Charlie Pridham writes
Our fruit cage is under going a major overhaul at present. about 20 years
ago I exchanged the normal fruit cage netting on the sides with chicken
wire of about three quarter inch dia holes. Worked really well as I was
able to grow clematis over the sides without damaging the netting.

Right, what I want to know is how big can the holes be before black birds
and thrushes get in? I would quite like to let in the tits, wrens and
other pest eaters so do not want to put back the same size unless I have
to.
The roof continues to be the standard woven fruit cage netting

I had a similar thought to Spider - have a look at the guards they sell
for bird feeders - these are designed to let in small birds and exclude
starlings, so the spacing on these would seem to be ideal.
--
Kay