The cost of virulence: an experimental study of egg eviction by brood parasitic chicks

Hatchlings of some virulent brood parasitic birds have evolved to eliminate host offspring. We experimentally studied the dynamics and potential costs of the egg eviction behavior of hatchlings of the common cuckoo Cuculus canorus in broods of common redstarts Phoenicurus phoenicurus, a cavity nesting host. Eliminating the labor of egg tossing per se improved the cuckoo chick's growth during the eviction period by similar to 20-30%. Evictor cuckoo chicks recovered from the cost of egg tossing to fledge at similar masses compared with solitary chicks, although they did so at older ages. Foster parents fed evictor chicks less often compared with nonevictors. Feeding frequencies by hosts to evictors correlated negatively with eviction effort as evicting chicks often appeared to ignore fosterers offering food. Nest cup steepness was negatively related to eviction success and positively to age at first eviction. We propose that eviction behavior by cuckoo hatchlings is favored by selection because the costs of eviction are much lower than the costs of cohabitation with host chicks.