Feathers serve multiple functions. They help shed water, retain heat, and enable flight. However, the feathers must be kept clean and properly aligned to successfully achieve these goals. They also have another function.
Trumpeter Swans tend to breed in Central Alaska and in-land in Western Canada and the United States. They tend to winter along the Pacific Coast from Washington State and north to Alaska.
I have never taken any photos of Trumpeter Swans on Union Bay after January. I am not positive when they head north, but if you want to see them, I would not delay your visit.
In the West, their smaller cousins, the Tundra Swans, tend to leap-frog the Trumpeters during migration. They winter along the Pacific Coast in Washington State and points south into California while breeding along the northern coasts of Alaska and Canada.
Mitchell, C. D. and M. W. Eichholz (2020). Trumpeter Swan (Cygnus buccinator), version 1.0. In Birds of the World (P. G. Rodewald, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.truswa.01
(By the way, my friend Tom Brown pointed out that the application named 'Wildflower Search' is extremely helpful. Click on the highlighted link to see for yourself.