In fact, this is winter plumage. Juvenile plumage would have more heavily patterned upperparts and more orange on the underparts. Most juveniles have moulted into winter plumage by about October/November. It may be a female - the bill looks very long - but it is difficult to be sure.
It looks like one of the wintering population, with pale plumage, large in size and a long bill with an extensive pink base. Most birds at Mai Po at the moment are migrants and most are in breeding plumage. These are smaller than the wintering population, but there are still a few of the larger birds around.
