I was particularly interested in seeing this enigmatic species. For many years its nest was unknown. Nests were ultimately discovered in the early 1970's in the inhospitable Atacama Desert of Northern Chile. Nesting season is November through January. The adults stay with the chicks during the day, but one adult heads for the coast at night to forage. Its white head is characteristic of breeding plumage. The brownish cast to its wing coverts and especially the lack of obvious white secondary tips indicate that this bird is second-cycle, presumably a non-breeder.
This species was formerly included in the genus Larus, but now moved to Leucophaeus. New genetic data indicated that inclusion of these gulls in Larus would make the genus paraphyletic. This new arrangement is followed by most authorities including Clements, IOC and Howard & Moore. However Birdlife International and HBW persist in placing these gulls in Larus. Canon PowerShot SX50 HS
References:
Burger, J., Gochfeld, M., Garcia, E.F.J. & Boesman, P. (2018). Grey Gull (Larus modestus). 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/53966 on 8 February 2018).
Gray Gull (Leucophaeus modestus), In Neotropical Birds Online (T. S. Schulenberg, editor). Cornell Lab of Ornithology, Ithaca, New York, USA. retrieved from Neotropical Birds Online: https://neotropical.birds.cornell.edu/Species-Account/nb/species/grygul
Howell, S.N.G. and Dunn, J. (2007) A Reference Guide to the Gulls of the Americas. Peterson Field Guides, New York.