Top three are adults in breeding plumage. Bottom two are first cycle birds. Formerly lumped with Olrog's
Gull (L. atlanticus) under the name Band-tailed Gull, Belcher's Gull is endemic to the Humboldt Current
of Western South America. Olrog's has a stouter bill, blacker upperparts and more white in its secondaries. Olrog's
also has a white body than Belcher's. However the ranges of the two species do not overlap. Canon PowerShot SX50
HSReferences:
Burger, J., Gochfeld, M., Kirwan, G.M. & Garcia, E.F.J. (2018). Belcher’s Gull (Larus belcheri). 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/53963 on 21 January 2018).
Howell, S.N.G. & Dunn, J.L. (2007) Gulls of the Americas. Houghton Mifflin, Boston.
Howell, S.N.G., Lewington, I. & Russell, W. (2014) Rare Birds of North America. Princeton Univ. Press