Direct Estimates of Breeding Site Fidelity and Natal Philopatry in Brood Parasitic Brown-Headed Cowbirds Molothrus ater

Abstract

Avian obligate brood parasitism is a specialised life history strategy that may impact the dispersal of juvenile and adult parasites when compared with non-parasitic (parental) bird species. In contrast to expectations, however, several brood parasites show a territorial spacing system while breeding, including breeding site fidelity within and across years. In comparison, data are sparse on the extent of natal philopatry in brood parasites. We estimated minimum levels of breeding site fidelity and natal philopatry in the generalist parasitic Brown-headed Cowbird Molothrus ater in two subsequent years of colour-ringing known-sexed adults and nestlings. Adults' breeding site fidelity was moderate and similar to previous reports on this species and on other non-parasitic temperate zone passerines. We recorded lower estimates for natal philopatry compared to adult Cowbird breeding site fidelity, but this still fell within the range typically reported for offspring of other North American, migratory, and parental songbirds. These results suggest that social parasitism as a reproductive strategy does not in itself appear to impact patterns of philopatric behaviours of Brown-headed Cowbirds and perhaps other brood parasites.