Egg shape variation across the distribution of the partially migratory fork-tailed flycatcher Tyrannus savana

The evolution of egg shape across birds has been associated with breeding ecology, body shape constraints and nest microclimate, among other factors. We model the effects of migratory status, climate, clutch size and egg volume on egg shape variation over the distribution of fork-tailed flycatchers Tyrannus savana. Although migratory status and climatic variables appear to be influencing intraspecific egg shape, these effects are not significant when accounting for nest identity as a random factor (i.e. eggs from the same clutch are more similar than to other clutches). Moreover, the differences that we observe in egg shape are not explained by variation in egg size. Finally, within a breeding population of migratory fork-tailed flycatchers, egg shape does not vary with respect to egg-laying order and/or female wing length (standardized by weight). Egg shape is highly variable within populations of fork-tailed flycatchers but not within clutches, suggesting that female traits, apart from migratory status and wing morphology, constrain egg shape variation.