Asymmetrical signal content of egg shape as predictor of egg rejection by great reed warblers, hosts of the common cuckoo

The size, patterning and coloration of bird eggs may signal different information content to nest owners, mates, predators, hosts, or brood parasites. Recent studies suggested that the pigmentation at one pole of the typically asymmetrical avian egg plays a critical role in the discrimination of own and foreign eggs by several host species parasitized by the common cuckoo (Cuculus canorus). Typically, both eggshell maculation and background colour are more consistent on the blunt pole, and hosts react more strongly to experimental changes in coloration of the blunt pole compared to the sharp pole. However, it remains unclear whether the asymmetrical shape of natural eggs per se enhances the behavioural responses of hosts to foreign eggs. To evaluate the salience of asymmetrical egg shape, we studied reactions of a rejecter cuckoo host, the great reed warbler (Acrocephalus arundinaceus), to artificial shapes of model eggs painted a non-mimetic blue colour. Artificial eggs with two blunt poles were rejected significantly more often than those with a single blunt pole or two sharp poles. These results corroborate the hypothesis that the different egg poles have different signal salience and may have implications for the evolution of diversity of not only egg coloration but also of egg shape in the arms race between hosts and brood parasitic birds.