From baby blue to black-speckled beige, bird eggs are as varied as the species that lay them. Why bird eggs come in so many colors is still something of a mystery, but new research identifies a reason for a common trend among birds, especially those up north: their dark-colored eggs.
Previous research on tropical birds has shown that darker hues help guard against predation — they are harder for hungry neighbors to see. Deep pigments in eggshells also contain light-activated molecules that protect against infections. Now, a paper published in Nature Ecology and Evolution suggests that birds up north lay dark eggs for another reason: The color helps keep the growing chick warm.
Read more: Why do some birds lay blue eggs?
The research team found that eggs are browner for birds nesting in cold climates with little sunlight. The need for this adaptation makes sense. Darker colors absorb and hold onto heat better than lighter colors, and this phenomenon — darker hues in colder habitats — has been found all over the animal kingdom.
Color Me Warm
Scientists have suspected this pattern would hold true for birds since the 1800s, says Daniel Hanley, a behavior ecologist at Long Island University Post. But the confirmation drives home just how many environmental factors bird eggs have to contend with if they're going to be successful. “There are a lot of pressures that influence the way you make a life as a bird,” Hanley, a paper coauthor, says.
Researchers have long assumed that birds in hotter climates were making tradeoffs when they laid dark eggs. The shells might deliver predation and disease protection, but surely those colors also ran a higher risk of making the egg too hot. After all, eggs have to stay around 90 degrees Fahrenheit to hatch. If internal temperatures get too high, the egg cooks. But because disease and hungry neighbors are less common in colder habitats, Hanley and his team wanted to know what might be deciding egg color in those regions.
The team mapped breeding range and egg color data from 634 species to visualize what hues showed up in different climates. The map showed a trend of increasingly darker eggs toward the highest latitudes, where sunlight becomes more scarce. Darker colors also appeared in some hot and humid regions too, like parts of Indonesia, where survival challenges like predation and disease are particularly strong.
The suspected rationale for darker eggs — improved ability to hold onto heat — played out in Hanley and his team’s lab, too. The group exposed different colored chicken, duck and quail eggs to sunlight and found that darker shells kept their heat longer than lighter ones.
Hanley wants to see if this lab scenario plays out in the wild. He next plans to use a thermal imaging camera to monitor temperature change in different colored eggs incubating in nature. He’s also curious about whether brood parasitism — or when birds lay their eggs in another nest to trick other parents into raising their young — is influencing egg color, too.