We have over a dozen nest boxes in our 1.5 acre garden at Woolaston. For the fourth year, Stock Doves are nesting in a kestrel-design box fixed to a shed wall where squirrels find access more difficult.
This year, for the first time, we have nuthatches in one box. There seem to be two chicks nearly ready to fledge (24th May 2015)
It is a larger box than my standard tit boxes. Internal dimensions are approx. 
30cm high at front x 15cm deep x 10 cm wide. After woodpecker damage I nailed on a new ply entrance, still enlarged by the woodpecker. The nuthatches have added mud above the entrance to seal it and have also adjusted the entrance size and shape to make it downward sloping inside and a little smaller.
The nest is made from large pieces of thin bark from our paper-bark maple. They must have struggled to pull them through the entrance hole.
David Priddis