One of the most highlighted bird identification challenges is Cooper's Hawk versus Sharp-shinned Hawk. The two are similar in both shape and plumage, and there's even some size overlap. Separating these accipiters is much easier with birds sitting on a post or in a tree, but it's almost a whole different case with flying birds. I am still not at the point where I can safely ID either species in flight, so this post will cover identifying them when perched. At the end, I have a short Cooper's vs. Sharp-shin photo quiz for those who want to "put to practice" what they have learned.
Without a doubt, identifying Cooper's and Sharp-shin is simplest with perched adults. All you truly need is a good view of the bird's head. I always think of adult Cooper's Hawks as having dark contrasting "caps", while adult Sharp-shinned Hawks have a uniform gray "hood." In other words, Cooper's has a dark, slate gray cap, followed by a pale nape and cheek, then the same slate gray coloration on its back and wings. Sharp-shin has gray starting at the head and continuing down the nape, back, and wings. The adults of both species have red eyes.
Above, two spreads from The Crossley ID Guide to Raptors. The first is Cooper's Hawk, the second is Sharp-shinned Hawk. Click each image for a larger photo.