Birds that lay their eggs in open nests and in colder climate tend to have darker eggshells than those with more enclosed nests or warmer habitats, a study finds.
The darker pigmentation helps the eggs to absorb more heat from the sun and maintain their target incubation temperature for longer, said US researchers.
The findings may help explain the variation seen in bird egg colouration and patterning, the caused of which had previously been unclear.
WHAT ARE EGGSHELLS MADE FROM?
Eggshell is formed from calcium carbonate and special proteins that help confer strength to the shell.
In a typical laying hen, for example, it takes around 20 hours to form a shell.
Pigmentation is added to the outside of the shell by special structures in the bird's oviducts.
As eggs are normally lain blunt-end-first, this side is normally subjected to more pressure — and therefore stronger colours — on its journey.
Birds that lay their eggs in open nests and in colder climate tend to have darker eggshells.
The possible drivers of egg colours are varied and contradictory.
For example, dark pigments tend to absorb more heat than their lighter counterparts, which could suggest dark eggs would be favoured in cooler regions.
On the other hand, darker eggs better shield against harmful UV rays, which are typically more intense in warmer regions.
They also have stronger anti-microbial properties, which are more useful in warmer climates — together suggesting the opposite might hold true.
Similarly, one might expect the conspicuous nature of lighter eggs to be a disadvantage in warmer regions, where predators tend to be more abundant.
To settle the matter, biologist Phillip Wisocki of the C.W. Post Campus of Long Island University and colleagues measured the brightness and colour of eggs, from 643 species of birds, preserved in the collection of various natural history museums.
The authors then mapped these colourations onto each species' known geographic breeding area — and found a pattern.
'Birds living in cold habitats, particularly those with nests exposed to incident solar radiation, have darker eggs,' the researchers wrote in their paper.
In contrast, eggs lain in warmer climes, or in cavities or enclosed nests tended to be lighter in colour.
Next, the team exposed chicken, duck and quail eggs — which various colours and brightnesses — to direct sunlight.
They found that eggs with a darker pigmentation were able to maintain their incubation temperatures for longer periods of time than their lighter counterparts.
'This evidence suggests that egg pigmentation could play an important role in thermoregulation in cold climates,' the researchers wrote.
Whereas, they added, 'a range of competing selective pressures further influence eggshell colours in warmer climates.'
