Transitions between life history stages are fitness-limiting events that depend on environmental and individual characteristics. For altricial birds, fledging from the nest is a critical shift in development with direct impacts on survival, yet it remains one of the most understudied components of avian ontogeny. Even less is known about how brood parasitism affects the fledging process in host nestlings. The prothonotary warbler (Protonotaria citrea) is a host of the obligate brood parasitic brown-headed cowbird (Molothrus ater). We tested whether the presence of parasitic nestlings negatively alters host fledging by experimentally parasitizing nests with a cowbird (heterospecific parasite) or a warbler (conspecific parasite) egg, comparing them to non-manipulated control nests, and monitoring them using radio-frequency identification (RFID) systems. As expected, in heterospecifically parasitized nests, warblers were smaller, fledged at older ages, and had greater overall fledging latency compared to conspecifically parasitized nests. There was no such impact of conspecific parasitism relative to controls. Warbler nestling size predicted the age and order of fledging, with larger nestlings fledging earlier. Nestlings fledging at earlier ages fledged later during daytime hours. Cowbirds and last-fledged, smaller warbler chicks spent the most time in the nest entrance before fledging. Finally, although male warbler nestlings were larger than females, there were no sex effects or effects of extra-pair status on fledging. Our study shows that while conspecific parasitism has no detectable effect on host nestmates, heterospecific parasitism impacts host size and fledging phenology, which may influence post-fledging survival of parasitized broods. Significance statement In many species, juveniles undergo dramatic transitions in lifestyle as they age and become independent. For most birds, fledging from the nest is an important developmental shift, potentially impacted by their previous growth and early social experiences. One aspect that may affect fledging is brood parasitism, whereby birds lay their eggs into the nests of other birds who care for the unrelated young. Here, we determined experimentally if brood parasitism affects fledging of the prothonotary warbler, a species that always accepts eggs of the larger, obligate brood parasitic brown-headed cowbird. Our results show that while same-species parasitism has no statistical effect on the fledging of host warblers, cowbird parasitism causes delayed fledging in hosts, revealing a previously unappreciated, additional cost of brood parasitism upon host nestlings.
