I mentioned this as soon as it came out for early access. It looks like there is a very small chance of getting animal protein from mammals ( like 1/100), while every bird I've killed has dropped it. Someone mentioned that they intended the "animal Protein" to be a rare drop. I have no idea why. This is a very broken game and the crafting system is tacked on. With that said I just shoot birds when I see them and later when you get to the beaches you can pick up all kinds of shellfish and eggs which substitute for the animal protein. The only ration really worth crafting is the endurance type.