This photo shows the typical Eurasian subspecies with extensive yellow at the base of the bill, sometimes called Bewick's Swan (C. c. bewickii). It was formerly considered a separate species from the nominate North American race, often called "Whistling Swan." I was curious why the North American bird got to be the nominate subspecies. European birds described by Linnaeus or others typically have priority while North American birds were normally described later. However Linnaeus failed to recognize Bewick's Swan only describing the similar but larger Whooper Swan under the name Anas cygnus in his 10th edition of "Systema Naturae" in 1758 the starting point of modern zoological nomenclature. It was not until 1830 that Yarrell recognized Bewick's was different from Whooper, but by that time the Whistling Swan had already been described by Ord (1815). Thus Whistling Swan's name gets priority and nominate status according to the ICZN rules. Canon PowerShot SX60
References:
Carboneras, C. & Kirwan, G.M. (2019). Tundra Swan (Cygnus columbianus). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from https://www.hbw.com/node/52808 on 1 April 2019).
Limpert, R. J. and S. L. Earnst (1994). Tundra Swan (Cygnus columbianus), version 2.0. In The Birds of North America (A. F. Poole and F. B. Gill, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bna.89