By Emma Brennand
Nature reporter, BBC Earth News
Dark birds attract the opposite sex
Darker coloured pigeons are healthier, researchers in France have discovered.
A study of urban pigeons in central Paris has shown that birds with higher levels of the dark pigment melanin have stronger immune systems.
They are also better able to fend off parasites.
Writing in the Journal of Avian Biology, the researchers say the findings may help explain why different coloured birds have adapted to different environments.
Lisa Jacquin, and her colleagues from the National Centre for Scientific Research in Paris, carried out the research in conjunction with Dr Simon Ducatez from the Natural History Museum in Brunoy.
The researchers explored why birds of the same species are often coloured differently.
Feral pigeons are descendents of the Rock Dove
Pigeons are excellent navigators using the sun as a compass and local landmarks and even scents to find their way
Feral pigeons are found in 30%, 17% and 9% of Scottish, English and Welsh gardens respectively
By assessing the colouration and state of health of 195 free-living urban pigeons, they found that darker pigeons had lower concentrations of a blood parasite called haemosporidian. Their immune systems also responded faster to infection, compared to their pale-feathered cousins.
Currently there are two theories about why animals of the same species can be coloured differently.
First, it may be that their environment is causing the colour difference, a theory called the "exposure" hypothesis.
An alternative theory is that birds have evolved genes which code differently for melanin expression, a theory called the "genetic link" hypothesis.
"We tried to disentangle the 'genetic link' and the 'exposure' hypothesis in free-living feral pigeons Columba livia," Ms Jacquin reported.
Her team's research suggests that the birds may have evolved to produce higher levels of melanin in order to protect their immune systems.
This could also explain why there are higher populations of dark feathered birds in urban areas, where parasite prevalence is higher.
"The finding that immune responsiveness and parasite intensity correlates with colouration suggests that melanin-based colouration could play a role in sexual selection," explains Ms Jacquin.
So darker birds may be healthier and also appear more attractive to the opposite sex.