Rook

Corvus frugilegus

Appearance
Large black bird
Large grey-black bill, with bare grey-white skin around base
  
Listen to its call. 

I saw a large number of these birds in the fields along the river trail between Ware and Hertford. I determined they were Rooks for the following reasons: 1) "untidy" leg feathers, 2) bare grey-white skin at base of bill, and 3) Rooks are sociable birds while Carrion crows are usually found alone or in pairs.

Learn more about the Rook.