Evolution of eggshell structure during rapid range expansion in a passerine bird

1. Environmental factors such as temperature, humidity and partial oxygen pressure can affect avian eggshell structure because gas exchange across the shell must allow sufficient water loss while preventing dehydration of the embryo. Studies of species with known chronology of colonization of novel environments provide a powerful insight into the relative importance of ecological factors shaping the evolution of eggshell structure. 2. Here, we examined changes in eggshell structure that accompanied rapid range expansion of house finches (Carpodacus mexicanus) across North America. We analysed thickness and pore density in eggshells from three ecologically distinct populations: the native desert population in southwestern Arizona and two 30-year-old populations in the northwestern (north-west Montana) and southeastern (south-east Alabama) parts of the species’ range. We also conducted cross-foster exchanges of freshly laid eggs within and between the northwestern and southeastern populations to examine consequences of population differences in eggshell structure on embryo development. 3. Eggshell structure was most distinct in a recently established population inhabiting higher humidity environment (southeastern Alabama), where eggs were the largest, eggshells the thickest and pore density the lowest. Populations that experienced highly distinct ambient temperatures (southwestern Arizona and northwestern Montana) nevertheless had similar eggshell structure. These results were corroborated by experiments where humidity differences between cross-fostered nests had twice the effect on embryo survival compared to the effect of change in ambient temperature. Correspondingly, experimental egg exchanges between southeastern. Alabama and northwestern Montana populations were associated with fourfold increase in embryo mortality compared to within-population egg exchanges. 4. We document rapid evolution of eggshell structure in response to colonization of novel environments and establish the relative importance of environmental factors on avian eggshells. We discuss these results in relation to population variation in incubation behaviour and its ability to shield eggshell structure from the selection exerted by novel environments.