Bird eggs come in a dizzying array of colors. But from a global perspective, that diversity follows a simple pattern – the colder the climate, the darker the egg, new research shows.
Darker eggs absorb more heat than lighter ones, which could help developing chicks stay warm while their parents forage for food, according to the study published in Nature Ecology and Evolution.
Using museum collections of bird eggs, Biologists Hanley, Wisocki and their colleagues compiled data on eggs from 634 bird species from 36 of the 40 living orders of birds. They then analyzed the data against a global map, and found that the brightness and color of eggshells closely correlated with temperature, even after correcting for color similarities between closely related species.
Birds in “the far north, which tends to be colder, had darker, browner eggs,” Hanley says. Eggs became lighter and slightly bluer for birds living closer to the equator, though egg colors were generally more variable in the tropics.
The researchers suggest the trend may reflect adaptation to the cold: A dark egg, like a dark car parked in the sun, absorbs more thermal radiation from the sun than lighter eggs. Testing the theory, the researchers exposed white, brown and blue chicken eggs to direct sunlight and tracked heat retention. Sure enough, brown eggs warmed up faster and cooled down more slowly than the lighter eggs.