Cuckoos do not select redstart hosts of better quality despite potential growth consequences for nestlings

Common cuckoo, Cuculus canorus , females rely on host species to raise their young and therefore should benefit from targeting high-quality individuals that maximize their fitness. Empirical evidence for individual host selection is, however, mixed with some studies suggesting random choice. Nevertheless, it is possible that the lack of consistent evidence for host selection may be because spatio-temporal variation in host availability has rarely been accounted for, or because the implications of host choice on fitness outcomes have not been tested experimentally. Here, we combined long-term monitoring data with an experiment to examine whether cuckoo females parasitising common redstarts, Phoenicurus phoenicurus , target individual hosts of higher quality to optimize their nestlings' growth. We first explored the scope for cuckoos to choose, finding spatial and temporal variation in host nest availability and host quality (using completed clutch size as a proxy). Cuckoos may choose hosts at different spatial scales (i.e. near neighbours versus habitat patches), so we next investigated whether parasitism varied with host quality (1) across the study area and (2) among nests within putative breeding areas. However, we found no evidence that redstarts laying larger clutches were more likely to be parasitized. Finally, we conducted a crossfostering experiment to disrupt the cuckoo's choice of nest. Moving cuckoo eggs to nonparasitized nests and between parasitized nests had no effect on morphometric growth of nestlings (mass, tarsus and wing length). Nestlings raised by foster parents differing in quality (i.e. smaller/larger clutch size) from the original nest tended to grow faster but smaller in asymptotic mass and tarsus length. Together these results suggest that the potential fitness benefits of choosing high-quality hosts do not compensate possible costs of searching for hosts in this system, although using different proxies for individual quality could help resolve the complexities inherent to host choice. • Cuckoo females should target high-quality hosts to maximize their fitness. • We tested this hypothesis with long-term data and a growth experiment in redstarts. • Redstarts laying larger clutches were not parasitized more than their neighbours. • Cuckoo nestlings grew similarly in nests regardless of parasitism by cuckoos. • Cuckoos crossfostered between nests of differing quality grew faster but were smaller.