If we find a lost animal we must return it to its owner (seen most recently on Bava Metzia 30, but I know this is codified elsewhere and said directly in D'varim). I also understand that we must care for it until that happens (I've heard this in the name of the Rambam, for one). While waiting, is one allowed to derive benefit from the animal, if doing so doesn't decrease the animal's value?
For example, a cow that is not milked when it needs to be suffers some pain. Therefore, it seems that you would be required to milk the cow as part of caring for it. Having done so, is the milk yours?
A different example: after a hen lays eggs the bird is not further affected if you do or don't take the eggs. Are the eggs yours?
I'm not asking about working the animal (for example using a found ox to plow your field), which could lead to the animal being injured so might be a different case.