CONICET | Buscador de Institutos y Recursos Humanos

Avian brood parasites lay their eggs in the nests of other species, leaving the hosts to care for the parasiticoffspring. The bookkeeping hypothesis predicts that, in order to reduce competition between parasiticnestlings, female parasites should keep a mental inventory of host nests that they have already parasitizedand avoid laying multiple eggs in the same host nest. However, selection against repeat parasitismshould be weaker when host nests are limited, or when hosts are able to rear more than one parasiticnestling. Here we use microsatellite genotyping of parasitic eggs to test whether female screamingcowbirds, Molothrus rufoaxillaris, avoid repeatedly parasitizing nests of their primary host, the greyishbaywing, Agelaioides badius, in Argentina. Parasitism rates were extremely high (96.5% of 57 hostclutches were parasitized with an average of 5.7 cowbird eggs each), indicating that host nests arelimited. Although eggs laid by the same female showed moderate spatiotemporal clustering, individualfemales rarely laid more than one egg in the same host clutch (2 of 57 clutches, 26 of which containedmultiple genotyped cowbird eggs). Females were much more likely to lay subsequent eggs in differenthost nests than to return to the same host nest. We found no evidence for kin structure among femalecowbirds parasitizing the same host nest, which were no more closely related than chance would predict.These results suggest that female screaming cowbirds frequently lay eggs in host nests that have alreadybeen parasitized by unrelated females. However, they typically lay just one egg per host clutch, eventhough greyish baywings are capable of rearing several nestlings. Since screaming cowbird laying is oftenpoorly synchronized with that of their host, avoidance of repeat parasitism may be adaptive if it allowsfemales to spread the risk of failure among multiple host nests.