What Has Devoured My Victoria Plum Leaves?

[Stargazer]

My jonagold apple tree has an record number of small apples beginning to grow but my two victoria plum trees have barely 10 plums between them.
The leaves look as though they have been eaten down to almost skeleton-like in places, particularly on the higher branches. Any idea what this might be down to as it is the first time it has happened?

[Stargazer]

I am not sure that slugs are so abundant at the top of the trees and I have not seen any there. I thought some sort of caterpillar but haven't seen any of those either.

[Stargazer]

could it be wood pigeons - as there are many around where I live?

[tonyav]

Could be winter moth.
http://www.growsonyou.com/question/show/83568

[megfitz]

Pigeons !! they did the same to my Victoria Plum tree last year,this year we threw odd bits of old pea netting over the top branches as a deterrent. It worked and some of the leaves regrew . I have been treating the tree for Plum Moth for the last couple of months as per R.H.S. advice.( they also agree it's pigeons.) this year I have a bumper crop and have made supports for the branches. So don't give up hope ..there's always next year.