Eggshell Maculation is Not a Signal of Maternal Condition in Two Species of Rail

Abstract

Eggshell pigments are costly to produce. The sexual signaling hypothesis predicts that the degree of pigmentation in eggshells is positively related to maternal quality. Thus, females in better condition are expected to lay brighter or more maculated eggs. To test this, we quantified the extent of maculation in eggshells of King Rails (Rallus elegans) and Common Moorhens (Gallinula chloropus chloropus) and related this to female body condition. We identified key variables for body condition using principal components analysis of morphometric measures. To evaluate which combination of variables provided the best index of body condition in each species, a series of linear models were ranked using AICc. To estimate the area of surface pigment on each egg, color egg images were converted to binary and we calculated the percent of black pixels within a scaled oval superimposed over the images. We ran linear regressions of mean percent pigmentation on maternal body condition. Neither female King Rails nor Common Moorhens in better body condition laid more maculated eggs. Thus, the sexual signaling hypothesis was not supported. Common Moorhen eggs were slightly larger, on average, and their eggshells had more pigment per unit area and per egg than King Rail eggs.