Bill, beak, neb, nib denote the jaws of a bird together with their projecting horny covering.
Bill is the general term and is used inclusively by ornithologists for such a structure; in popular usage, however, bill suggests a structure that is straight and often flattened or long and slender (as in the duck, swan, hummingbird, crane, heron, sandpiper, or snipe) or one that is short, stout, and conical (as in the cardinal bird or hawfinch).
Beak is associated with striking or tearing and is the usual term for a structure, characteristic especially of birds of prey, in which the tip of the upper mandible has a sharp downward curvature and overhangs the lower mandible (as in the eagle, vulture, or hawk).
Neb and nib are equivalent to bill or beak chiefly in dialect or poetry but derive from this use their commoner extended sense of a jutting or pointed thing or part.