Those are celery leaves. In general, the stalks are eaten (by people) more than the leaves. The leaves are stronger-flavored; I love them in soup.

When I was a kid, we fed celery (leaves from the older stalks, plus whatever scraps we trimmed off) to my brother's guinea pig.

Thank you! It's actually guinea pigs I have too. I was wondering if they could eat them, but I guess you've answered that already.

They could also be parsnip leaves. I note that groundhogs like them, so for your pigs it should make no difference.

Probably Chinese celery. CC grow tall wit a lot of leaf branches and a lanky stem. I like its flavor better than traditional celery. I have tried to grow it from store bought (by rooting) but did not get good results.

It could be lovage too, very similar looking to celery.
Grows tall.