Small bird, loud voice

Went for a walk at lunchtime around the campus and lake. This very small bird, a wren I think, was shouting from the top of this small tree. I didn't manage to get an in-focus picture of it singing. 
I used the Merlin app while I was walking as the bird song was so loud and diverse. It picked up Robins, Chiffchaff, Blackcap, Wren, Blue tit, Song thrush, Egyptian goose, Canada goose, Blackbird, Moorhen and Great tit. 
Such a great environment to be able to escape into for a lunchtime stroll.