If you are raising a baby you don't really need to feed it, they mature into an adult faster and their food drains slower than average creatures, so if you wanna see something weird you can leave a recently hatched baby argy in your breeding pen, then after a few hours come back and there will be a wild (or more appropriately unclaimed passive) adult argy sitting in your breeding pen. If you let it out it will wander on land until it reaches water which will make it fly, it won't attack anything and won't react when attacked, and lastly you can still claim it. If you kill it though it will still say "you killed argentavis (level ??? argentavis)"