Birds living in cold climates and with open nests tend to have eggs with darker shells, finds a study in Nature Ecology & Evolution. 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 wide range of different colours and patterns, but the major drivers of this variation are unclear. For example, darker pigments absorb more heat than lighter pigments, so darker shells may be favoured in cold regions, but they also filter harmful ultraviolet radiation, which is stronger in warmer regions. Similarly, darker pigments have stronger anti-microbial properties and may also be favoured in warmer, humid areas. However, lighter pigments are conspicuous to predators which tend to be more abundant in hot regions.
Daniel Hanley, Phillip Wisocki and colleagues examined global patterns in eggshell colouration by measuring the brightness and colour of eggs from 634 different species, sourced from natural history museum collections. The authors mapped the patterns onto each species’ geographic breeding range and found that eggs are significantly darker when both temperature and solar radiation are low, and where nests are built openly on the ground, rather than in cavities or cup-like nests.
The authors 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 eggshell colouration.
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