Once the Christmas Bird Count has come and gone, memorable birding experiences can be hard to come by in the dead of winter. What tops the list for me is the avian activity that conveniently takes place right outside my kitchen window at the bird feeders. The number of birds using my feeders this winter is way down. It took me a while to catch on, but the reason eventually became clear. In past years a marauding Cooper’s hawk has been the culprit — sometimes a beautiful adult with blue-gray plumage, and sometimes a brown-backed youngster. Their stealth tactics are worthy of awe and their speed a little frightening.
This winter, however, it is a smallish sharp-shinned hawk that rules my avian neighborhood. I cannot really imagine that one of the Cooper’s hawks would tolerate this predator competitor, so they must have moved on or perished. That either hawk can survive the winter by catching other birds is impressive. Imagine the reflexes!
This is a bird identification problem we encounter regularly here on the Vineyard: how to distinguish a Cooper’s hawk from a sharp-shinned hawk. If you are looking through binoculars at a perched bird, or you have a photo with good detail, you have a better chance of making the right call.
In fall, winter and spring, either species can be expected here. During the summer breeding season there are a number of active nesting sites for Cooper’s hawks on-Island. Not so, that I know of, for sharp-shinneds. (I am leaving out the third and largest American accipiter, northern goshawk, because it is only rarely and briefly encountered on the Island.)
There is a whole list of features to look for in order to distinguish between these two very similar accipiter species. Unfortunately, those features are sometimes variable and most useful only when the two species can be directly compared, which almost never happens, except when the hawks are on the wing and migrating up to, over and away from the Gay Head Cliffs in the fall. In other words, the distinguishing features are not just relative, but elusive.
Bird books will give you all the features to compare, but doing so is quite difficult if you do not already have a baseline of experience with one accipiter or both. Even observers with considerable experience may be flummoxed by a quick look at one of these hawks and have to settle for the conclusion that it was an “accipiter species.” It happens, especially when the bird’s size falls in the middle of the combined size range for both species. They say there is not actually a measurement overlap between a large female sharpie and a small male Coop’s, but that subtle difference is so hard to call in the field.
The features typically compared are overall size; head, neck and beak size; projection of the head beyond the wing’s leading edge in flight; how straight that leading edge is in a glide; how fast and how stiffly the wings beat; how steady the bird seems in a stiff breeze; the relative thickness of the legs; and the relationship of the tail length to the length of the wings and of the body.
As to plumage, there are a couple of hard-to-see differences: Adult Cooper’s hawks show more definition between the color on the top of the head and that of the nape; an adult sharp-shinned’s coloration on the top of the head and its nape is the same or close to the same, on a smaller, more rounded head. Also, the breast feathers of a juvenile Cooper’s hawk are apt to have defined dark streaking, whereas the feathers on a sharpie’s breast show less distinct blobs of lighter color. Plumage comparisons also point to how wide the white terminal band across the tail feathers is — wider for the Coop’s. This feature is going to show more in birds with fresh plumage and not so much when the tail feathers are worn. A lot is also made about whether the end of the folded tail has the rounded outline of a Cooper’s or the squared-off corners of the sharp-shinned. The individual tail feathers of the Cooper’s hawk are rounded and of unequal length and the sharpie’s are mostly squarish, but beware the differences that wear and posture can make to this rule.
There is also one behavioral difference to note: Cooper’s hawks may perch out in the open, on a pole or power line, but sharp-shinneds rarely do.
There are all those factors to consider. I usually look for the features in this order of priority: overall size first, then the relative length of the tail in flight (the Cooper’s is much like a northern harrier in this respect, but with shorter and rounder wings), the overall impression of bulk and strength, thickness of the legs, if that can be seen, and, lastly, plumage details. All of these marks are pretty much useless if the observer has little previous experience looking at accipiters, so, if you do not, you may as well get started.
If this particular identity crisis is one that you have written off as too frustrating, think about giving it another shot when you have the opportunity. It’s better to be prepared for the challenge with a list of things to look for at the moment of encounter, than to hope to remember salient factors accurately after the fact.
We see most accipiters on the Island as fall transients winging over the Gay Head Cliffs. In the summer, when we know that Cooper’s hawks are nesting, and even where some of the nests are located, the adults can be very cautious and stealthy in the vicinity of a nest and next to invisible in thick foliage. It turns out that one of the best places to study them may be in the winter within eyesight of your own bird feeder.
Felix Neck reports finding a northern gannet on their beach on March 8 and that there are now two eggs in the barn owl home. Liz Loucks and Luanne Johnson spotted six brant off the north shore last Sunday and report friends having seen an immature bald eagle near Squibnocket. Also on Sunday last, Matt Pelikan observed horned grebes on the outer harbor in Vineyard Haven and watched a northern harrier cruise right over Five Corners. He also mentioned seeing a Cooper’s hawk strafe the collection of rock pigeons that frequent the Martha’s Vineyard Hospital.
Lanny McDowell photographs Martha’s Vineyard birds, writes about them and makes large prints of his most compelling images. Visit online lannymcdowellavianart.com and ottgallerymv.com.
Please report your bird sightings to the Martha’s Vineyard Bird Hotline at 508-627-4922 or e-mail to email@example.com.