This is an adult in nonbreeding (basic) plumage. These gulls range along the coasts of Southern South America but they bear a strong resemblance to the Black-headed Gull (Chroicocephalus ridibundus) of the Western Palearctic. In fact, the two were considered conspecific by Hellmayr. Brown-hooded has a slightly darker mantle, narrower dark subterminal band on its primaries, more dark at the bases of its outermost primaries, and distinct white primary tips cf. Black-headed.
Most authorities including Clements, IOC, HBW and Howard & Moore consider this species monotypic, but Howell & Dunn (2007) classify birds from northern Argentina to southern Brazil as the nominate race while C. m. glaucodes with more white in its wing-tips is found throughout the rest of the bird's range. Under this classification, these birds are C. m. glaucodes. In 2007 SACC moved this species along with other small hooded gulls from Larus into the genus Chroicocephalus in an effort to keep Larus monophyletic. This classification has been adopted by most authors with the notable exception of HBW and Birdlife International.
Canon PowerShot SX50 HS.
References:
Brown-hooded Gull (Chroicocephalus maculipennis), In Neotropical Birds Online (, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA. retrieved from Neotropical Birds Online: https://neotropical.birds.cornell.edu/Species-Account/nb/species/brhgul2
Burger, J., Gochfeld, M. & Garcia, E.F.J. (2018). Brown-hooded Gull (Larus maculipennis). 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/53994 on 3 May 2018).
Howell, S.N.G. and Dunn, J. (2007) A Reference Guide to the Gulls of the Americas. Peterson Field Guides, New York.