Conditional response by hosts to parasitic eggs: the extreme case of the rufous-tailed scrub robin

Fitness costs imposed by the common cuckoo, Cuculus canorus, on its hosts select for host defences such as nest defence and egg discrimination. The efficiency of both types of defence varies among host species, host populations and even seasonally within a host population. We examined changes in egg recognition as defence against cuckoo parasitism in the rufous-tailed scrub robin, Cercotrichas galactotes, both seasonally and over a decade. The rejection rate decreased from 64.7% to zero within 10 years following the cuckoosʹ disappearance from the area. In this host species, egg recognition without rejection had previously been reported and in this study, using nonmimetic model eggs, we found that, in the absence of cuckoos, females lost their motivation to reject despite maintaining their discrimination capacity. During presentations of dummy cuckoos and other species, rufous-tailed scrub robins did not recognize the female cuckoo dummy as an enemy and its presentation near the host nest did not increase egg rejection, the rejection rate remaining very similar regardless of the dummy species placed near the nest. We suggest that other cues such as the sight of cuckoos flying across the breeding area or directly to the nest are responsible for a fine-tuned assessment of the risk of parasitism in this host species. The sharp decrease in rejection rates according to perceived risk of parasitism reported here reflects phenotypic plasticity, an adaptive mechanism that enables animals to adjust their behaviour to a rapidly changing environment.