Give a Hoot Don’t Pollute!

Genetics of Adaptation Graduate Seminar

By Lindsay Chaney

Woodsy the Owl (US Forest Service icon circa 1970) is not the only owl concerned with the environment.  Recently the tawny owl of Europe is calling out hoots to scientists about global climate change.  Over the past 48 years there has been an increased number of brown tawny owls across Finland, likely as a direct result of milder winters.

The tawny owl (Strix aluco) is found across temperate regions of Europe.  It can be heard calling hu ..... hu-hooooo as it hunts through the night for voles and other small rodents.  There are two colors of the tawny owl, grey and brown (Fig 1).  Feather color is determined by level of the reddish-brown pigment pheomelanin present in feathers at birth.  The tawny owl remains this color throughout its life, regardless of age or sex.  Scientists have found that the grey color is dominant over the brown color, meaning there would most likely be more grey colored tawny owls than brown in a population (Fig 2). 

In southern Finland, scientists have monitored 250km² of land and tawny owl nests for over 33 years.  Year after year researchers have observed these owl populations through counting, color identification, and behavioral studies of the owls.  Recently, they have seen an increase in the number of brown owls.  From 1961 to 2009, over 3,239 tawny owls had been observed and the frequency of the brown owl has increased by about 2.5% per year.  Why was this? 

One possibility is that the owls have a color preference for mates. This occurs for many bird species, such as the peacock.  Females prefer male peacocks with the biggest, most ornate tail feathers.  When they preferably choose the big tail males, their sons have big tails as well, and there is an increase in the number of big tailed male peacocks.  This is an example of sexual selection, natural selection taking place due to preference in mate.  So, do the tawny owls prefer the brown owls as mates?  This is not the case.  Through studies of the owls, there has not been indication of any mate preference.

Another possible explanation is immigration.  If additional brown owls joined our study population, there would be an increase of the number of brown owls in the study population.  In these studies, the researchers catch the owls, take measurements and attach a harmless metal bracelet around their legs with an identification number. This provides critical information on every bird, and which members are new to the group.  By doing so, scientists have ruled out immigration as a key influencing factor. 

If not mating preference or immigration, what has caused this change?  Scientist began turning to the climate.  They developed five different prediction models to determine what accounts for the increasing prevalence of the brown tawny owl.  They noticed that the snow depth in the area has been steadily decreasing for the past 30 years.  Is this the key that is changing the color compositions of the owls?  Data showed that there is a difference in survival based on owl color.  When there are high snow years, the brown owl is not as likely to survive (Fig 3).  The model confirmed this idea with 99.4% accuracy.

Researchers offer three possible explanations for why brown owls would increase with the milder winters. One, the brown owl is a target of prey.  The larger eagle owl preys on the tawny owl.  In snowy conditions the brown tawny owl would be more contrasted against the white snow, while the grey tawny owl would be camouflaged.  If there is less snow, due to climate change, that would mean less brown owls being eaten.  This seemed likely until it was discovered that the eagle owl does not hunt by vision. 

A second scenario is that the brown owl has different energy requirements than the grey owl.  This would require the brown owl to hunt and eat more food, such as voles.  With the vole population in decline, this would be difficult in high snow years, causing the brown owl to decrease survivorship. 

The third potential explanation is that feather color is genetically linked to genes that provide a better immune system.  With milder winters, this would allow brown owls to survive more successfully, allowing their numbers to increase. 

Regardless of the specific cause of change, it is clear that the brown tawny owl is becoming more common throughout Europe.  In 1960, brown owls made up 11-15% of the population. In 2010, their frequency increased to nearly 50%.  The change in the population of tawny owls is an example of modern day evolution taking place as result of the change in climate.  This is one of the clearest examples we have seen thus far of nature responding to climate change impact. As global warming takes place it causes a domino effect.  The environment changes, resulting in changes in animal survival (natural selection).  This, in turn, causes a change in the genetic composition of the animals and which traits are passed on to the next generation.  This leads to the animal becoming better suited to the new environment (adaptation).  Thus, climate change does have a significant impact, even on the animals in our own backyard. 

Woodsy the Owl had wise counsel for us when he said, “Give a Hoot, Don’t Pollute!”  By following his advice, we may be able to stop or mitigate the trend of our warming planet. And next time you see a grey tawny owl in the wild, pull out your camera.  If climate change continues we may only have the brown owls left.

Source: Karell, P. et al. Climate change drives microevolution in a wild bird. Nat. Commun. 2:208 doi: 10.1038/ncomms1213 (2011).