I've got my spring cabbages covered with netting so it's not pigeons. All the leaves are eaten, from the outside edge of the leaf. Can't see any caterpillars.
slugs?
also, when i turned the leaves over, they were covered with a grey 'film' and lots of cabbage white fly. but they sit underneath the leaves, so you may not have seen them?
I would agree, slugs and snails. On the plot today, noticed on of my lovely kales, eaten where it has been touching the ground.