Wednesday, October 25, 2017

Great Horned Owl (Bubo virginianus) colour in Manitoba, Canada

I live in Manitoba, Canada within the range of the subarcticus subspecies of Great Horned Owl. Of the many hundreds I have seen here, where they are common and easy to see, especially in the Prairie Pothole region, the vast majority have a pale colouration. The bird on the right of the photo collage below would be a fairly typical example in my neck of the woods though some are paler than this, some darker, and some have slightly warmer tones, i.e. more warm buff or even a rufous wash to the pale areas of their plumage. Every so often I find a Great Horned that is so warmly coloured (like the one in the photo on the left of this collage) that it makes me wonder if it could be an individual of the nominate subspecies that has dispersed north. I am especially curious when I find such birds in fall at a time of year when dispersal is most likely to occur, especially natal dispersal. Natal dispersal is when young owls gain independence from their parents and vacate the territories and home ranges of their parents in search for a place to call their own. Of course they have a lot in store for them ahead, including surviving the toughest time of their lives, their first winter, before they can claim a territory and find a partner. Natal dispersal (the distance from an owl’s birth place to the site of its first reproductive effort) is sometimes short but can sometimes be hundreds of kilometres. Dispersal during food shortages can sometimes also be distant. Fall/winter is therefore the time when one might chance upon an admixture of subspecies as a product of dispersal. It might even be possible that such far dispersing birds find a mate of a different subspecies (subspecies can by definition interbreed) and to produce “intergrade” offspring but it is extremely difficult to prove that for a species with such a clinal range of colouration as the Great Horned Owl (from the super dark birds of the west coast to some pale northern birds that could almost be mistaken for a Snowy Owl to some of the intensely rufous birds of the wet and warm parts of the range). We have seen some fascinating pairing of light and dark birds in Alberta, for example. The rufous individual Great Horned shown in the left photo of this collage was photographed recently just 175 metres away from the site where the other photo was taken and the markings reveals that this bird is not one of the local nesting pair. My best guess is that this bird is a dispersing individual whose presence will not be tolerated for very long by the resident guardians of our cottontail-rich neighbourhood. With this colouration, it is tempting to speculate that this bird comes from within the range of B. v. virginianus but we will likely never know.      

For a little context, the rufous bird on the left is being mobbed by crows and on alert for that reason. The pale bird on the right was perched near the river but within sight of a well-used public trail and was keeping an eye on the passers-by, including one with a small dog (dogs are often perceived as a threat to the young instead of a potential snack if you are wondering). Both photos are taken at a distance of greater than 50 metres and heavily cropped.


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