By Emma Brennand
Earth News reporter
The owls are evolving to cope with increasing temperatures
Tawny owls turn brown to survive in warmer climates, according to scientists in Finland.
Feather colour is hereditary, with grey plumage dominant over brown. But the study, published in the journal Nature Communications, found that the number of brown owls was increasing.
As winters become milder, the scientists say, grey feathered tawny owls are likely to disappear.
This study indicates that the birds are evolving in response to climate change.
Dr Patrik Karell from the University of Helsinki, who led the study, gathered together data from long-term tawny owl studies carried out across Finland over the last 30 years.
The owls can be split into two plumage-based categories - brown or grey.
The colour of a tawny owl's plumage does not change throughout its lifetime, so Dr Karell and his colleagues were able to use the data to create "colour maps" of breeding pairs and their offspring.
The maps showed that plumage colour was hereditary; pairs with grey plumage had the grey "version" of the gene that coded for plumage colour, so they produced grey offspring.
In the case of mixed colour breeding pairs, the grey colour trait was "dominant", which meant that an owlet that inherited both grey genes and brown genes would be likely to have grey plumage.
Tawny owls have two plumage colours, grey (left) and brown (right)
The team examined tawny owl data, which was compiled by amateur bird ringers from the Finnish Museum of Natural History.
This revealed that, in years when winter weather was particularly severe, there was a higher mortality rate in the brown owl population.
This could be because brown owls were more visible to predators when there was thick snow cover.
Previous genetic studies have also suggested that brown owls' may have other disadvantages compared to their grey counterparts, including weaker immune systems and higher metabolic rates - meaning they need to forage more in order to survive.
But as the winters have become warmer, and snow cover has been reduced, the brown tawny owl populations have greatly increased.
Dr Karell told BBC News that the brown owls, which used to form 30% of the tawny owl population in Finland, now make up 50%.
"Its survival has improved as winters have become warmer," he said. "In other words, climate-driven selection has led to an evolutionary change in the population."
The results also suggest that a changing climate could, in some species, reduce the number and variety of characteristics that can be inherited.
If the grey owls disappeared from the "gene pool", for example, there would be only one version of the colour gene to be found.