snails? slugs?

I'd also go for snails/slugs.

Do they eat ferns? Does anything eat ferns??

Mine are the same this summer I think it's caterpillars

Caterpillars have been eating my ferns this year (been a good year for Caterpillars - I have removed 100's) - not found any on the Dicksonia thank God!

Thank you one and all. I am also convinced it is caterpillars, last night I walked up the local allotments,
and they have been the biggest pest this year according
to the dedicated gardeners,having eaten all sorts of flowers,cabbage. etc.
Slugs and snails I have a routine to get rid of them.!!!
In answer to Bulbaholic, I have NEVER had my ferns
eaten before,that is why I was at a loss to know what it
was. Thanks once again.Wellie

I would say it is a bad year for slugs as it has been so dry since April. Defo Caterpillars have been a real pest - Have seen the same problem across the UK in other forums.