Trumpeter Swan (Cygnus buccinator)
Very unusual individual with that yellow on the bill. Was it a captive specimen at a zoo? If yes, I suspect it is probably a hybrid with Whooper Swan http://davesgarden.com/guides/birdfiles/go/873/ (which has even more yellow on the bill).
Thank-you for pointing that out Resin, I think it is a whooper swan or at least a hybrid. It was at the W. K. Kellogg Bird Sanctuary; where some of the birds are captive but most come and go as they please. So, it's hard for me to know whether it was captive or not.
How do I go about removing this image so I can keep BF accurate, because I only just joined in on that guide last night?
Thanks again for pointing it out.
Upon further investigation, do you think it could be a tundra swan? That's another swan they have at the sanctuary and the photos I find show it with varying degrees of yellow on the bills. Or maybe a young whooper? Do their bill colors develop with age?
Too large and long-billed for a Tundra Swan, they have a more goose-shaped head with a slightly shorter bill. I'd say it is a Whooper × Trumpeter Swan hybrid, this hybrid is known in captivity (it can't happen in the wild as the two are on different continents!).
The bill colours don't change with age, other than when the yellow first develops in young birds about a year old; once they get the yellow, the pattern of it remains the same for life (it can even be used to recognise individuals)
Probably a good idea to leave the photo in, as an educational thread - anyone sufficiently interested to notice the difference in the bill pattern will click on the photo and read this discussion.