No immediate or future extra costs of raising a virulent brood parasite chick

Parental care is an adaptive behaviour increasing the survival of young. Virulent brood parasites, like the common cuckoo Cuculus canorus, avoid the parental care and leave the care for their nestlings to hosts. Although raising a cuckoo is always costly because it kills host’s progeny, to date it is not known whether raising of a brood parasite itself represents an extra cost affecting host’s fitness. We quantified costs of rearing a cuckoo nestling in the most frequent host, the reed warbler Acrocephalus scirpaceus. We measured changes in the host physical (body mass) and physiological conditions (stress levels quantified via heterophils/lymphocytes ratio) within the one breeding attempt (immediate cost) and re-trapped some of these adults in the next breeding season to estimate return rates as a measure of their survival (future cost). In contrast to universal claims in the literature, raising a cuckoo nestling did not entail any extra immediate or future costs for hosts. This counte...