Probably the Grey Heron Ardea cinerea, Héron cendré.
HAM 2 2 375 p I know a hawk from a handsaw.
When Hamlet finds out that Rosencrantz and Guildenstern have been sent to spy on him, he tells them:
I am but mad north-north-west. When the wind is southerly,
I know a hawk from a handsaw
(HAM 2 2 374-5)
While there is no doubt that the overall meaning of what Hamlet says is that he is not as mad as he appears, the precise meaning of "hawk" and "handsaw" is not so clear.
Two interpretations can be found:
1. Handsaw is a corruption of hernshaw an old name for the heron. Hamlet is then saying that he can distinguish between two birds. This is consistent with his princely status as his knowledge of falconry is then assumed. TILLEY (H226) registered this famous sentence along with: "[...] her wits do walke Yet doth not knowe a Buzzard from a Hawlke" (Breton, Pasquil's Foolscap: Wks., I 20). If at first sight the meaning is the same - being able or not to distinguish between two birds - on closer examination, in the light of the customs of falconry to "knowe a Buzzard from a Hawlke" means that one is able to make the difference between a bird of prey which is suited for the purpose of hawking and a baser bird which is not (see BUZZARD).
Whereas to "know a hawk from a [heron]" means, with much more pertinence in the context, that one is able to recognize the hunter from the hunted. HARTING, about hawking at herons, says this (p. 222-3):
[it] was thought to be 'a marvellous and delectable pastime,' and in all the published treatises upon falconry, many pages are dedicated to this particular branch of the sport. Not only were herons protected by Act of Parliament but penalties were incurred for taking the eggs, and no one was permitted to shoot within 600 paces of a heronry, under a penalty of £20 (7 Jac. I. c. 27).
2. "Handsaw" is a carpenter's saw and "hawk" is a plasterer's hawk, the mortarboard on which plasterers carry their plaster. Within a more popular frame of reference the same idea, being able to distinguish between the obvious, is expressed.
Once again, consciously or not, Shakespeare provides us with what Peter GOODFELLOW calls here a "witty quibble", and above that with chameleon imagery that can adapt itself to any kind of audience, from nobles to craftsmen.