Shared parental care is costly for nestlings of common cuckoos and their great reed warbler hosts

Obligate avian brood parasitism typically involves one of 2 strategies: parasite chicks are either 1) virulent and evict all other eggs and nest mates to be raised alone or 2) more tolerant and share foster parental care with host chicks for some or the entirety of the nestling period. We studied the consequences of experimentally forced mixed broods of age-matched one common cuckoo (Cuculus canorus) and 2 great reed warbler (Acrocephalus arundinaceus) chicks. In these broods, both cuckoo and host chicks grew slower than did either individual cuckoos or great reed warblers in broods of 1 parasite or 3 host chicks, respectively. Video records showed that in mixed broods, cuckoo chicks received feedings less frequently than the 33% predicted by chance at 4 days of age but parental food allocations increased to chance levels at 8 days of age. The consistent patterns of lower growth rates arose even though chicks in broods of 1 parasite and 2 hosts received the largest prey items per feeding. In addition, several other measures of parental provisioning also did not predict species and brood-specific differences in nestling growth rates across the different treatments. However, variation in begging displays and its specific costs on host and parasite chicks in the different nest treatments were not quantified in this study. We conclude that young of nest mate-evictor common cuckoos benefit from the sole occupancy of host nests in part owing to an initial competitive disadvantage for parental care in broods with age-matched great reed warbler chicks.