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.
Sources:
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).