the nestling stage.But other selective pressures also act on parasitic nestlings (e.g., to make them cryptic) and these may sometimes be responsible for parasitic nestling mimicry.The most common host adaptation is believed to be the rejection of foreign eggs.The most common counter-adaptation by parasites seems to be mimicry of the host' s eggs.Although egg-related adaptations have received much discussion (Baker 1942, Southern 1954, Friedmann 1964, 1968, Payne 1967, Smith 1968, Rothstein 1971, and others), large-scale experimental studies of host adaptations are lacking.Especially critical is the scarcity of conclusive evidence for the basic assumption that some hosts reject foreign eggs.The experiments of Swynnerton (1916, 1918) and Rensch (1924, 1925) were among the first to demonstrate rejection but were too few to allow quantitative analyses.The present study examines adaptations in the actual and potential hosts of the Brown-headed Cowbird (Molothrus ater), the only brood parasite [2501