Experimental increase in eviction load does not impose a growth cost for cuckoo chicks

Abstract: Chicks of many avian brood parasites evict their hosts’ eggs within 48 h of hatching. This behavior eliminates competition inside the nest and is beneficial for the fitness of the parasite. Several studies have proposed that this behavior is costly for the parasitic chick and may limit opportunities for cuckoos to exploit hosts with large clutch sizes. We tested whether increased eviction effort was associated with reduced growth in cuckoo chicks by artificially increasing the clutch size of superb fairy-wrens, the main host of the Horsfield’s bronze-cuckoo in Australia. Contrary to theoretical predictions, chicks that evicted a larger number of eggs did not lose mass. Instead, they had higher growth rates than chicks that evicted fewer eggs. This unexpected result suggests chicks might be able to use compensatory growth to overcome the costs of egg eviction, perhaps by increasing their begging rate after evicting more eggs. Our results, combined with previous evidence, suggest that brood parasites may not be constrained by the clutch size of their hosts, resulting in a broader set of potential hosts. Furthermore, laying larger clutches might not be an effective host defense against brood parasites. Significance statement: Avian brood parasites lay their eggs in the nests of other species. Many parasitic chicks, like cuckoos, avoid competition inside the nest by evicting all the other eggs laid by the host. It is generally assumed that this innate behavior is costly for the cuckoo chick, and that evicting more eggs requires greater effort and higher physiological costs. In this study, we use the system of the superb fairy-wren and the Horsfield’s bronze-cuckoo in Australia to test whether evicting more eggs results in decreased growth in parasitic cuckoos. We found no evidence to suggest that evicting more eggs compromises growth in Horsfield’s bronze-cuckoos. In fact, chicks that evicted more eggs grew significantly faster than chicks that evicted fewer eggs, suggesting that chicks are able to compensate for the initial effort of evicting more eggs.