Note to Twelfth Night, 3.1.64-65, "And, like the haggard, cheque at every feather / That comes before his eye."

A "haggard" is a hawk captured when it is an adult. Such hawks are hard to manage, and likely to suddenly go off mission and "cheque," or turn, and go for other targets of opportunity, or even chase a false target, such as a feather. In comparing the Clown's foolery to a haggard, Viola is admiring the Clown's constant alertness to every opportunity to make a jest out of both acceptable and unacceptable targets which will amuse those for whom he performs.

Clown with Tabour from mural Roskilde Cathedral, 1464