Although I do not like to make photos to nests of birds, in this case I think that is good to see the reforms made in a nest of Great spotted woopecker by our protagonists.
The photos I have made by means digiscoping techniques, reason why the observed distance has been the sufficiently good like so that the birds were not bothered.
As we can see this is a hole of a nest Great Spotted Woopecker (Dendropocos major).
Here we can see since the reform has been made, reducing with mud the inlet diameter of the hole, until obtaining the suitable one.
Here we have the authors of this peculiar adaptation, the Nuthatch (Sitta europaea).