What is "treecreeper"

n. Any of various small passerine birds of the family Certhiidae which live in woodlands throughout the Northern hemisphere.

The treecreepers are a family, Certhiidae, of small passerine birds, widespread in wooded regions of the Northern Hemisphere and sub-Saharan Africa. The family contains ten species in two genera, Certhia and Salpornis. Their plumage is dull-coloured, and as their name implies, they climb over the surface of trees in search of food.