Colour characteristics of the blunt egg pole: cues for recognition of parasitic eggs as revealed by reflectance spectrophotometry

Variation in appearance of parasitic and host eggs is an essential trait in the coevolutionary struggle between a brood parasite and its host. Here we examined the role of colour characteristics in the egg recognition in blackcaps, Sylvia atricapilla, assessed by (1) human vision and (2) spectrophotometry, including UV wavelengths. Since colour and spotting pattern within eggs are often highly variable, we also assessed three different parts of blackcap eggshell separately. We investigated the possibility that a lower intraclutch variation in the parasitized clutch would positively affect egg recognition. In addition, we predicted that egg rejection would depend on contrast between host and parasitic eggs. However, we found that neither intraclutch variation nor mimicry if assessed by humans had significant effect on host rejection. Multivariate analyses of spectrophotometric data revealed, however, that the likelihood of egg rejection significantly increased with decreasing intraclutch variation in blue chroma. Furthermore, probability of egg rejection significantly increased with decreasing contrast between parasitic and host eggs in UV wavelengths and with decreasing brightness of host eggs at the blunt egg pole. Accepted foreign eggs were significantly darker at the blunt pole than acceptors' own eggs. The paradox that rejected eggs had lower contrast could be explained by the fact that the contrast in UV spectra is associated with brightness of the blunt egg part. Our data suggest that colour characteristics of the blunt egg part may play a major role in the recognition of parasitic eggs in the blackcaps.