Darker eggs have their purpose

An egg’s ability to maintain temperature within strict limits is critical to the survival of a developing bird embryo, but the role that eggshell colour plays in maintaining thermal balance has been a long-standing question.

Now, a study published in Nature Ecology & Evolution has found that birds living in cold climates and with open nests tend to have eggs with darker shells. The darker pigmentation allows the egg to maintain its internal temperature for longer when exposed to the sun, the research suggests.

Bird eggs come in a breathtaking range of colours and patterns, but the major drivers of this variation are unclear.

Darker pigments absorb more heat than lighter pigments, so it follows that darker shells may be better in cold regions. But darker pigments also filter harmful ultraviolet radiation, which is stronger in warmer regions.

Similarly, darker pigments have stronger anti-microbial properties, which would point towards suitability in warmer, humid areas. However, lighter pigments are more obvious to predators, which tend to be more abundant in hot regions.

Phillip Wisocki from Long Island University Post, US, and his colleagues – including Phillip Cassey from Australia’s University of Adelaide – sourced the eggs of 634 different bird species from natural history museum collections. 

They developed global patterns in eggshell colouration by measuring the brightness and colour of the eggs, then mapped the patterns onto each species’ geographic breeding range.

They found that eggs are significantly darker when both temperature and solar radiation are low, and where nesting takes place on open ground, rather than in cavities or cup-like nests.

The researchers then exposed chicken, duck and quail eggs of varying colours and brightness to solar radiation. They found that darker eggs were able to maintain their incubation temperatures for longer than lighter-coloured eggs.

Taken together, these findings strongly suggest that thermal regulation may be the main factor determining an eggshell’s colour.

And that shouldn’t be the end of the story, the researchers suggest.

“Our study provides insight into the environmental pressures that have shaped the global distribution of colours and should serve as a call to reinvigorate this line of research and investigate patterns of colour expression in other organisms,” they write.