Cuckoo host use

The breeding success of brood parasites relies on the host’s ability to raise their offspring. Brood parasites might cue on host phenotypes to parasitize high-quality individuals to maximize the likelihood of their offspring fledging. However, parasite preference for high-quality hosts will be dependent on the availability of having multiple nests as options to parasitize. Using long-term data from the common redstart (Phoenicurus phoenicurus), a regular host of the common cuckoo (Cuculus canorus), I tested if cuckoos selectively parasitize redstarts based on phenotypic proxies of individual quality. Morphometrics of breeding redstart adults, redstart and cuckoo nestlings were measured, and body condition was calculated for all. I first evaluated the extent of availability of redstart nests to cuckoos during each parasitism event. Host nest availability is a requirement when testing brood parasite host preference because brood parasites can only be selective when multiple host nests are available within their territories. I found that for at least 75% of nest parasitism events (59 of 78 nest), cuckoo females had other nests available to be parasitized within 1 km. Second, I tested the link between nestling size and the phenotypic traits of the parents and found that larger redstart adults raised larger redstart chicks, and females with higher body condition raised redstart chicks with higher body condition. Lastly, I tested the likelihood of redstart individuals being parasitized as a function of their traits, and the consequences for cuckoo nestlings in terms of their size and condition. I found that once the host nest availability was considered, redstart females with higher body condition had a higher likelihood of being parasitised. However, I found no evidence that cuckoo nestlings benefit from being reared by redstart females with higher body condition. As brood parasites have limited options for impacting their offspring’s development, non-random host use based on host quality seems may be particularly important for parasite’s fitness.