こんな寒くても、東京に鴨がたくさんいます。新宿御苑で。
Ducks are still a common sight in Tokyo, wherever there’s clean water. Don’t they get cold in winter? Also in Shinjuku Gyoen.
Ducks keep city dwellers company at ponds and rivers
Tokyo Green Space
an unexpected view of the world's largest city
Tokyo Green Space 
I did a quick research on those ducks because they look different from regular カルガモ. Those with black heads, white bellies, and yellow eyes, are キンクロハジロ (Aythya fuligula). They come to Tokyo during winter and go away in April. http://en.wikipedia.org/wiki/Tufted_Duck