Wednesday, December 13, 2017

Sharp-shinned and Cooper's Hawks


Fig. 1: Who am I? (Photo: Jerry McFarland)
The Sharp-shinned Hawk (Accipiter striatus) and Cooper's Hawk (Accipiter cooperii) are two closely related birds of prey, and pose an identification challenge that some ornithologists used to consider unsolvable.

These two species belong to the genus Accipiter, which contains about 50 other species worldwide. Only one other accipiter, the Northern Goshawk, is found in the USA and Canada. Accipiters' short, rounded wings and long tails are well-adapted for maneuvers in forested habitats.

In today's post we'll discuss how to differentiate this duo while perched; many of these traits can be used for in-flight ID as well. One of the most important themes in this post is that no field mark is reliable on its own. Thus, let a majority of traits lead you to an identification. If there's no majority, it's better to be uncertain than inaccurate!

In this post I will often refer to Sharp-shinned Hawks as "Sharpies" and Cooper's Hawks as "Coops."  Enjoy!

Shape

These species are structurally different, but may have quite similar plumages both as immatures and adults. Thus, shape is the best place to start in making an identification.

Overall proportions
Figure 2: Immature birds: Cooper's Hawk (left) and Sharp-shinned Hawk
Cooper's Hawks are long and lean with a larger, blocky head that projects out from a more evenly proportioned, "tubular" body. Coops may appear bulkiest at the belly. The tarsi (tarsometatarsus) and toes of a Cooper's Hawk are thicker and appear sturdier, not quite so thin between the joints.

Sharp-shinned hawks have a small, rounded head plopped with an ice cream scoop onto a heavy-shouldered, thin-waisted body. In a relaxed position like in Figure 1, Sharp-shinned Hawks can seem neckless. Sharpies' toes and namesake tarsi are thin, looking almost like toothpicks in comparison to the rest of the bird. Some refer to Sharpies' toes as "aye-aye toes."

Beware that momentary changes in posture can affect these impressions, so photos may be deceiving! For instance, the head of a Cooper's hawk can look quite small when turned at an angle, and an alert Sharp-shinned Hawk may look particularly long-necked.

Head shape

Figure 3. Head shape comparison in adult Cooper's (left) and Sharp-shinned Hawks

Cooper's Hawks have large, blocky heads with a "corner" of raised feathers at the back. In profile, the culmen--the top of the upper mandible--creates a smooth line with the crown of the head, forming a "roman nose;" however, this impression may be diminished when the crown feathers are erect. The eyes may seem proportionally closer to the bill and smaller in proportion to the size of the head. These features, along with a prominent orbital ridge, lend a fierceness to the expression of the Cooper's Hawk.

Sharp-shinned Hawks have smaller, rounded heads. The dainty, parakeet-like bill interrupts the curve of the crown. Sharpies are frequently described as "bug-eyed;" their eyes seem to bulge with concern. This trait is due to a less pronounced orbital ridge, proportionally larger eyes, and more central eye placement. The wide-eyed look can be exacerbated by a prominent yellow ring of featherless skin around the eye, a feature that is not as common in Coops.

Tail shape

In these two hawks, the difference in length between the outer and inner rectrices (tailfeathers) is a helpful clue, but one must account for variability, molt, and seasonal wear.

Figure 4. Typical immatures: Cooper's Hawk (left) and Sharp-shinned Hawk
Cooper's Hawks show graduated rectrices: the outer pairs of feathers are shorter than the inner pairs. This graduation causes the tail to look deeply rounded when spread. However, wear throughout the fall, winter, and spring can shorten the inner rectrices, reducing the amount of graduation. Some extreme individuals have barely-graduated tails.

Sharp-shinned Hawks may totally lack graduation in their rectrices, though some birds can show slight graduation, especially younger females (source). When slightly spread, a typical Sharpie's tail looks square. Beware that when spread wide enough, the tail can still appear rounded.

Furthermore, the shape of the outer rectrices themselves is different: the outer pair of feathers is more sharply cornered in Sharp-shinned Hawks, whereas Cooper's Hawks' outer pair is more rounded at the tip. In practice, this contributes to a slightly more squared tail shape for Sharpies.

Finally, Cooper's Hawks show proportionally longer tails than Sharp-shinned Hawks. If the tail seems stubby or appears generously long, you'll be that much closer to identifying these confounding birds. Note that immatures of both species have longer wing and tail feathers than adults of the respective species; it is theorized that this difference helps to compensate for younger birds' lack of flight experience and strength.

Plumage

In both species, adults have a gray or blue back and top of head, orange or grey cheeks, and an orange-barred underside. Immatures have a brownish back, streaky brown head often with lighter eyebrow, and a white underside streaked with brown (though beware of misleading fresh and darkly marked individuals). Immatures wear this plumage for about a year before molting into adult plumage in late spring and summer.

Immatures have yellowish or greenish eyes that transition to red as adults. Some individuals go through this change earlier or later than anticipated; plumage is a more reliable characteristic for aging.

White tail tip

One of the most commonly used--and misused--traits to distinguish Cooper's Hawks and Sharp-shinned Hawks is the width of the white tail tip. This feature is more subtle than presence or absence, and using it correctly requires taking account of a variety of exceptions.

Fig 7. Variation in tail tip width in Cooper's Hawks (at left: topbottom)
and Sharp-shinned Hawks (at right: topbottom). Top: juveniles. Bottom: adults.
A Cooper's Hawk in fresh plumage has a thick white tail tip (Fig. 7, at left). This is bordered by a blackish subterminal band (Fig. 7, lower left), or in some juveniles, by a dark gray area that blends into the outermost black band (Fig. 7, upper left). Sharpies often show a white tip on the tail, but the tip is narrower compared to a fresh Cooper's. Adults have a dark subterminal band (Fig. 7, lower right), whereas juveniles nearly always show a light subterminal band (Fig. 7, upper right)

The lack of a white tail tip is useful for identifying Sharpies in the fall only: by the spring, both species may have lost their tail tips due to feather wear. However, the presence of a white tip in the fall doesn't mean the bird is a Cooper's!

The size of a tail tip is best judged when it is not backlit and using a topside view of the tail, as in the photos in Figure 7. Again, juvenile Sharp-shinned Hawks often show white tip next to a light gray subterminal band (Fig. 7, upper right). When backlit, this light gray band can appear whitish, making the bird appear to have a thicker white tip.

Juvenile Plumage

Breast streaking

Aside from the shape of the bird, breast streaking is the feature most commonly used to differentiate immatures of these two species.

Classic immature Sharp-shinned Hawks have light brown, smudgy streaking, whereas typical Cooper's Hawks have dark brown, thin "tear-drop" streaking. On average, Sharpies will show denser streaking that continues farther down the belly, though there is some overlap on this feature (source with good example images). For good examples of typical juveniles, check out Figures 2 and 4.

Fresh Cooper's Hawks often show a golden backdrop to the streaking at the top of the chest, but this wears off as the bird progresses into fall and winter. An example of this feature is visible in Figure 4.

But these birds aren't conformists! Beware of plumage pitfalls.

Fig. 5: Two confusing immature birds. Cooper's Hawk at left, 
Sharp-shinned Hawk at right (used with permission of Tim Rains)

Take a look at the bird on the left. It is certainly a Cooper's Hawk by its large, squared-off, projecting head and thick tarsi and toes. But its streaking is dense, smudgy, and rust-tinged.

Now examine the bird on the right. Notice the unmistakable structure of a Sharp-shinned Hawk: it is neckless with a small, rounded head, a parakeet-like bill, broad shoulders, and thin tarsi and toes. Additionally, this photo was taken at Denali National Park in Alaska--about 1,000 miles from the typical range of Cooper's Hawks. But this bird has dark brown tear-drop streaking and substantially graduated tail feathers.

To avoid this confusion, it may help to assess the evenness of the streaking. Often, Coops' markings get gradually thinner and sparser at the bottom of the belly, whereas streaking on Sharpies is thick all the way down the belly, save a few thin streaks near the vent.

Head & nape details

Figure 6. Head details of Cooper's Hawk (left) and Sharp-shinned Hawk immatures
Cooper's Hawks tend to have a lighter, tawny crown and nape with dark streaking, contrasting with a  solid dark back. Sharp-shinned Hawks either lack this feature or have a reduced version of it; in general their crowns are closer to the color of their backs, and their napes are less contrastingly streaked. This trait is visible in Figure 4. For more examples, check out this size-adjusted comparison from a blog post by bird identification expert Jon Ruddy (source).

Both species may or may not show a light supercilium ("eyebrow"). On average Sharpies have larger, whiter supercilia--a minor point at best.

Another useful point is the amount to which the throat contrasts with the auriculars and malar (the "cheeks"). Sharpies' throats tend to lack contrast, often being thoroughly streaked in orange or brown. A contrasting white throat, perhaps with a single vertical dark line through the center, is a feature typical of Coops. Some Coops do show more densely streaked throats.

Finally, while not exactly a "plumage" detail, Sharp-shinneds often show a striking lemon-yellow eye, whereas Cooper's Hawks' eyes are typically a paler greenish yellow. If the eye color really pops, you likely have a Sharpie in your sights.

Adult Plumage

Head & nape details

Fig. 8: Adult Cooper's (left) and Sharp-shinned Hawks
Adult Cooper's Hawks have caps, whereas adult Sharp-shinned Hawks have hoods. On a Cooper's Hawk, the dark gray color on the top of the head and back is interrupted by a light-colored nape. Sharp-shinned Hawks, however, have a dark nape, so the color seems continuous, as if the bird is wearing a hooded jacket.

Unfortunately, this feature can be tough to distinguish if the bird's head isn't in profile. Individual variation is also at play--some Sharp-shinned Hawks have very dark feathers on the nape, and some have a slightly lighter coloration here, especially on the outer edges of the nape, farther down the shoulders.

Additionally, adult Sharp-shinned Hawks seem more likely to show whitish streaks on the supercilium, though Cooper's may show this feature as well, e.g. this individual.

Sex differences

In both species, males are more slaty-blue, whereas females have a less colorful, brownish tinge. Additionally, males of both species commonly show darker gray on the head than on the back.

Mature male Cooper's Hawks have a number of plumage features that distinguish them from all other birds. Their caps may be dull blackish instead of gray, contrasting with the gray back. While male Sharp-shinned Hawks may also have a darker head than back, they never show a black cap. Additionally, only mature male Cooper's Hawks show gray auriculars ("cheeks," the feathers covering the ears); Sharp-shinned Hawks and female Cooper's Hawks have orange auriculars. The Cooper's Hawks in Figures 3 and 8 show these features.

Final notes

Size as a field mark

Although there is little overlap in the size of the species--Sharp-shinned Hawks are uniformly smaller than Cooper's Hawks--there is considerable variation within the species. As in many birds of prey, males are smaller than females. This dimorphism is so strong that a male Cooper's hawk can be about the same size as a female Sharp-shinned hawk!

Furthermore, without a known reference point, size is an unreliable field mark and can be misleading in the field. Instead of following your gut when you think a bird looks big, ask yourself what makes you think it looks big: the slow wingbeats? The thick tarsi? These are more reliable indicators of species than an impression of size.

Tough calls

You may find certain field marks hard to judge. For instance, many folks become frustrated when trying to decide if an accipiter's tarsi are thick or thin. It's okay to just stick to the markings that you feel confident about! Once you have used the more familiar field marks to confidently identify an individual, study the more challenging traits. Eventually the pieces will fall into place.

Finally, although most individuals can be identified with experience and a good look, even raptor experts leave some birds unidentified. Momentary conditions, the bird's posture, and confusing individuals with a mix of traits all add to the uncertainty. Certain birds are better left unidentified; as Kenn Kaufman says, "Sometimes the right answer is 'I don't know.'"

Fig. 9: Who am I? (Photo: Len Blumin)

Resources used

My knowledge of these topics was substantially influenced by the writings of Jerry Liguori online and in his books Hawks at a Distance and Hawks from Every Angle. Several subtle points, including information about rectrix shape and sexing adult birds, came from Kenn Kaufman's Advanced Birding. Learning these traits was possible thanks to the guidance of Facebook's Hawk ID forum, an excellent resource for novice and expert birders alike.

Many thanks to all of the photographers whose Creative Commons-licensed photographs give this article much of its value. Some photographs were cropped and rotated/flipped to allow for better comparison between individuals.

3 comments:

  1. The bird above looks like a Sharpie to me.

    ReplyDelete
  2. Figure 1 looks like a Cooper's and Figure 9 looks like a Sharpie?

    ReplyDelete
  3. Thank you for acknowledging me, that was thoughtful. You will go far in this field with your positive attitude and approach!

    Cheers,
    Jerry Liguori

    ReplyDelete