Timing is everything: accurate parasitism timing is critical to cowbird nestling success

Abstract

Obligate brood parasitism is a reproductive strategy whereby the parasitic parent leaves its offspring to be raised by a host of a different species. This has resulted in specialized adaptive behaviours selected to improve the survival of parasitic progeny. The timing of when to leave a parasitic propagule with the host can be paramount to the parasite’s success, and erring in this decision could have dire fitness consequences. Brown-headed cowbirds, Molothrus ater, are North America’s most widespread avian obligate brood parasite and, as generalists, successfully parasitize over 170 host species. Female cowbirds are cryptic and difficult to monitor, and much of how they make parasitic egg-laying decisions is not well understood. Despite the diversity in potential host species, cowbirds nearly always parasitize nests during a narrow window of the host’s egg-laying period just before the onset of incubation, resulting in their offspring typically hatching before the host’s own young. How important is it to get this timing right, and what are reproductive costs to parasitizing nests ‘late’ (i.e. after the onset of host incubation)? We simulated three different parasitic egg-laying timing decisions by adding cowbird eggs to the nests of a locally common host, the prothonotary warbler, Protonotaria citrea, at different stages of the host’s laying and incubation periods and monitored the subsequent effects on cowbird hatching success, hatching asynchrony relative to host eggs, nestling size, haematocrit, a health proxy, and survival. Our experiment established the costs of delayed parasitic egg laying: late egg addition produced cowbird nestlings that hatched at the same time or later than their host nestmates, had smaller same-age body size and showed markedly lower survival compared to those placed in the pre-incubation window, supporting our hypothesis that parasitism timing is a behaviour under strong selection in this, and perhaps other, obligate brood parasites.