Willow Warbler - Phylloscopus trochilus
A small species of summer migrant, usually seen on willow trees, where they have a very simple call that sounds like a mouse squeaking "who-wheeee".
These birds are very well camouflaged, and are more likely to be heard than seen. They can be confused with other warblers on sight, so their song is the most reliable way of identification.
Willow Warblers feed on willow pests.