Behavioural and evolutionary ecology of avian brood parastism

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Obligate brood parasites lay their eggs in nests of other species of birds, thus depending on the hosts to rear their offspring. As most hosts produce fewer young, adaptations for successful parasitism are countered by increasingly sophisticated adaptations by hosts that minimize the effects of parasitism or the likelihood that it will occur. Focusing primarily on parasitic cowbirds in North America, with comparative work on cuckoos in the old world, we are studying this interaction from the standpoints of parasite and host. Once believed to parasitize any nests they happened upon, cowbirds are now known to use some hosts more than others; this is not surprising because cowbirds lay fewer eggs than previously believed. They seem to select only certain hosts, perhaps the best ones. Studying cowbirds and hosts in different habitats, we are exploring the idea that selective host use increases a cowbird's reproductive success. With experiments, we are identifying attributes of hosts that may cause cowbirds to lay non-randomly: (1) nest defense and acquisition of requisite behaviour, (2) nest placement, (3) tolerance of parasitism, and (4) host quality. We are comparing differences within and among species that influence host use, such as size, aggressiveness at nests, parasitism of already-parasitized nests (and its conservation implications), egg-discrimination capabilities, and costs of acceptance versus rejection of cowbird eggs. We will focus mainly on host tolerance and host quality, building in part on our discovery 15 years ago of a small species that ejects cowbird eggs. Experiments will extend work on limits of how hosts see cowbird eggs and their own eggs relative to their abilities to recognize foreign eggs, and on bill-size constraints, answering the pervasive question of why so many host species tolerate costly parasitism. Knowing the minimum physical requirements for ejection and host-specific costs of acceptance is essential for understanding whether equilibrium or lag better explains acceptance in each host species. Rejection costs may explain acceptance in some hosts (equilibrium), whereas lag explains it in hosts that cannot recognize cowbird eggs.