BIT OF AVIAN ANATOMY is a term used to describe the physical features of birds. The crossword solutions for this term are BILL (4 letters), QUILL (5 letters), and ATOM (4 letters). BILL refers to the beak of a bird, which is used for eating, drinking, and grooming. QUILL is another word for feather, which helps birds fly and keep warm. ATOM is the smallest unit of matter, and it is also used to describe the tiny bones in a bird's wings.