Bird eggs come in a dizzying array of colors and patterns; each an evolutionary study of adaptation to a specific habitat. There is nonetheless a global trend.
“The survival of a bird’s developing embryo depends on the egg’s ability to stay within strict thermal limits.” ~ American evolutionary biologist Daniel Hanley et al
Worldwide, egg coloration follows a single thermal rule: the colder the climate, the darker the egg. Darker eggs absorb more heat quicker than lighter ones. The few minutes of warmth a dark egg provides its embryo inside gives its caretaker time precious time to forage. Doubtlessly avian mothers can calculate their eggs thermal limits. Evolution is often finely tuned.
Phillip A. Wisocki et al, “The global distribution of avian eggshell colours suggest a thermoregulatory benefit of darker pigmentation,” Nature Ecology & Evolution (28 October 2019).
Jonathan Lambert, “Bird eggs laid in cold climates are darker, which may keep eggs warm,” Science News (28 October 2019).