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from Gopher contains these words:

In message , Charlie
Pridham writes
Many thanks for the rest I will look at the various bird sites as
suggested 2" sounds in the right area. Helen your 8cm sounds a bit large
at around 3" but maybe lost something in the conversion/translation.

I agree almost totally with Charlie however I noticed last year that the
black woven mesh with which I surrounded our blueberry section had been
penetrated by blackbirds. Observation proved that they pecked and picked
at an obviously suspect part and created their own entry...

I had exactly the same experience last year trying to protect
gooseberries. That netting might work as the top of a cage where the
birds find it harder to get any purchase but is useless low down. It's
also surprising just how small an opening a bird that size can get
through. They only ever think of getting in and have no clue about
getting out again. I wouldn't use any smaller mesh than an inch,
one-and-a-half at the most, if that size exists. I believe two inches
would be as an open window, and the smaller birds like finches will get
through the one inch mesh; they're really nothing but air and fluff.