This photo shows two different subspecies of Tundra Swan. The birds with extensive yellow at the base of the bill are Bewick's Swan (C. c. bewickii), the subspecies expected in Europe and Asia. Unexpected was the slightly smaller bird with much reduced yellow confined to its lores. This subspecies is called Whistling Swan (C. c. columbianus) and normally ranges through North America. These two were formerly regarded as separate species. Bewick's is said to average slightly smaller, but in this photo they look bigger perhaps raising the specter of C. c. jankowskyi a supposed large race of Bewick's from East Asia generally regarded is invalid.
I was curious why the North American bird got to be the nominate subspecies. European birds typically have priority because North American birds were 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