The Nuthatch is fairly easy to recognise. It is steely grey-blue above, pinky buff below and has a prominent black eyetstripe, white chin and dark, dagger-like bill. Nuthatches are busy and agile birds, typically seen working their way down a tree trunk searching for food.
If you see a pair of birds together you might be able to make out the extra reddish brown markings down the flanks of the male bird, just under the wing.
Mud is plastered around the entrance, side and roof of the nest cavity. When using nest boxes, they usually plaster mud around the entrance hole and boxes should therefore be opened with care.