Experts led from the Long Island University Post measured the brightness and colour of eggs, from 643 species of birds, preserved in the collection of various natural history museums.
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.'
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