Northern California numbers vary from year to year. Prior to the 1950's this species was very rare in Northern California, but it is now regular north to Oregon sometimes even reaching Washington State and British Columbia in invasion years. Surges are usually correlated with warm-water El Nino events. These large terns breed mostly in the Sea of Cortez, particularly Isla Rasa, and at a few colonies in Southern California. After nesting they disperse along the coast. Controversial records of this species breeding in Western Europe have recently been confirmed by genetic testing. This species was formerly included in the genus "Sterna" and thought to be closely related to the Royal Tern (T.maximus). However recent molecular research places it as sister to the Sandwich Tern (T. sandvicensis). Canon SX60 HS PowerShot.
References:
Bridge, E.S., Jones, A.W. & Baker, A.J. (2005) A phylogenetic framework for the terns (Sternini) inferred from mtDNA sequences: implications for taxonomy and plumage evolution. Mol. Phyl. & Evol. 35(2): 459–469.
Burness, Gary P., Kara Lefevre and Charles T. Collins.(1999).Elegant Tern (Thalasseus elegans), The Birds of North America (P. G. Rodewald, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America: https://birdsna.org/Species-Account/bna/species/eleter1
Dufour P, Pons J-M, Collinson J M, Gernigon J, Dies J I, Sourrouille P & Crochet P-A. 2016. Multilocus barcoding confirms the occurrence of Elegant Terns in Western Europe. Journal of Ornithology.
Gochfeld, M., Burger, J., de Juana, E. & Garcia, E.F.J. (2017). Elegant Tern (Thalasseus elegans). 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 http://www.hbw.com/node/54014 on 22 August 2017).
Hudon, J. and A. H. Brush. 1990. Carotenoids produce flush in Elegant Tern plumage. Condor 92:798-801.