Cooper's Hawk or Sharp-shinned: the field marks that survive contact
Half the identification advice for these two only works on a bird sitting still at forty feet. Here is what still works when it is crossing your yard at speed.
There is a bird that goes through your yard three times a winter, takes a House Finch off the feeder, and leaves before you have the binoculars up. It is an accipiter. It is either a Cooper’s Hawk or a Sharp-shinned Hawk, and telling which is the most reliably frustrating identification in North American birding.
The frustration is not because it is subtle. It is because most of the advice assumes a bird that is sitting still and lit from the front, and the bird you actually get is a shape crossing a gap between two hedges.
The marks that do not survive
Worth naming, so you stop reaching for them:
Size. In principle Cooper’s is crow-sized and Sharp-shinned is jay-sized. In practice a big female Sharp-shinned and a small male Cooper’s overlap almost exactly, and there is nothing next to the bird to compare it to anyway. Size works when both species are in the same tree, which happens roughly never.
Eye position. Cooper’s has a larger head with the eye set further forward. This is real and it is diagnostic and you cannot see it on a bird in flight.
Leg thickness. Sharp-shinned legs are famously pencil-thin. Excellent mark. Requires a perched bird at close range in good light, at which point you probably did not need it.
The marks that do
Tail tip
The single best flight mark. Cooper’s has a rounded tail, with the outer feathers shorter than the central ones, and a broad pale terminal band. Sharp-shinned is square or notched, and the band is thin or absent.
The catch: a fanned tail looks rounder than it is. Judge it when the tail is closed, which is most of the time in a straight-line flight, and be suspicious of any bird you only saw banking.
Head projection
Watch the front half of the bird, not the back. Cooper’s leads with its head: the neck pushes the head well out in front of the wings, and the silhouette reads like a flying cross. Sharp-shinned tucks in, so the head barely clears the leading edge of the wing and the front of the bird looks blunt.
This one works at distance, in silhouette, against a grey sky. It is the mark worth practising.
Flap cadence
Both do the accipiter thing of flap-flap-glide. Cooper’s flaps are slower, with stiffer wings, and the glide is long. Sharp-shinned flicks: faster, looser, with an almost buoyant quality, and the glides are shorter. Once you have seen both in a week you stop being able to un-see it.
Chest streaking, on adults only
An adult Cooper’s has fine, clean rufous barring. An adult Sharp-shinned looks smudgier and more diffuse across the flanks. Do not use this on juveniles at all; juvenile streaking overlaps completely and it has fooled everybody at some point.
The honest answer
Sometimes it stays an accipiter. That is a legitimate entry in a notebook and it is a legitimate entry in a checklist, and writing “accipiter sp.” is a far better habit than guessing and reinforcing the guess.
The birds you do call are the ones where two independent marks agreed. One mark is a hypothesis. Two is an identification. That rule is worth more than any individual field mark on this page.
Where to practise
A feeder in winter, from inside, with the binoculars already on the windowsill. Accipiters hunt feeders because feeders concentrate prey, and a bird that comes back on a schedule is worth ten birds seen once. The second visit is when you start noticing the head projection, because you are no longer spending the first two seconds being surprised.