Common cuckoos do not mimic the size and shape of host eggs

Often considered a textbook example of coevolution, common cuckoo (Cuculus canorus) eggs are among the best-studied eggs in the world. Female cuckoos belong to genetically distinct host-races, each laying a specific egg type. When host species evolved to reject cuckoo eggs from their nests, cuckoos evolved better egg color and pattern mimicry. In this study, we asked: have cuckoos also evolved eggs that are well matched to host eggs in size and shape, and is the match better for highly discriminating hosts? We used digital image analysis to quantify the sizes and shapes of ~1230 eggs laid by ten European host species and their respective cuckoo host-races. We found that there is some variation in egg size and shape among host species. By contrast, different cuckoo host-races lay eggs that are—on average—similar in size and shape. This generic “one size and shape fits all” cuckoo egg morph is a poor match to most host egg sizes but a good match to most host egg shapes. Overall, we showed that host discrimination behavior was not correlated with the degree of egg size or shape similarity. We concluded that cuckoo females have not evolved egg size or shape mimicry. Alternative explanations for egg shape similarity include biophysical constraints associated with egg formation and selection for incubation efficiency. Finally, to place our results in a broader context, we compared the egg shapes of the common cuckoo and its hosts to those of three Australian parasitic cuckoo species and their hosts.