These gorgeous birds are often spotted at bird feeders, appearing out of nowhere to take out a sparrow or finch in a sudden explosion of feathers. Sharp-shinned Hawks and Cooper's Hawks have extremely similar plumage and are acknowledged to be one of the hardest raptor identifications in the US. Unfortunately, they are also one of the most likely a beginner will find themselves wanting to identify. Cooper's hawks are bigger, on average, than most "sharpies", and the difference can be stark. However, this field mark is confounded by the fact, common to many raptors, of a pronounced sexual dimorphism with respect to size. Surprisingly, the female of the species is considerably bigger than the male, often as much as 1/3 bigger. What this means in practice is that a female Sharp-shinned Hawk can be almost as large as a male Cooper's hawk. Given the practical difficulty of judging size in the field, chances are unless you have a female Cooper's or a male Sharpie, where the size difference is pronounced, this isn't going to be sufficient to make a firm identification.
There are a number of other field marks that can be used to aid identification. Sharpies have a pronounced square edged tail, whereas the tail of a Cooper's is rounded. Received wisdom says that if you can see a clear square edge, it is almost certainly a Sharpie, however, if the tail is rounded it could due to variation in how the tail is being held, and is therefore not a conclusive feature. Other factors that can be helpful, include, the amount of white at the tip of the tail, the uniformity of the bands in the tail, whether the head seems flattened or rounded, whether there is contrast between the cap of the head and the nape of the neck, the relative thickness of the legs, length of tail in proportion to the body, and position of eye relative to the rest of the head. The more of these field marks that can be spotted the more firm the identification will be.
Given that quick-fire introduction to accipiter identification, I'd like to introduce you to today's conundrum. I have some pictures of an accipiter that I spotted to today, and I'm going to walk through my thoughts as I attempt to identify this bird. I'm going to be candid, I'm still thinking about this identification problem, and as with many accipiters, the most honest conclusion may be that this was an unidentified accipiter species. Before settling for that unsatisfying climax, lets see what can be observed about the bird in question.
My first few of the bird was from some distance away, high up in a tree, and backlit so that there were few plumage details visable. The following pics have been tweeked to show more detail than was visible through my binoculars:
In the first two pictures above, the head seemed rounded in a way I might associated with a Sharp-shinned Hawk. This, in combination with my initial perception of a fairly small size, albeit from a considerable distance, coloured my initial gut reaction that this was probably a Sharpie. Not too many additional field marks visible from these shots, although, there does seem to be a distinct contrast between the nape of the neck and the cap in a few of the pictures, a feature associated with the Cooper's Hawk, not the Sharpie. I could almost convince myself that the tail is pretty squared off ... although there does seem to be a degree of rounding at the corners. I wouldn't want to bet a beer on it at this stage.
After some application of my highly refined stealthy birding skills I was able to get some closer shots, although unfortunately, only from directly behind. A profile view of the beak and forehead would have been a great help, as would a view of the legs.
From this angle the tail seems distinctly more rounded than before, at least to me. There is a sizeable white terminal band also, both features pointing to an identification as a Cooper's Hawk. The length of the tail, relative to the body, also seems to me to be more in line with a Cooper's proportions. As noted previously, there still seems to be a distinct contrast between the nape of the neck and the crown of the head, again pointing towards a Cooper's.
So ... all things considered, I'm going to put my neck on the line and say I think this bird is a Cooper's Hawk. Hopefully, some other experienced birders will weigh in and share their opinions, preferably explaining their line of reasoning. Tackling these kinds of identification problems can be daunting, and with good reason. However, I maintain, that even if you don't manage to figure out what species you are looking at, you will definitely look more carefully and see more about the bird if you give it a good try. That can't be a bad thing!