Within-individual consistency and between-individual variation in the shapes of eggs laid by tree swallows (Tachycineta bicolor)
Bird eggs come in a wide variety of shapes and recent large-scale studies across species have renewed interest in understanding the factors that generate and maintain this diversity. However, these advances largely overlook the fact that egg shape also varies dramatically within species: between populations, between individuals, and between eggs produced by the same individual. We measured the shape and size of 1382 eggs produced by 205 tree swallows (Tachycineta bicolor) in 259 nests with many females having eggs measured in two or three years. We found that intra-specific variation in the asymmetry and ellipticity of tree swallow eggs spanned the range of typical egg shapes from 69% of species reported in a recent comparative study. Variation in shape and size was largely explained by individual identity, as the repeatability of egg shape and size was remarkably high both for eggs produced within a clutch and for clutches produced in successive years. All characteristics of mother and daughter eggs were positively correlated, although with a sample size of only 15 pairs these relationships were not significant. Female mass and structural size were correlated with the size of eggs produced, but not with their shape. Older females produced eggs that were larger, more asymmetrical, and less elliptical; this pattern was driven by a combination of both longitudinal changes in egg characteristics as females aged and by differential overwinter survival of females with particular egg characteristics. We found no evidence that any aspect of shape or size that we measured was related to nestling measurements or fate. Finally, we found that the patterns of covariation in aspects of egg size and shape between-individuals differed from that observed within-individuals, suggesting that the mechanisms driving egg shape variation might differ across these levels. A complete understanding of avian egg shapes will need to incorporate variation at multiple scales and we discuss the interpretation of our results in light of recent large-scale comparative studies that focus only on mean species egg shapes.