Treecreeper Certhia familiaris

The Treecreeper, also called the Common or Eurasian Treecreeper outside of the UK. British Isles subspecies illustrated. Relatively common but most elusive small bird which spends much of its time climbing tree trunks in search of insects (it tends to climb up, then fly to the base of another tree to climb up; a small bird walking down a tree is likely to be a nuthatch, which rarely climbs up). It is well camouflaged and runs round the far side of the trunk out of sight from approaching people/predators.
The Treecreeper has a thin, curved bill, a white chin and front and pointed tail feathers.