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

► Scrub robins show conspicuous decline in defences both seasonally and over a decade. ► In absence of cuckoos motivation to reject was lost but not discrimination capacity. ► Cuckoo dummy presentation did not increase rejection rate. ► Assessment of parasitism risk could be from sight of flying cuckoos. ► Decrease in rejection rate according to parasitism risk reflects phenotypic plasticity.