Egg discrimination in the Australian reed warbler (Acrocephalus australis): rejection response toward model and conspecific eggs depending on timing and mode of artificial parasitism

In a coevolutionary arms race between an interspecific brood parasite and its host species, both are expected to evolve adaptations and counteradaptations. We studied egg discrimination in the Australian warbler (Acrocephalus australis). This species is currently not significantly parasitized by the seven species of cuckoo for which it is a suitable host. However, experimental brood parasitism in the warbler revealed a fine tuned egg discrimination response towards non-mimetic and conspecific eggs, the first such evidence in an Australian passerine: (1) non-mimetic eggs were significantly more often rejected than conspecific eggs; (2) only non-mimetic dummy eggs were rejected selectively, whereas rejection of conspecific eggs entailed a rejection cost; (3) replacement of a host's egg with a conspecific egg during egg laying resulted in a significantly higher rejection rate than after the day of clutch completion; (4) by contrast, rejection rate after addition of a conspecific egg was independent of nest stage; (5) conspecific eggs introduced into a clutch during the egg laying period led to a significantly higher nest desertion rate and a lower egg ejection rate than after the day of clutch completion; and (6) addition of a conspecific egg led to egg ejection while egg replacement with a conspecific egg led to nest desertion. The fact that this species responds differentially toward different modes of artificial parasitism suggests that its egg discrimination has evolved to minimize the costs of rejection and parasitism. The ability to reject highly mimetic conspecific eggs may explain the current paucity of brood parasitism in this species. The significance of this for brood parasite-host coevolution is discussed.