Grey wagtail, Motacilla cinerea.
The species looks similar to the yellow wagtail but has the yellow on its underside restricted to the throat and vent. The grey is associated with its back and head. Breeding males have a black throat. They are usually seen on open marshy ground or meadows capturing ground insects. Like other wagtails, they frequently wag their tail. On the RSPB red list due to its declining numbers. A pair were nesting regularly for many years at the Highgate end of Parkland Walk where a leaking water main provided all year round damp muddy ground. Another pair (and a fledgling) were spotted feeding at another mains leak in Muswell Hill in 2017.
Photo: © Simon Olley