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 i ts host species, bath are expected to evolve adaptations and counteradaptat ions. 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, experime ntal brood parasitism in the warbler revealed a fine tuned egg discriminati on response towards non-mimetic and conspecific eggs, the first such eviden ce 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 rej ection 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 dw of clutch completion; (4) by contrast, rejection rate after addition of a conspecific egg was independent of nest stage; (5) conspecific eggs intr oduced into a clutch during the egg laying period led to a significantly hi gher nest desertion I-ate and a lower egg ejection rate than after the day of clutch completion; and (6) addition of a conspecific egg led to egg ejec tion while egg replacement with a conspecific egg led to nest desertion. Th e fact that this species responds differentially toward different modes of artificial parasitism suggests that its egg discrimination has evolved to m inimize the costs of rejection and parasitism. The ability to reject highly mimetic conspecific eggs may explain the current paucity of brood parasiti sm in this species. The significance of this for brood parasite-host coevol ution is discussed.