Birdsmouth

Support the veneer stack on a ¾-in.-thick board with a narrow V notch cut into one edge. This board is called a birds-mouth, and the V notch provides clearance for the blade while the work is supported for cutting.

Source: wiktionary

After the wall and ceiling framing has been completed the erection of the roof takes place. Nail the four straight rafters selected as the first and last into position at the birdsmouth.

Source: wiktionary

birdsmouth, sally; in timber frame construction, a notch cut into the end of an inclined timber to receive a horizontal timber running perpendicular to it; used for the housing of rafters; other similar notches in other components: birdsmouth brick a special brick manufactured with an indented end, designed for use in decorative brickwork and at an internal obtuse corner in a brick wall. […] birdsmouth joint, birdsmouthed notched joint; in timber roof construction, a joint formed by notching the extremity of a rafter with a birdsmouth and fastening it to a wall plate; any joint similar to this.

Source: wiktionary

There are various ways of tying rafters to sidewalls, and the choices may vary depending on roof pitch. One of the most common is the use of “birdsmouths” cut into the rafter. A notch is cut into the rafter so that the rafter bears down flat upon the doubled top plate of stick framing, or upon the girt in heavy timber framing.

Source: wiktionary