Little Grebe

Tachybaptus ruficollis

Appearance:
Small water bird
Short, pointed black bill
Fluffy, whitish rear end 
Breeding plumage: (above)
   Dark back
   Rufous-coloured cheeks and sides of the neck
   Yellow gap at base of bill
Non-breeding plumage:
   Brownish-grey back
   Pale buff on lower quarters
Juvenile
   Yellow bill with a small black tip
   Black and white streaks on cheeks and sides of the neck

I saw this little fellow while strolling around Regent's Park yesterday.  It was a gorgeous day and everyone, including the waterfowl, were enjoying the sunshine. 

Learning more about the Little Grebe.