It's a Hawaiian goose, or nene, gosling.
Hawaiian goose with gosling
Branta sandvicensis (Vigors, 1833)
Anatidae; Anseriformes; Aves; Chordata
London Wetland Centre
The hint in the photo was the half-webbed, or semipalmate, feet. The goslings are growing fast and are as of yesterday almost the same size as their parents.