Patterns of variation in eggshell colour of Mountain Bluebirds (Sialia currucoides) provide mixed support for the sexually selected eggshell colour hypothesis

Abstract

Determining the functional significance of ornamental traits and whether they are shaped by sexual selection requires understanding how they vary among individuals, and their relationship with other measures of quality. Conspicuous eggshell coloration is a trait that may have evolved through sexual selection as a signal of female quality. This hypothesis is based on the possibility that the pigment used to colour blue-green eggs is limited and potentially costly to allocate to eggshells. We investigated the signalling potential of blue-green eggshell colour by quantifying patterns of within- and among-clutch colour variation in Mountain Bluebirds (Sialia currucoides) to determine if pigment was limited and if egg colour traits are intrinsic to individual females. We also explored relationships over two breeding seasons between eggshell colour and other female traits and investment in eggs. Eggshell colour was highly repeatable within clutches for most measures, and relatively consistent between breeding events in different years, indicating that eggshell colour is an intrinsic trait. Eggs in second clutches within a season had less saturated blue-green colour than in first clutches, and within clutches, later-laid eggs were brighter but generally less blue-green in colour than earlier laid eggs, providing evidence of pigment limitation. However, associations between eggshell colour and other female traits and investment in eggs were less clear, and inconsistent among study years. Our findings suggest that there is potential for blue-green egg colour to function as a signal of female quality, but further study is needed to determine if blue-green eggshell colour is a truly informative signal.