Originally Posted by Julie50
Not 100% sure from the photo, but nuthatches make the entrance holes smaller using mud stuck to the tree branch/trunk. You can see some both above and below the hole in your picture. When the juveniles are about to fledge they often remove some of the mud to make the hole larger.
Could be that!
did not know they removed some, when they are about to fledge :-)
This hole is just on the side of a walking path, so I came across it few weeks ago, when they (or she) was still mudding up the hole. Therefore, I'm not so sure that that is mud pieces. In the part further away from its beak, you can see some curly thing, bit shiny, which gave me the impression of the tail of some (moth?) insect or the like.