Birds that live in colder areas and build open nests mainly lay eggs with dark skins. American and Australian researchers write in the scientific journal Nature Ecology & Evolution that this is because the dark pigment ensures that the eggs heat up better and retain heat in sunlight.

Birds must actively regulate the temperatures of their eggs by hatching and at the same time they protect their future young against predators. However, they also have to leave the eggs to look for food.

In colder areas it is more difficult to keep the temperature at the right height, but at the same time there is less danger from predators. The parents spend less time monitoring the temperature of the egg, but more often. It now appears that the color of the egg probably also helps to maintain the temperature in the egg.

Scientists examined the eggs of 634 different bird species, which they could obtain from natural museums. The eggs of species that live in colder regions with less strong sunlight and that often also build open nests were considerably darker than the other eggs.

After this analysis, the researchers took the eggs from chickens, ducks and quail with colors ranging from light to dark. The eggs were exposed to solar radiation and the temperature was recorded. These tests showed that dark eggs could retain the heat longer than lighter eggs.

By combining these results, the scientists conclude that the color determines the heat balance in the egg.

Temperature of egg important for embryo

How an egg handles temperature can be very decisive for the embryo inside it. How well a bird embryo can survive depends on the temperatures that the egg faces.

With turtle eggs, the temperature even determines the sex. The warmer an egg is, the greater the chance that a female will crawl out later. A turtle embryo, for example, can determine its own sex by sitting on the warmer or colder side of the egg.

Global warming seems to be putting a population of green sea turtles in Australia in trouble. The sand is getting warmer, causing a female to crawl out of 90 percent of the eggs. The artificial cooling of these eggs may help to save the balance in the population.

See also: Cooling turtle nests can help to determine gender populations