According to RSPB the most common bird in Britain is the Wren, with at least 8.5 million breeding areas in the UK.
They live in many different areas and their sing can be easily heard, although they tend to look for cover and are not as frequently spotted as the woodpigeons or the sparrows.
For more information about the Wren please visit the following link:
http://www.rspb.org.uk/wildlife/birdguide/families/wrens.aspx