Parasitic cowbird development up to fledging and subsequent post-fledging survival reflect life history variation found across host species

Abstract: Generalist avian brood parasites commonly serve as a model system to test for plasticity in offspring growth and behavior under various host environmental and parental regimes. While past research has provided compelling evidence that developmental rates differ among hosts, the fitness consequences of such variation remain unclear. In hosts, carryover effects from the nesting to the post-fledging stage are critical in driving differential post-fledging survival within and among species, as well as the evolution of songbird life histories. Consequently, offspring of brood parasites may be subject to the same carryover effects and life history constraints observed in host species. Applying a pre- to post-fledging carryover effect framework to avian brood parasites may therefore provide novel insights into parasite life histories as well as parasite-host interactions. We assessed the potential influence of host carryover effects on the development and survival of juvenile cowbirds using empirical data from 10 realized host species in east-central Illinois, USA. We found that life history variation among juvenile cowbirds reflects variation in tradeoffs between mortality risk and nestling period length as well as pre- to post-fledging carryover effects found across host species. Cowbirds from host nests with higher mortality rates had shorter nestling periods, fledged with less developed wings, and exhibited higher rates of post-fledging mortality. Cowbird mass at fledging also predicted post-fledging survival, but was not associated with other life history traits. Our results provide novel links between juvenile growth, development, and mortality that help to explain differential survival in parasitic young across host species. Significance statement: Offspring of avian brood parasites develop at different rates based on the host environment in which they are raised, but the fitness consequences of such plasticity are poorly understood. Parasitic offspring may be subject to carryover effects from the nesting to the post-fledging stages and other life history constraints operating in host systems. We assessed the potential influence of host carryover effects on the development and survival of juvenile brown-headed cowbirds (Molothrus ater) using empirical data from 10 realized host species. Reflecting pre- to post-fledging carryover effects found across host species, cowbirds from host nests with higher mortality rates had shorter nestling periods, fledged with less developed wings, and exhibited higher rates of post-fledging mortality. Our results provide novel links between juvenile growth, development, and mortality that help to explain differential survival in parasitic young across host species.