birds-mouth

from The Collaborative International Dictionary of English v.0.48

Bird's-mouth \Bird's-mouth`\, n. (Arch.)
   An interior angle or notch cut across a piece of timber, for
   the reception of the edge of another, as that in a rafter to
   be laid on a plate; -- commonly called {crow's-foot} in the
   United States.
   [1913 Webster] Bird's nest