I got down on my knees the other day and groped under the pond edge. Sure enough, the mallard et al are pecking (billing?) in under the bank edge. I could slide my fingers in a couple of inches where the undermining was happening.
Seems that I'll need to consider a solid pond edge / margin, in the form of bricks or paving slabs / stones or re-cycled bits of broken concrete or the like. If I don't do something, the pond will get bigger than I want it to, as the undermined bank collapses downwards on itself, from time-to-time.
I wondered if - like hens - ducks need grit for their crops, so 'mining' for grit and stones from the pond edge is a sort-of natural phenomenon.
