As can be seen by these new images (hopefully) this is way to big to be a duck hybrid but must be a goose hybrid of some type the problem is what species ie - Ross's Snow Goose x Comb Duck as been mentioned on another site).
I saw at least 3 myself in different places on my visit (12/04/2014)
No, its a duck hybrid- Muscovy x Domestic Mallard type e.g. Aylesbury. The Muscovy drake lends the greater size plus you are only seeing it here in comparison wih wild-type mallards which are a lot smaller than domestics, so that it looks enormous. The bill colour and markings would be from the Muscovy parent also.
This cross is infertile and sometimes called a 'Mulard'.
Many thanks both never gave Muscovy duck a thought so Muscovy x Mallard hybrid it is Mulard goes down as Lifer in the Hybrid list for me again thank you.
I think its often due to cost- when the exotic waterfowl get killed or gradually die off they get replaced by some cheap Domestics which for most people still fit the bill- 'ducks'. I agree they don't make an attractive or interesting display though.