Some females lay their eggs in other birds nest, leaving these foster parents to rear genetically unrelated offspring and pay fitness costs associated with rearing parasitic young. This is known as brood parasitism, and provides a unique model system for studying co-evolution and decision-making in the wild. Co-evolutionary theory has long assumed that parasitic eggs very similar to the hosts’ own would be less likely to be detected and removed, while eggs that were more different would be more likely to be detected and removed. Recent research has found that not all hosts use this decision-rule, and that brown foreign eggs are more likely to be rejected than foreign eggs that are blue-green, despite the absolute perceived difference from host eggs. This systematic removal of brown eggs should select for ever-bluer parasitic eggs and host eggs that are bluer and more pigmented than parasitic eggs. We tested whether the great reed warbler, a regular host of the common cuckoo, was more likely to reject brown eggs than blue-green eggs. Then, using a dataset of eggshell reflectances of great reed warbler and cuckoo eggs from the same nest, we tested whether host eggs were bluer and more pigmented than their parasites’.
Results/Conclusions
We found that great reed warblers were biased toward rejecting brown over blue-green eggs (R2 = 0.17, β = 0.11 ± 0.04, N = 57, P = 0.002), which is consistent with other hosts tested thus far. Within host-parasite pairs, host and parasite eggshell coloration (t18= −4.1, P < 0.0001) and brightness (t18= −8.3, P < 0.0001) were related, such that eggs of hosts were both bluer and more deeply pigmented than those of parasites (Kuiper test = 3.8, P < 0.01). These findings have important implications for co-evolutionary arms races because they illustrate selection pressure on host eggs to be ever-bluer within the avian visual color space. We found that cuckoo eggs were less blue-green and less deeply pigmented than host eggs and suggest that these ‘intermediate’ eggshell appearances may provide parasites with an advantage to exploit more individual hosts or host races. These findings suggest that hosts may not be free to evade parasites in any phenotypic direction, but rather, may be experiencing strong selection to adapt bluer colors that they themselves are more likely to accept.