Great Bittern Botaurus stellaris is the name of this bird.
I was with Mike Kilburn - consummate birder - in Mai Po at the end of last year when he pointed out this typically very secretive bird. It flew about a bit, and he explained that you normally only see single birds in flight, and that they like the extensive reedbeds of Mai Po where their plumage blends well with the reeds.
click on the images below for a larger view.
This lovely bird was actually joined by another - a rare sight that had Mike smiling from ear to ear.
Mike also explained this strange behaviour they call "star gazing" were the bird points its bill straight up. Probably to help it blend in even more.
Here it is coming into land in the reed beds.