A sum of its individual parts? Relative contributions of different eggshell regions to intraclutch variation in birds

The rejection of eggs of brood parasites in several species of hosts is based on cues only at the blunt pole (BP) and not at the sharp pole (SP) of the foreign and own eggshell. We investigated whether intraclutch variation is confined to a specific egg pole in species where the extent of intraclutch variation in the overall egg appearance is known to positively covary with either egg rejection rate or the probability of being parasitized. For the two poles separately, we analysed intraclutch variation of eggshell brightness and blue chroma. We quantified intraclutch variation as the standard deviations of these colour metrics, instead of their coefficients of variation which would represent a statistically flawed approach. Pooling measurements of brightness across the whole egg surface led to statistically non-significant results and masked positive correlations of BP brightness with egg rejection or parasitism risk, respectively. In contrast, patterns of blue chroma were important across the whole egg. Thus, the traditional whole egg 'averaging' approach may mask biologically important effects of intraclutch variation when the variation and potential signalling functions of egg appearance are confined to a specific egg part (brightness). However, analyses based on only BP and SP eggshell region specific data may also lack the power to detect effects of phenotypic traits that do not vary between egg poles (blue chroma). We advocate the use of a combination of region-specific and whole-eggshell based colour metrics and manipulations in cognitive, perceptual, and ecological studies of foreign egg rejection.