Gress, Carol, Jacob W Byers, Emily Jennings, and Craig Ely. 2013. “Behaviour of Wintering Tundra Swans Cygnus Columbianus Columbianus at the Eel River Delta and Humboldt Bay, California, USA”. Wildfowl, April, 38-51. https://tidsskrift.dk/Wildfowl/article/view/153540.