Bengaluru: Birds living in colder parts of the planet in open nests tend to hatch eggs with shells that are darker in colour, a new study has found.
The dark pigmentation allows the egg to absorb more sunlight — maintaining its internal temperature for a longer period of time when exposed to the sun, according to the study published in Nature Ecology and Evolution journal Monday.
Bird eggs have always come in a multitude of colours, patterns and sizes, but researchers didn’t understand the reasons behind it so far.
Dark pigmentation in biology helps enable increased absorption of sunlight. Plants and animal species that cannot regulate their body temperature and depend on external sources of heat tend to have this kind of pigmentation, and such creatures are usually found in colder climates. This is often called Bogert’s rule in ecology.
Dark pigmentation on egg shells comes with several confusing contradictions that researchers haven’t yet managed to fully understand.
Dark shells absorb heat and help with thermoregulation of the embryo inside. But they also filter out the sun’s harmful UV rays, which are highly beneficial in warmer regions.
Light-coloured shells, meanwhile, are very noticeable to predators and attract their attention, and predators are much more abundant in warm regions.
Dark shells contain pigments, which exhibit strong antimicrobial properties when activated with sunlight. This makes it seem natural that darker eggs would be found in warm, humid areas that pathogens love, but this isn’t so.
To solve this flip-flopping mystery, American researchers catalogued and examined global colour variations and patterns from 634 different species found in natural history museums. They then mapped the patterns onto the individual species’ breeding geography — where their breeding grounds lie, how far they travel and where they hatch eggs.
Ultimately a very specific pattern emerged.
The team found that birds hatched darker eggs in regions where both temperature and solar radiation are low. Darker eggs are found more when nests are built openly on the ground or in cup-like nest structures, or any other kind at all, in colder regions. In cavity-type nests, darker eggs were found in both cold and hot regions. They also appear in extremely dry parts of the world.
Then, to understand the effect of solar radiation on darker shells, they exposed chicken, duck and quail eggs of varying colours and brightness to solar radiation. The survival of a hatchling depends on the embryo safely staying within thermal limits inside its protective egg. Researchers found that darker eggs were able to maintain their incubation temperatures for a longer time than lighter-coloured eggs. Thus, extremely hot and humid locales also tend to have dark egg shells.
Additionally, the researchers also discovered that bird eggs tend to be bluer when in cup or cavity-type nests in cool and humid regions.
A combination of all these findings enabled the researchers to conclude that thermal regulation is most likely the main factor that determines the colour of an egg shell and their geographical spread patterns.