Welcome to the American Ornithological Society 2018 Annual Conference. We are pleased to have you join us at the lovely Hilton El Conquistador Resort in Tucson, AZ.
Patterns of animal growth and development vary widely and drivers of variation are often unclear. Differences in altricial bird nestling development have been linked to differences in food availability and predation risk, but current models of development largely ignore the cost of brood parasitism. Brown-headed Cowbird (Molothrus ater) brood parasites remove host eggs from the nest, disturb host parental behavior, and produce offspring which compete for food with the hosts' nestlings. We hypothesize that this costly avian brood parasitism plays a role in the evolution of host nestling growth and development strategies. We located and monitored 80 nestling-stage nests of three grassland-obligate songbirds that are known cowbird hosts: Grasshopper Sparrows (Ammodramus savannarum), Dickcissels (Spiza americana) and Eastern Meadowlarks (Sturnella magna). We measured the growth of the nestlings' (N=159) tarsi, wings, masses, bills, and feathers every other day for a total of 358 measurement events. The presence of cowbird nestlings did not change the mean structural growth of the hosts, but did increase the variability in growth between siblings. For host nestlings, faster growth resulted in earlier fledge dates, but for cowbird nestlings faster growth resulted in later fledge dates regardless of host. Finally, host nestlings in parasitized nests fledged earlier than nestlings in non-parasitized nests, though the magnitude of the effect varied between host species. Our results indicate that cowbird parasitism does affect songbird growth and development and we suggest further work on development include the potential effect of brood parasites.
