In article ,
wrote:

Often cats that have lived exclusively outdoors all or most of their
lives have no idea what to do when presented with a litterbox with the
expectation that they pee/poop in it. They are used to going in dirt and
have no idea what litter is. Your best option, and one that I have used
many times successfully, is to put dirt from outside in the litterbox.
Putting one of the kittens poops in the box should help as well.
Gradually add litter to/remove dirt from the box until it is changed
over enough that you can dump the contents and use plain litter.

Good idea. Thanks.

Priscilla