All extant variation in the avian eggshell color is generated through two pigments: biliverdin producing blue-green and protoporphyrin producing brown. Two recent hypotheses propose that eggshell pigments have adapted to filter harmful solar radiation (“pigment as parasol”) and that pigment concentrations are adapted to reflect incident heat (“dark car”). Because light intensity varies across the globe, these hypotheses can be tested via a biogeographic approach. In the tropics, the average egg color should be bluer and less pigmented compared to higher latitudes because biliverdin absorbs more harmful UV light than protoporphyrin, and because lighter pigments absorb less thermal energy from the sun. To test these predictions, we used average eggshell color data from >600 species across the full phylogenetic diversity of birds. For each species we calculated photoreceptor quantum catches for the typical UV-sensitive bird. We generated coordinates simulating nests within each species’ breeding range at one nest per 1,000 km2. We then used discrete equal area global hexagon grids to calculate the average color within 1050 sampling areas spanning the globe, each accounting for phylogenetic relatedness. Finally, we used least-squares models accounting for spatial auto-correlation to compare these data to average temperature, and solar and ultraviolet intensity.
Results/Conclusions
We found that average egg color is more blue-green and brighter in the tropics than at higher latitudes. Specifically, tropical eggs were 9.3% bluer (χ²=37.6, p<0.0001) and 36.0% brighter (χ²=34.9, p<0.0001) than those at higher latitudes. These findings suggest that egg pigmentation plays a role in photo-protection and thermoregulation. When examining these patterns within individual nest types, we found that ground nesting birds are 10.0% browner (χ²=47.1, p<0.0001) and 33.0% darker at higher latitudes (χ²=35.7, p<0.0001), while cup nesting birds showed no significant color pattern (χ²=2.7, p=0.26) but were 26.8% darker (χ²=16.8, p<0.0001), and cavity nesting birds are not significantly different in color (χ²=2.7, p=0.26) or brightness (χ²=3.3, p=0.19). This provides strong evidence that sunlight and nesting strategies are important selection pressures driving egg pigment evolution.