Blackcap (Sylvia atricapilla)
The Blackcap is a migrant warbler that breeds on the Common during the Spring/Summer period after arriving from its wintering grounds in the Mediterranean and West Africa. They are a small bird (similar size to a chaffinch) which is mostly grey in colour except for the colour of its cap which is black in males and chestnut brown in females. The Blackcap has a beautiful fluting warbling song which you can hear at the below link: Blackcap Song
Over recent years there has been an increase in numbers of overwintering Blackcaps in the UK that visit from a different population from Germany and Northeast Europe. It is entirely possible that you may now encounter one during the Winter months.