Can you benefit from a lost animal while waiting to find the owner?

Question

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.

Answer 1

Babba Metziah 28b - 29a discusses the issue of reimbursing the finder for his expenses in caring for the lost animal.

The idea is based on Devarim 22:2. The Mishnah points out that the verse tells us to return the lost object to its owner. So, infers the Mishnah, there must be something left to return. Therefore, an animal is not kept under the finder's care to rack up expenses, because then there would be nothing left to return!

The Gemara then goes on to discuss various animals that earn their keep or not. In general, animals that can be worked to pay for their own feed, are worked and fed (up to 12 months, then sold). Animals that cannot earn their own keep are sold after a short time. The money belongs to the original owner, not the finder. However, the Gemara questions if the owner may borrow the money meanwhile.

Rambam, laws of gezeilah and aveidah 13:15-19 codifies this Gemara into law.

Cows and donkeys may be rented out for work. Chicken eggs may be sold. The proceeds are used to feed the lost animals. After 12 months, the Rambam says that the value of the animal is assessed, and the finder becomes a partner with the original owner.

Other animals have 3 month, 30 day, and 3 day limits until they are sold, because it is too expensive to feed them. So too, anything that is produce of the animal belongs to the original owner. However, if it will rot, it should be sold.

The money may be used by the finder as a loan (apparently until the owner comes seeking his lost livestock). If someone finds a wallet, he cannot use the money. The reason the finder of an animal (who then sells it later) may borrow the money, is as an acknowledgment of his troubles dealing with the live animal for his brother.