Shape is one of the most important field marks you should consider when identifying a bird. While color and plumage condition will fluctuate, shape and proportion are much more consistent throughout the year and between individual birds of a given species.
That being said, the shape of a bird can change at any given moment due to the way they hold their feathers.
This Cooper’s Hawk had just bathed and was helping their feathers dry by fluffing them out. From a distance, this bird appeared to show a broad breast with rusty barring and a broad, banded tail. The drooping wings made the tail seem shorter. These are all great marks for a Red-shouldered Hawk.
After a while, the bird smoothed their feathers down, revealing a slender build and a long rounded tail typical of a Cooper’s Hawk. So shape alone is enough to identify this bird, but we had to observe the bird long enough to see what their shape actually was. This is another reason to take the time to study every bird you come across. A quick glance or a single photo can be misleading. (How many times have plastic bags been identified as Snowy Owls?)