Darker eggs may be at an advantage in colder regions as they are more heat-absorbent

Scientists have cracked the mystery of why bird eggs are different colours, with new research suggesting it boils down to temperature.

The appearance of bird eggs is rooted in two pigments, one of which is greenish and the other reddish-brown. Different concentrations of these pigments, together with the underlying white calcium carbonate of the shell, give rise to colours ranging from deep blue to greenish-white and rich brown.

But what influences the appearance of the eggs that different birds lay has been a topic of hot debate, with theories including the need for camouflage from predators, mitigating the harmful effect of UV light on the embryo’s DNA, helping birds recognise which eggs are theirs – and even that different colours may have different antimicrobial properties.

Researchers say while these factors might still play a role – particularly at a local level – a key influence at a global scale is temperature.

“Eggs in these colder places are darker brown and then as you go towards the tropics and even the temperate zone there are just more competing selection pressures, so there is more variation in colours,” said Dr Daniel Hanley, a co-author of the study from Long Island University.

With global temperatures rising, he said one concern was the impact on organisms such as birds that have traits adapted to particular climates.

Writing in the journal Nature Ecology and Evolution, Hanley and colleagues report how they analysed the colour and brightness of one egg from each of 634 species of bird from around the globe, and looked at the breeding range for each species.

The team then took the average for egg brightness and colour across the communities of birds in a given area and created a map of the results. Hanley said most species had eggs with a certain appearance, even though there could be some variation between different females.

The map reveals that darker eggs tend to be more common in regions with lower solar intensity and hence colder, while eggs are more variable, albeit generally brighter, in areas with higher solar intensity and hence warmer. The trend held even when different nest types were considered.

While cooler places are generally linked to browner eggs, a variety of colours from brown to blue are found in warmer regions.

Although levels of UVB radiation are also higher in areas of higher solar intensity, the team say this is unlikely to be driving the results since darker blue-green eggs would be best at blocking such light, but eggs in higher-radiation areas were generally brighter than in chilly ones. In any case, they add, eggshells are generally poor at letting light through.

The team say it makes sense for the brightness and even colour of eggs to be linked to temperatures. Darker, more heat-absorbent eggs could be at an advantage in colder regions since the embryo within an egg needs to be kept within a narrow temperature range but has no way of regulating this itself.

To back up their theory, the team took eggs from different chicken breeds and put them in sunlight. They found that darker ones heated faster than brighter ones and their surface temperature remained higher for longer.

They add that the variability in eggshell colour and even brightness in warmer regions may be down to less of a need to absorb heat, and more related to other competing factors such antimicrobial properties, predation risk or eggshell strength, which could possibly favour different colours.

Prof Simon Griffith, an expert in avian ecology at Macquarie University in Australia who was not involved in the research, said the study showed how birds across the world had adapted the colour of their eggs to the challenges imposed by environmental conditions.

But he said questions remained. “The study examined eggshell brightness overall, but this can be influenced by both the background colour and the amount and type of spotting or streaking,” he said. “This study does not shed any light on why some eggs are spotted, some are streaked and some have a uniform colour.”