Off for another caterpillar

I noticed yesterday that one of our nesting boxes has been re-occupied. Two Great Tits are whizzing in regularly with green caterpillars. They are quite nervous and immediately stop if I get too close. So this shot was taken from a distance and blown up, hence the lack of detail.

I googled what type of green caterpillars are fed by Great Tits to their young, and the answer was the caterpillars of the Winter Moth. This seems to be a catch-all phrase for a family of moths whose easiest to recognise characteristic is that they turn the leaves of fruit trees to something more like fine lace. I asked MrHCB if anything was punching holes in our fruit tree leaves, and he said "Oh Yes, especially the plum tree leaves". He had been thinking of spraying the trees to kill whatever was doing it. But I have persuaded him that the Great Tits are doing the job for him and it would be a pity to deprive them of the food source.