Virulent brood parasites refrain from arduous parental care, often kill host progeny and inflict rearing costs upon their hosts. Quantifying the magnitude of such costs across the whole period of care (from incubation through to parasite fledgling independence) is essential for understanding the selection pressures on hosts to evolve antiparasitic defences. Despite the central importance of such costs for our understanding of coevolutionary dynamics, they have not yet been comprehensively quantified in any host of any avian brood parasite. We quantified parasite-rearing costs in common redstarts Phoenicurus phoenicurus raising either parasitic common cuckoo Cuculus canorus or their own chicks throughout the complete breeding cycle, and used multiple cost parameters for each breeding stage: incubation, brooding and feeding effort; length of parental/host care; parent/host body condition; and heterophil/lymphocyte ratio (stress-level indicator). Contrary to traditional assumptions, rearing the parasite per se was not associated with overall higher physiological or physical costs to hosts above the natural levels imposed by efforts to rear their own progeny. The low parasite-rearing costs imposed on hosts may, in part, explain the low levels of known host counter-defences in this unusually frequently parasitized cuckoo host.

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Brood parasite–host interactions during the incubation and nestling stages have been well studied, but the post‐fledging period remains virtually unknown. Using radiotracking, we provide the first detailed data on post‐fledging interactions between the Common Cuckoo Cuculus canorus and its only regular cavity‐nesting host, the Common Redstart Phoenicurus phoenicurus. Cuckoos raised alone (‘solitary’) fledged at higher mass, with higher wing and tarsus length and started to fly at a younger age than Cuckoos raised alongside young Redstarts (‘mixed’). However, a further 23 fledging and post‐fledging parameters measured at five pre‐determined times (fledging, first‐flight, predation, starvation, independence) did not differ between solitary and mixed Cuckoos. In addition, none of the parameters measured during the post‐fledging period (growth, dispersal distances, number of flights) differed between solitary and mixed Cuckoos. Redstart fledglings from non‐parasitized broods (‘solitary’) showed generally similar fledging and post‐fledging parameters to fledglings reared alongside a Cuckoo (‘mixed’). Surprisingly, there were no significant differences in post‐fledging predation rate, starvation or overall survival rates between mixed and solitary Cuckoos or mixed and solitary Redstarts. Thus, during the post‐fledging period, mixed Cuckoo fledglings successfully compensated for the poorer performance experienced during the nestling stage whereas mixed and solitary Redstarts did not differ in any measured parameters. This suggests that the regular occurrence of mixed broods in this host–parasite system – which is unique among the many Cuckoo hosts – is evolutionarily stable for both hosts and parasites.

Studies of brood parasite-host coevolution have generally assumed that hosts invest more parental care to rear parasite progeny than their own offspring: this view was taken for granted in cases when a parasite chick (e.g. the common cuckoo Cuculus canorus; hereafter 'cuckoo') was dramatically larger than a host chick (small passerines in the case of cuckoos [1]). This seemed obvious because a cuckoo fledgling weighs as much as the whole family of a host, i.e. all chicks and both male and female fosterers combined (own unpublished data). Recently, we reported results that question this view: rearing the cuckoo was not associated with overall higher immediate rearing costs to common redstarts Phoenicurus phoenicurus (hereafter 'redstart') above the natural baseline levels that are imposed by efforts to rear their own progeny anyway [2]. Additionally, we suggested that such low costs may partly help explain low levels of host counterdefences in this host. In their comment, Yang et al.[3] do not contradict our main conclusions (rearing the cuckoo is not extra costly) but disagree with our suggestion that surprisingly low immediate rearing costs can be relevant for the evolution of host defences. Here, we address this criticism.Yang et al.[3] claim that individual hosts that are successfully parasitized by cuckoos do not contribute their genes to the next generation, which provides a strong selective pressure on hosts, whereas an excessive parental care for parasite progeny can hardly drive the evolution of anti-parasitic adaptation in hosts. Further, [3] argue that even if the immediate costs of rearing a cuckoo chick do not exist, parasitism itself should be sufficient to promote anti-parasitic defences in hosts.We agree that the loss of the host's own progeny after successful parasitism is the most important cost that provides a strong selective pressure on hosts to evolve defences. We also agree that the loss of progeny per se is sufficient to promote anti-parasitic defences in hosts even in the potential absence of immediate rearing costs. However, all this does not mean that other costs are irrelevant. Indeed, the lost progeny cost is the same or very similar in all regular cuckoo hosts because the cuckoo chick kills all its nest-mates in all host species [1] (with a single exception of redstarts); therefore such invariable lost progeny cost cannot explain the large variability in host defences. Therefore, we suggest that multiple costs-i.e. not only the lost progeny cost-play a non-negligible role in the evolution of anti-parasitic defences and can thus better explain variability in levels of anti-parasitic defences across various host species.Such costs include high demands to raise the cuckoo because host parents exhausted by care for the cuckoo more likely fail to transmit their genes in potential next breeding attempts when caring for their own progeny (i.e. the trade-off between the current and future reproduction). Specifically, in the redstart, [2] reviewed factors considered by previous studies as impo...