Trumpeter or Tundra Swan?

So I saw my first swans of the season and I found myself in my annual state of swan confusion. What are the differences between between Tundra and Trumpeter Swans again?

Where I live, there is greater probability in a birder coming across a Tundra Swan (see eBird frequency chart). The first thing I look for is bill coloration. If I see some yellow on the lores, almost automatically I clinch the i.d. as a Tundra Swan. If it's all black, then it is probably a Trumpeter, but still could be a Tundra, so we need to look at additional characteristics...which characteristics I seem to forget from year to year and  have to re-learn.

Fortunately, there is some great information in the popular field guides and online to get me back up to speed. I converted a couple bad images to black and white and used some yellow and orange highlights to emphasis certain traits and relearned that I had been seeing Trumpeter Swans. Here's my reasoning...

 On Trumpeter Swans, where the forehead feathers come down to intersect with the bill, there is more of a point. Whereas on Tundra Swans, this area is less dramatic and rounded. The Trumpeter Swan also has a portion of flat-headedness as compared to the more rounded heads of Tundra Swans. The black bill tapers gradually as it approaches the eye of the Trumpeter, whereas a Tundra Swan eye has a more detached from the bill appearance. Though I struggle with this last tip in the field, it is helpful when reviewing photos...The Trumpeter has more of a straight line edge of the bill to the cheek whereas a Tundra Swan often shows more of a curve right at the gape.

For more great online resources to learn how to differentiate Trumpeter and Tundra Swans, you can visit the following:

Sibley Guides (and another Sibley diagram)

BirdChick