Want to see an enduring source of scientific dispute and perplexity? Go to your refrigerator, open a carton of eggs, and look at their speckles.

Ubiquitous, deceptively simple, and in many species beautiful, bird-egg coloration has been studied for more than a century. Its function is only partly understood, however, and most researchers may have ignored something fundamental: what’s happening inside the egg.

In “An embryo’s eye view of avian eggshell pigmentation,” a sprawling review of dozens of studies published Sept. 14 in the Journal of Avian Biology, biologists Golo Maurer and Phillip...