The message
from Malcolm Ogilvie contains these words:

I'm in SW London where we have 3500 or so ring necked parakeets and it is
thought that they take over to the nesting holes of other birds, so perhaps
this could be another reason for it happening. We are now regularly
getting
up to 6 RNPs on our bird feeders.

They certainly are competing for nesting holes and, unlike the
woodpeckers, can't make their own.

Not even curved holes?

--
Rusty Hinge
horrid·squeak&zetnet·co·uk
http://www.users.zetnet.co.uk/hi-fi/tqt.htm