Date: 3/11/17 8:51 pm From: Joel Geier <joel.geier...> Subject: [obol] Re: Swan ID
About using size to distinguish between Tundra Swans and Trumpeter Swans, others have hinted at this (as in Lars' mention of accipiters) but perhaps someone needs to state it more clearly:
Males of both species are usually noticeably larger than females. A large Tundra Swan male can be as big as a Trumpeter Swan female.
The two species will occur side-by-side and sometimes intermingled in daytime feeding situations, in the Willamette Valley. They sort themselves out again at the end of the day as family groups fly off together on their way to nocturnal roosts, but during the day they can mix it up a bit.
They differ in their feeding preferences so there is usually some segregation when you see them in grass fields (Tundras grazing on the higher ground and Trumpeters rooting around in the swales.). However when you're viewing a flock through a scope, foreshortening of the view field can make them look much closer to each other than they really area.
The best traits (other than voice if you're so lucky as to hear them vocalize) are the relation of the eye to the bare skin just ahead of it as Dan Heyerly described, and after that the angle that the bare skin/feathered cheek border makes with the base of the lower jaw.
The Trumpeter Swan Society swan ID sheet (which someone else mentioned) illustrates numerous other characteristics that can help in situations where the facial details are hard to ascertain (distance, heads tucked, etc.). Back shape, neck proportions, etc. I personally like to use multiple characteristics rather than rely on any one thing.