The cuckoo lays its eggs in the nest of an unwitting host species and after hatched, the parasite nestling often evicts the host eggs or young, ensuring maximum resources for itself. While some hosts recognize and destroy the cuckoo egg, defensive action against hatchlings has been underinvestigated. In the 13 March Nature, Naomi Langmore and colleagues at the Australian National University, Canberra, show that, in response to parasitism, the Australian superb fairy-wren stops feeding the bronze-cuckoo chick and abandons the nest, a strategy that can be outwitted by the right cuckoo call (Nature, 422:157-160, March 13, 2003).

Langmore et al. showed that once brooding had started, the decision to abandon an egg clutch hinged on seeing the female cuckoo at the nest, rather than the egg itself. Once hatched, the decision to abandon nestlings depended in part on the number of chicks in the nest — females abandoned not only single cuckoo chicks, but also single chicks of their own brood. By comparing acceptance rates for two cuckoo species with different appearances and begging calls, they showed that auditory cues, not appearance, was the critical factor for acceptance of lone chicks by the host. Horsfield bronze-cuckoo chicks, which accurately mimic the superb fairy-wren chick, were abandoned 60% of the time, while the poorer mimic shining bronze-cuckoos were abandoned 100% of the time.

Describing their findings as another ratchet upward in the evolutionary arms race between cuckoos and hosts, the authors conclude, "Our experiments demonstrate that host discrimination against evictor-cuckoo nestlings is possible, and suggest that it has selected for the evolution of nestling mimicry in bronze-cuckoos." Further studies will determine if the host's ability to discriminate against cuckoo chicks is innate or acquired.