The muscle turns black as it dries out and is exposed to air, go by smell and how bird is feeling as well as spreading of blackness to gauge necrosis. That wound will heal much faster if stitched but it won’t hold sutures at the moment, you missed the initial window, but you’ll get another chance in a few days to a week, what you want is a nice bed of granulation tissue (proud flesh to you horse folks), a moist pinkish stuff that will cover the raw surfaces and will heal together if it touches other granulation beds. Have somebody hold the bird, cut away the dried and dead edges (you want skin or muscle, fat and granulation tissue won’t hold sutures) until they bleed. Use thread or fishing line. Pull edges together but don’t put too much tension on the string or they’ll break. Use single sutures and secure with two square knots. Aim to cover the deep exposed muscle first don’t worry about little skin defects. Continue good wound care until the skin covers everything.