My favorite spot to watch for Tundra Swans, Cygnus columbianus may seem a little unusual; it's the NE Minneapolis neighborhood I live. Every year I usually hear and/ or see one flock go by, usually at night, and more often in the spring. It's one of my favorite signs marking the passage of the seasons. Seeing them at night gives them a somewhat ghostly or other worldly cast. The passing flock's calls sound wild and out of place in the city.
I don't think I've ever seen Tundra Swans on the ground or swimming in Minneapolis, just migrating past. But I can be a pretty lazy birder, maybe a swan or two that I have identified as a Trumpeter Swan, C. buccinator has actually been a Tundra Swan - they can be difficult to tell apart. If I see a flock of swans in the spring or fall I call them Tundra Swans (they usually give themselves away by their call). If I see a pair, anytime of the year, I call them Trumpeter Swans. I did a little research on the eBird site to see if this generalization is supported by the observations of others. I compared year-round sightings of the two swan species in Hennepin County. The data seems to support my generalization: Trumpeter Swans are seen more often, in smaller numbers, and throughout the year. Tundra Swan sightings are most likely in the spring and fall and in larger groups. Of course this is a simplification of the data presented at eBird, but it I think it gives some credibility to my lazy swan identification.