That’s according to this article which suggests the owls are evolving to deal with increasing temperatures. Feather colour in owls is hereditary with grey plumage being more common than brown. Scientists in Finland have studied the owl and found that as the winters become milder the grey owls are more likely to disappear. It seems when the winter weather is more severe there is a greater mortality rate in the brown owl population possibly because they are more visible to predators when there is thick snow cover. The brown owls although they have  other disadvantages such as a weaker immune system have greatly increased as the winters have become warmer whilst the grey owls have declined. If the decline continues and the grey owl becomes removed from the gene pool then eventually only the one colour variation could remain.