Yosef Karo, Shulchan Aruch — 30: The law regarding a fractured skull
“ But with waterfowl such as geese and ducks (Rashba and teshuvat haRosh 20:9) , even if the skull is perforated any amount, it's Treifah.A land bird that was bit by a weasel on its head, or was struck on wood or stone: he puts his finger on the side of the hole and presses his finger there, or puts his hand in its mouth and presses there; if the brain doesn't ooze out, it's certain the membrane of the brain isn't punctured and it's kosher. ”
