How many Cooper's Hawk photos have you seen identified as Sharp-shinned Hawks based on their "skinny legs?" I know I have seen too many! Do Sharp-shinned Hawks have skinny legs….of course. But, Cooper's can have skinny legs as well (especially males).
Here are a few photos of Cooper's Hawks, check out the legs on each and decide for yourself if you would be able to ID them based on leg thickness only. Besides, there are many other traits that are more useful in separating the 2 species (i.e. head shape and size, plumage, tail length and shape, etc.). Oh yeah, be careful of using the large-eyed vs. small-eyed look as well, it can be tricky.