(Eurasian) Treecreeper
Certhia familiaris

From a distance, these small birds look like a mouse running up a tree, but if you can get a closer look, you’ll probably see one of these. They hop up tree trunks looking for caterpillars in the bark and then fly down to the base of another tree to start climbing again. On the continent there is an almost identical relative called the Short-Toed treecreeper, but the differences are very small (shorter toes, longer beak and a slight variation in plumage). This is a juvenile not long fledged, as can be seen from the slight yellow patch in the corner of the mouth and the brighter colours in the plumage.
 

1: Kelvingrove Park, Glasgow, 30/05/2005.