COS 59-3 - Collision risk, instead of rolling-off risk, may explain the form-function association between bird eggs and nests

Abstract: Eggs and nests are two critical traits that allow birds to breed in most corners of the earth land. However, their evolutionary interactions remain inconclusive because the form-function association between the two traits is obscure. Here we combined evolutionary models with mechanical engineering approaches to examine the interactions between egg variables and three nest characteristics—nest attachment, site and structure—for thousands of species across the avian phylogeny. In particular, we tested two hypotheses—egg collision and rolling-off hypotheses—to examine the evolutionary responses of eggshell stiffness and shape to variations in nest characteristics. Results showed that the collision hypothesis can explain the associations between eggshell stiffness and all three nest characteristics. Eggshell stiffness was higher in nests located on more unstable sites or with more unstable attachment, suggesting that birds building unsteady nests tend to produce eggs with high resistance to collision damage in nests. In addition, eggs were stiffer in nests with higher levels of enclosure, probably because they have a higher chance of smashing against one another inside enclosed nests. In contrast, the rolling-off hypothesis had relatively low power to reflect the egg-nest relationship, as we only found that more unstable attachment approaches were associated with less spherical eggs. While median eggshell stiffness remained similar across avian evolution, interspecific variations increased likely due to an expansion of nest niches. Particularly, passerines produced eggs with high stiffness, which requires extra energetic investments but allows them to use unstable or enclosed nests. Our study highlights an overlooked, but critical, connection between nest and egg characteristics and illustrates how their trade-offs influenced the evolutionary success of birds.