In what is birdsmouthing, or birdmouthing, used?

Question

AConstruction, e.g. a cut to seat a rafter on wall structure

BHunting ducks

CScandinavian folk music

DPlaying some wind instruments

Select from the options above.

Correct Option: 1  

Explanation:
Construction, e.g. a cut to seat a rafter on wall structure
The construction technique is to cut a right angle corner out of the rafter or other member, resembling an open bird's mouth.