I wish I had photos to illustrate this post, but I don’t, it’s just something that’s been on my mind.
I’m not sure I’d ever laid eyes on a dark-phase Red-tailed Hawk before I moved to Eastern Oregon. Now I see them all the time. (At first, I’m embarrassed to admit, I think I mistook a few of them for Golden Eagles. Western raptors are still a new thing for me.) Click here for a photo of a typical Red-tailed Hawk, and here for a photo of the dark variety. Same species, two very different-looking birds.
A number of other Buteo raptors also have dark and light morphs – Swainson’s Hawks, Rough-legged Hawks, and Ferruginous Hawks are all found here and all include both dark and light birds. This variation is genetic (think of hair color in humans), and while dark birds are generally less common that light birds in every species, this varies by geography – you’re more likely to see a dark Red-tailed or Ferruginous Hawk out west, more likely to see a dark Rough-legged Hawk back east. What I want to know is, why? Is natural selection at work here, and if so, how are different color morphs adaptive for different regions?
One possible explanation that comes to mind is Gloger’s Rule, a zoological principle that states that within a species, darker animals will be found in more humid climates. My undergraduate ornithology professor studied how this applies to birds, and in fact he’s cited in the Wikipedia article linked to above. He found that, at least among Song Sparrows, there is more bacteria in birds’ feathers in humid climates than in dry ones, and having more pigment in the feathers makes them more resistant to bacterial damage. Unfortunately, I don’t think this applies to dark-morph hawks – otherwise why would dark Red-tails be more common here on Oregon’s “dry side” that in the comparatively rainy Midwest?
Of course, this could come down to random chance, some kind of founder effect where the start of a new population just happened to include more or less dark birds. But that’s boring, so I kept looking.
My next step was to do a quick literature search to see if I could find any other theories about color variation in hawks. (Someone else must have wondered about this at some point, right? Oh God I’m such a nerd.) Back in 1980 someone studied a population of Red-tailed Hawks in Arkansas that included both light and dark individuals, and they found that different-colored birds hunted in different habitats, selecting perch sites where they best blended in – light hawks hung out in open areas, while dark hawks preferred dense shade. Color variation let the species as a whole use more habitat types more efficiently.
I also found that this ties in with something called the image-avoidance hypothesis. If light-colored predators are what’s most common, that’s what prey animals will learn to avoid, and dark birds within a population might be more successful hunters as a result. Eventually the dark birds will become more common, prey animals will start paying more attention to them, and some sort of equilibrium will be reached.
This still doesn’t explain the geographical variation, as far as I understand it. Why are dark Red-tailed Hawks more common here in open rangeland than in shady Midwestern forests? It seems like it should be the opposite, and I’ve circled back around to a question that doesn’t really seem to have an answer.
Oh well. I had fun looking, and sometimes that’s the whole point.
- Burtt & Ichida, 2004. Gloger’s rule, feather-degrading bacteria, and color variation among Song Sparrows. The Condor.
- Preston, 1980. Differential perch site selection by color morphs of the Red-tailed Hawk. The Auk.
- Rohwer & Paulson, 1987. The avoidance-image hypothesis and color polymorphism in Buteo hawks. Ornis Scandinavica.