Hooded merganser (Mergus cucullatus). 

One pair and a trio of hoodies in the main garden. The male during breeding season has a large white crest with a black border, remainder of the head is black.  Neck and mantle black with greenish sheen, extensions to the mantle continue as two bars down each side. The females nest in raised nest boxes near to the waters edge and the egg clutch size is 8-12 glossy white eggs