Fixed Rejection Responses to Single and Multiple Experimental Parasitism in Two Fringilla Hosts of the Common Cuckoo

Previous studies have shown that the tendency to reject parasitic eggs among certain hosts is strongly dependent on the degree of similarity with own eggs, whereas other conditional cues do not affect rejection decisions. This paper examines whether two such hosts, the closely related brambling and chaffinch, show a different tendency to reject parasitic eggs if they are multiply parasitized. Some individuals were experimentally parasitized with both a non-mimetic and a low-intermediate contrasting egg in the same breeding attempt. The non-mimetic egg was rejected almost without exception. In chaffinches, the low-intermediate contrasting egg was introduced shortly after rejection of the non-mimetic egg whereas in bramblings, both eggs were introduced simultaneously. A control group consisted of individuals that were parasitized with one low-intermediate contrasting egg. There was no significant difference in the tendency to reject the low-medium contrasting eggs between the experimental and control group in any of the species, implying that the same acceptance threshold is applied to each parasitic egg independently. Moreover, the rejection rate of non-mimetic eggs was high (> 90%) regardless of whether the egg was introduced alone or together with a low-medium contrasting egg. The results are discussed in relation to recent studies with great reed warblers Acrocephalus arundinaceus that obtained contrasting results in similar experimental designs. The different degrees of flexibility displayed by Fringilla and great reed warbler hosts are likely to reflect differences in both the perception and action component of the recognition system.