Tweets,

The state of Washington alone has over 40 members of the American
Pheasant and Waterfowl Society, a number of whom breed Mandarin Ducks (and
many other waterfowl species). Some breeders allow their birds to nest
under more-or-less natural conditions. In such cases, unless they capture
and pinion the ducklings in a timely fashion, the immatures could fly away
and grow up to become essentially wild birds. Breeders also may allow
their adults to remain full-winged and to remain outdoors under netting,
which may or may not be secure.

Mandarins are common enough in private captivity that it's almost
inevitable that some may get away and be seen in the wild.

One of the largest private aviaries in this region has an informative
website:

http://www.greatnorthern.net/~dye/farm.html

I don't mean to suggest, however, that these folks may be the source of the
Mandarins (or any other wildfowl) being seen out there.

Cheers,
------------------------------------------------------------
P W (Bill) Smith
Grays Harbor, Washington USA
birdman at techline.com