CONSTRAINTS ON EGG DISCRIMINATION AND CUCKOO HOST COEVOLUTION

To understand the co-existence of rejection and acceptance of cuckoo e ggs within a host population, the mechanism of egg discrimination and the cost-benefit balance of rejection behaviour were investigated. At a study site in central Japan, rejection rate of cuckoo, Cuculus canor us, eggs by great reed warblers, Acrocephalus arundinaceus, was 61.5%. An analysis of host response to natural and experimental parasitism w ith real cuckoo eggs, cuckoo egg models and painted host eggs indicate d that: (1) hosts are more likely to reject eggs that look different f rom their own; (2) almost all individuals (94%) can reject highly non- mimetic eggs, suggesting that there are few, if any, true accepter gen otypes in the host population; (3) hosts usually reject by egg ejectio n; (4) during the host-laying period, the day of parasitism does not a ffect host response; (5) egg types that were rejected at lower rates a lso took longer to be rejected; (6) acceptance was more likely to occu r among mid-season breeders which consist of a higher proportion of yo unger females in the host population. Two experiments indicated that p revious exposure of a host to its own eggs affects its rejection behav iour, suggesting that a learning mechanism (an imprinting-like process ) is involved. Parasitized nests from which the cuckoo egg was experim entally removed, or ejected by hosts, fledged more host young than nes ts in which the cuckoo egg was accepted. Hosts that deserted parasitiz ed nests were likely to re-nest, and the success of re-nests was high. Costs due to breakage of host eggs occurred in only 3.5% of successfu l cuckoo egg ejections. a cost-benefit model of egg rejection suggests that under some circumstances, the cost of recognition errors may exc eed that of parasitism. Egg variability within a clutch was higher amo ng younger females. Some hosts rejected painted eggs and conspecific e ggs based on differences that may occur naturally within variable clut ches of other individuals. It is suggested that host egg variability i s a major constraint on the learning mechanism of egg recognition. Acc ordingly, the cost of mistakenly rejecting an odd egg from the nest se lects for greater tolerance towards divergent eggs in young breeders, and justifies a prolonged learning mechanism in which a host can learn to recognize the variation range of its own eggs. The co-existence of rejection and acceptance within the host population can therefore be explained as a compromise between the cost of parasitism and the cost of recognition errors, rather than as an evolutionary lag. This explan ation is particularly pertinent where the cuckoo has evolved mimetic e ggs and where the parasitism rate is low.