A Field Guide to Aging and Sexing Rough-Legged Hawks

Am I becoming obsessed with Rough-Legged Hawks?  Perhaps, but for good reasons I swear!  There is relatively little known about the year-round ecology and life history traits of this high arctic breeder.  This is all the more surprising when you consider how common this raptor can be on it’s wintering grounds in the continental United States.  Most of what is known about Rough-Legged Hawks has been inferred from small scale studies on their wintering grounds (my Master’s thesis research included), with few studies documenting their breeding and migration ecology.  I hope to be able to study these beautiful raptors in greater detail soon, but for now I thought I’d take some time to familiarize some of you with field techniques I use to age and sex the species.

Rough-legged Hawks are unique among hawks (also called Buteos), in that they can be aged AND sexed fairly reliably using plumage characteristics in their feather patterns.  Most hawks can only be aged (juvenile vs. adult), using plumage characteristics.  Let us start with the bird pictured in Photo 1.  First, we see this individual has the trait common among all Rough-legged Hawks in it’s dark “wrist”, or carpal, patches (A).  All Rough-legged Hawks have these, although they can vary quite a bit between individuals.  The second trait we see that is also common among all Rough-legs is the subterminal, or sometimes terminal, tail band (C).  This individual is an adult male, which are unique by sometimes have multiple tail bands, as we see this individual has a faint second tail band above it’s well-defined subterminal band.  Lastly, we can see perhaps the trait that most often distinguishes adult males from females, which is the lack of a dark “belly” band across the lower abdomen (B).  There is considerable variation among adult male plumages (which are not all covered here!), so lets look at two more individuals:

We can see that the adult male in Photo 2 is fairly similar to the adult male in Photo 1.  A few key differences to note:  The first being that the male in Photo 2 has a considerably darker belly than the male in Photo 1.  In fact, Photo 2 almost makes it appear as if this individual has a belly band (an adult female and juvenile trait), however the picture is backlit, so keep in mind that light can play tricks on you.  The second difference between Photo 1 and 2, is that the adult male in Photo 2 has a much more well defined second tail band (A).  Adult males can sometimes have anywhere from 1 to 5 (or more) tail bands.

The last adult male I want to share with you is pictured in Photo 3 and is vastly different from the previous two individuals.  This adult male has heavily “mottled” feathers (or feathers with markings on them) on its underwings, as well as chest and belly.  These feathers are so heavily marked that the characteristic carpal patches of Rough-legs almost fade into these markings, but still remain slightly darker than the rest of the underside of the bird.  We can also see another trait common among all Rough-legs (except some dark morphs), which is the presence of a white-rump patch at the base of the top-side of the tail (A).  This can lead some to easily confuse Rough-legs with Northern Harriers, which also have a conspicuous white-rump patch.  Lets quick re-cap the two best defining characteristics of most adult males:  1) lack of a well-defined, dark belly band, and  2) more than 1 tail band (sometimes).  Now lets move on to adult females:

Right away we see a difference in Photo 4 from all the previous birds.  This bird has a clearly defined, dark belly band (A), coupled with a single well defined subterminal tail band (B).  These two traits help identify adult female Rough-legs.  Photo 5 is another adult female, where we can again see the dark belly band, along with the single subterminal tail band (C).

Adult females are most often confused with juvenile Rough-legs, which cannot usually be sexed unless you have them in the hand.  This confusion results from the dark belly band  common to both adult females and juveniles.  A juvenile Rough-leg:

The biggest difference between juveniles and adult females is the lack of a well defined tail band in juveniles (A).  We can see in Photo 6 that this bird has a broad, dusky tail band that gradually fades into the rest of the cream colored tail.  Another key characteristic of most juveniles is a general lack of mottling, or markings, on their underwing feathers (B).  To see this more readily, lets compare Photos 4 and 6 side-by-side:

Now lets look at another juvenile that is considerably lighter, and has almost no tail band at all (very light):

Photo 8. Juvenile Rough-Legged Hawk. Photo by Neil Paprocki.

So now you are an expert in aging and sexing Rough-Legged Hawks right?  Well maybe.  As with most things in life, nothing here is absolute.  Some key considerations and difficulties can arise:  1) These are all pictures of birds in flight, and perched birds are often times much harder to age and sex, although the belly band of juveniles and adult females is still conspicuous,  2) There is evidence in the scientific literature (Clark & Bloom 2005) that some adult males can exhibit adult female characteristics (namely a dark belly band, among others) after their first feather molt, so they are still retaining those juvenile belly band feathers while exhibiting other adult traits, and  3) Adult females can often have varying degrees of belly bands, with some showing lighter bellies indicative of adult males, while some can also show more than one tail band!

Hopefully this will be useful to some of you for the remainder of the winter, as Rough-Legs will sometimes stick around on their wintering grounds as late as April/May before heading back to the arctic to breed.  But you can certainly use this guide for next winter!  I’d welcome any and all photos or questions you have on Rough-Legged Hawks.  I personally love seeing the wide-range of plumages this species can show.  Please direct them to my e-mail at neil@wildlensinc.org.  Below are some useful references I have used to help me identify Rough-Legs, and other raptors.

• Wheeler B.K. and W.S. Clark. 2003. A Photographic Guide to North American Raptors. Princeton University Press.
• Liguori J. and D. Sibley. 2005. Hawks from Every Angle: How to Identify Raptors in Flight. Princeton University Press.
• Bechard M.J. and T.R. Swem. 2002. Rough-legged Hawk (Buteo lagopus). The Birds of North America Online (A. Poole Ed.). Ithaca: Cornell Lab of Ornithology.
• Clark W.S. and P.H. Bloom. 2005. Basic II and Basic III Plumages of Rough-Legged Hawks. Journal of Field Ornithology 76: 83-89.