The global distribution of avian eggshell colours suggest a thermoregulatory benefit of darker pigmentation.

The survival of a bird's developing embryo depends on the egg's ability to stay within strict thermal limits. How eggshell colours help maintain thermal balance is a long-standing and contested question. Using data spanning a wide phylogenetic diversity of birds on a global spatial scale, we find evidence that eggshell pigmentation may have been shaped by thermoregulatory needs.