Nearly any time you find a baby animal by itself, the correct thing to do is to leave it exactly where you found it. Most animals leave their young for long periods of time while they find food. Rabbits, for example, only feed their young once at dawn and dusk, and deer usually leave their fawns for many hours— sometimes even entire days— at a time! It’s almost always best to leave baby animals where you found them.
The key word being “almost!”
Baby opossums, called joeys, are an exception. Mother opossums aren’t very good at counting or keeping track of their number of young. When joeys leave the pouch and start riding on Mom’s back, one might fall and get left behind. It’s very rare for the mother to actually return for a joey that fell off!
Please contact a rehabilitator if you find a joey alone that is too young to be by itself. Most of the time, a joey over about seven inches is big enough to be independent and doesn’t need to be saved, while a joey under that size needs help.
Many of us have been lied to a time or to about how big seven inches is and don’t carry a ruler, so as a very general rule of thumb: if the opossum’s body (not including the tail) is larger than your hand, it’s okay to be left alone. If it’s smaller and is alone without mom, call a rehabilitator for help.
It’s usually best to leave baby animals alone for their mothers to come back, but opossums are an exception. Please be sure to get help for opossum neighbors in need!