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The following discussion on how to distingush the two swans from one another may be of interest. It is by Joe Gryzbowski, and it appeared  on the Oklahoma listserv. 

Identifying adult Trumpeter and Tundra swans can be tricky.  Both can have all black bills.  A bird with yellow in the lores will be a Tundra, but there is much variation in the amount of yellow, and it might not be a big enough area to detect at a distance.  Birds without yellow cannot be distinguished on that character, as many adult Tundras will have no yellow in the lores.  Of 8 Tundras I had last year in a group, I think only two or three showed yellow, one a pretty small area.
Most other characters given in the field guides are quite subtle.  The slope of the forhead and bill is straight in Trumpeters, and slightly dished in Tundras.  Similarly, the back edge of the black facial skin is straight(er) in Trumpeters, curved slightly forward in Tundras.   But look at pictures and see how many times you can tell what these are.  Its tricky.  Also, the eye is set further back and more separate from the angle formed by the back edge of the bill/lores and forehead in Tundras than in Trumpeters.  With experience....maybe; At a distance...forget it.

I like things I can measure rather than interpret.  When they are together in a mixed group, it is easy; Trumpeters are bigger by a fair amount (but need to have them next to each other).  

By themselves, a possible character is relative bill length.  The length of the bill from its tip to bottom edge (at gape) goes into the head (with bill) about twice in Tundras, more than twice in Trumpeters (about 2.3-2.5)--that is the bill is about half the head length in a line from tip of bill to back of head in Tundras, less than half in Trumpeters.  Trumpeters have a (proportionately) longer-billed appearance, but mostly because the snoz sticks out further, (although it is [proportionately] longer in Trumpeters if measuring from tip of bill along the culmen up to the forehead--which includes the black lores).  

Another good way is to compare lengths with that of ducks.  If you can take a Mallard and use its length straightened out from tip of bill to tail, it will go into a Tundra Swan length (also tip of bill to tail straightened out) twice, easily more than twice for a Trumpeter.  You can use other duck species. A Gadwall goes into a Trumpeter about 3X (clearly <3 in a Tundra), A Ring-necked Duck about 3X in a Tundra, more than 3X into a Trumpeter.  Mallard is the easier species with which to do this because it is bigger.  But again, they need to be next to or very near each other, and scoping can distort size comparisons of birds in front of or behind each other.  AND this may take some practice with pictures. (Its may be hard to tell how two people will do this--I started with string on pictures, or pencils and my thumbnail).

On young, Trumpeters are usually dirtier-plumaged than Tundras, but they both pale as the winter season progresses.  Bills and size still work though. 

This is my two cents, after struggling with these ID's myself.

It is quite possible that the group of swans observed in Custer County had both Tundras and Trumpeters in them.  Last year, I saw up to nine swans in Roger Mills County, 8 Tundra and one Trumpeter.  The Trumpeter was bigger, and obvious when looking for size comparisons between birds next to each other, but not just sticking out like a sore thumb.

Bob Fisher
Independence, Missouri
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