Nuthatches nest in existing holes, usually old woodpecker cavities, and cannot excavate their own. Entrances larger than needed admit starlings and other competitors, so the bird reduces the aperture with mud, applying it in beakfuls and letting it set hard. Cracks and crevices around the cavity are plastered too, further sealing the chamber.
The reduction is precise enough that the finished hole is close to the minimum diameter the adult can pass through. Where mud is scarce the birds use other pliable material, and the hardened plaster persists well beyond the breeding season.
The other habit that marks the species is its ability to descend a trunk head-first, which few other birds manage. It does so using one foot braced above and one below rather than propping on the tail like a woodpecker or treecreeper, which is why the nuthatch can work down a trunk and spot food that upward-climbing birds pass over.