Kit,
Even experienced birders struggle with the identification; so this is a great question. The identification problem is not restricted to these two species either. I've seen other hawks mistaken for these two species fairly often. Goshawk, Red-shouldered, and even Red-tailed have been confused by birders. We all need to admit when we don't know, and leaving a bird unidentified is okay (e.g. Accipiter sp. or Copper's/Sharp-shinned), especially if one is entering data for a Christmas Bird Count or eBird, for example.
I'll assume you saw either a Cooper's or Sharp-shinned and get on with some ways to tackle the identification. In short, you are on the right track: one needs to look at size (and shape) and markings (including coloration). A difficulty learning these outside of field guides is having good reference material and gaining experience in the field with known, correctly identified birds. Photos on the internet are frequently misidentified, and this makes trustworthy comparative material difficult to find. Try to study single birds that allow extended looks, like the one that caught a dove at your feeder. Learn to thoroughly describe characters so that you "see" them more readily on the next encounter. Don't worry too much about ID at first. Keep the detailed notes or photos and try to ID later (learning in the process).
Expectations are something to keep in mind. In the past, Cooper's Hawk numbers were much reduced and Sharp-shinned was the more expected species. That seems reversed now, with Cooper's twice as common in fields, wood edge, and the like (numbers that pass over hawkwatch sites are a different matter; let's keep this is about general encounters in winter). Complicating that is a change in Sharp-shinned status, it seems to be increasing as a wintering bird, mainly being seen at backyard feeders where they prey on birds. In general, I think Cooper's are under-reported and Sharp-shinned are over-reported, but it depends. Try to ask yourself why a bird under study is NOT a Cooper's, instead of approaching the identification from what you see that "makes it" a Sharpie. (Using this methodology of attempting to "reject" a tentative ID is good to practice.)
For a simple start, these websites may be helpful. They include a lot of shape and size differences that take some practice. The photos are probably reasonably useful for birds seen at feeding stations.
http://static.birds.cornell.edu/pfw/AboutBirdsandFeeding/accipiterIDtable.htm
http://www.allaboutbirds.org/guide/sharp-shinned_hawk/id
A few tips.
Juveniles (aka immatures) UNDERPART markings
SHARP-SHINNED: look for reddish tinted streaks that are fairly thick and spread across the underparts; the shape of these reddish streaks turn into spade-shaped markings down the flanks and belly.
COOPER'S: Relatively thinner and sparser blackish streaks that are shaped like elongated teardrops; these are concentrated across the breast and sparse to absent down the mid-abdomen. These markings can be brownish (not usually or ever as reddish as juv. sharpie though) and may be more spade-shaped at the extreme sides.
Adults (gray or blue-gray above; reddish barred below; red or deep orange eyes)
SHARP-SHINNED: small and rounded head with dark of crown continuous down back of neck and with back; pencils for legs
COOPER'S: paler nape accentuates hackled appearance (crown looks tipped up with squared off back of head); older males can be grayish in the cheeks, something not shown by Sharp-shinned.
Tail shape (both ages)
A key difference largely overlooked is the GRADUATED tail shape of Cooper's. On a perched bird, try to see the tail from the ventral (belly) side. Look at where the tips of the tail feathers lie. On a Cooper's, the outer tail feathers are much shorter and don't reach the tip of the innermost tail feathers (this is well shown in some of the photos at the above links). Sharp-shinned has all the tail feathers more or less even in length. This difference contributes to the oft mentioned "rounded" versus "squared" tail tip of Cooper's versus Sharp-shinned. Male Sharpies have even more square-shaped tips to their outer tail feathers, and this enhances the difference. Female Sharpies have more rounded outer tail feather tips, and this makes them, along with their larger body size, look more Cooper's-like. Small male Cooper's are probably most frequently called Sharp-shinned (but I've seen even experienced observers misidentify large female Cooper's as Sharp-shinned). This is probably the most frequent problem. In flight, even a small male Cooper's shows a largish head sticking out beyond the leading edge of the wings. Look for the tail shape too. Cooper's tend to show a broader pale tip to the tail, but this might be a relative character hard to judge when starting out (and it does wear away by spring).
I hope this helps you and others who have asked this question. Remember, it's okay to leave a bird unidentified.
Louis Bevier
Fairfield
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