(Common) Pochard
Aythya ferina

Mostly a winter visitor to Britain, although some breed here every year. They can dive deeply to find their food, but can also upend and dabble. The first two are females, the last two are males.
 

  

  

1-2: Black Swan Lake, Dinton Pastures, 02/01/2007.
3-4: Sandford Lake, Dinton Pastures, 06/03/2006.