Rough-legs at Rollins

Here's a nice find from Rollins Savanna Forest Preserve today:

Adult male Rough-legged Hawk (Buteo lagopus), Rollins Savanna Forest Preserve,
Lake Co, IL 1/29/2014

This is a Rough-legged Hawk (Buteo lagopus). They breed on the Arctic tundra of Alaska and Canada as well as Eurasia. This year has seen a major invasion in the middle of the continent, similar to the eastern Snowy Owls.

This guy is a light morph adult male. Rough-legged Hawks show a good deal of plumage polymorphism, with light, intermediate, and at least two dark morphs in addition to age and sex-related variation in each morph. Here's a dark morph bird for comparison:

Dark Rough-legged Hawk, Illinois Beach State Park, Lake Co, IL 10/29/2012

This is normal variation, in contrast with albinism or leucism, which arise from an inability to make or lay down pigments for some reason, as seen in this Red-tailed Hawk (B. jamaicensis):

Leucistic Red-tailed Hawk (Buteo jamaicensis), Illinois Beach State Park,
Lake Co, IL 11/9/2013

Buteos in general seem quite prone to this sort of polymorphism, with 15 of 29 species listed in Ferguson-Lees and Christie's Raptors of the World showing distinct dark and light morphs (1). Here in North America, there is a fairly consistent pattern associated with this -- eastern birds (except the largely neotropical Short-tailed Hawk (B. brachyurus)) don't show a dark morph. Of our western buteos, only Red-shouldered Hawk (B. lineatus) in California and Gray Hawk (B. nitidus) don't show one.  White-tailed Hawks (B. albocaudatus) in Texas do not have a dark morph, although there is a range of variation in juveniles, but the same species in South America does. Red-tailed Hawks in the east don't show a dark morph, even though dark Red-tails are rather common in the west. Even Broad-winged Hawks (B. platypterus) have a dark morph in the west -- they breed at the western edge of the species' range, at the eastern edge of the Canadian Rockies in northern Alberta. Swainson's Hawks (B. swainsoni) nesting on the central and eastern Great Plains are all light morph, whereas farther west we see dark morphs appearing.

This pattern is a puzzle. There is a general rule that populations in humid climates tend to be darker than those in arid climates, known as Gloger's Rule, but the dark morphs in the west tend to be in drier environments than the strictly light morphs in the east. The varied topography in the west does suggest a greater variety of potential habitats, but I've never seen the morphs correlated to habitats, and they don't appear to segregate that way in my experience.

There is also one species that bucks the trend: Rough-legged Hawks. Dark morphs in this species are most common in humid areas in Alaska and eastern Canada, which fits Gloger's Rule, but for some reason they are unknown in Eurasia. (2)

Polymorphism in buteos has been argued to be adaptive because prey species might not recognize those dark birds as predators. Roulin & Wink (3) found that polymorphic species preyed more often on mammals than non-polymorphic species, but Galeotti & Rubolini (4) found no evidence for this. They argued instead that species that lived in variable habitats are undergoing disruptive selection, which can maintain genetic variation in a population.  So we're still not sure why these polymorphisms are present in the first place, and what produces the geographic pattern we see is a mystery: Is there some sort of adaptive significance to dark plumage that we're missing? Is there some sort of sexual selection going on? Are we seeing the result of differing history of the two regions? Perhaps eastern populations underwent a population bottleneck during the last glacial maximum, and the dark morph alleles disappeared from their populations?

Biologists have learned some amazing things about the critters we share the world with, but if a bunch of big, highly visible charismatic birds can still hold on to such secrets, we clearly aren't going to run out of questions to ask anytime soon.

(1) Ferguson-Lees, James & David A. Christie. (2005). Raptors of the World. Princeton: Princeton University Press.

(2) Wheeler, Brian K. (2003). Raptors of Eastern North America. Princeton: Princeton University Press.

(3) Roulin, A., & Wink, M. (2004). Predator‐prey relationships and the evolution of colour polymorphism: a comparative analysis in diurnal raptors. Biological Journal of the Linnean Society, 81(4), 565-578.

(4) Galeotti, P., & Rubolini, D. (2004). The niche variation hypothesis and the evolution of colour polymorphism in birds: a comparative study of owls, nightjars and raptors. Biological Journal of the Linnean Society, 82(2), 237-248.