Flexible cuckoo chick-rejection rules in the superb fairy-wren

Recognition of brood parasitic cuckoo nestlings poses a challenge to hosts because cues expressed by cuckoos and host young may be very similar. In theory, hosts should use flexible recognition rules that maximize the likelihood of rejecting cuckoo nestlings while minimizing the risk of rejecting their own young. Our previous work revealed that female superb fairy-wrens Malurus cyaneus often abandoned nestling cuckoos and that the presence of a single chick in the nest was 1 trigger for abandonment because fairy-wrens also sometimes abandoned a single fairy-wren chick. Here we use a combination of 20 years of observational data, a cross-fostering experiment, and a brood size reduction experiment to determine the basis for individual variability in the chick-rejection rules of superb fairy-wrens in response to parasitism by Horsfield's bronze-cuckoos Chalcites basalis. We show that the decision to abandon a single chick is based on integration of learned recognition cues and external cues. Experienced females were relatively more likely to abandon a single cuckoo chick and accept a single fairy-wren chick than naive females. Breeding experience therefore facilitates the ability to make an accurate rejection decision, perhaps through learned refinement of the recognition template. In addition, fairy-wrens modified their rejection threshold in relation to the presence of adult cuckoos in the population, becoming more likely to abandon single nestlings with increasing risk of parasitism. By using these flexible rejection rules, female superb fairy-wrens are more likely to defend themselves successfully against exploitation by the cuckoo and are less prone to mistakenly reject their own offspring.