This was the most common small gull in the harbor. Immatures and adults in winter plumage lack the brown hood. They are distinguished from the similar Bonaparte's Gull by their thin red based bill. Black-headed Gull ranges across Eurasia but is replaced in North America by the similar Bonaparte's Gull. Interestingly, it is the Bonaparte's Gull who actually has a black head in breeding plumage. The Black-headed Gull's head is decidedly brown in breeding plumage. 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. These gulls bear a strong resemblance to Brown-hooded Gull (Chroicocephalus maculipennis) which ranges along the coasts of Southern South America. 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.
Canon SX60 HS PowerShot.
References:
Burger, J., Gochfeld, M., Kirwan, G.M., Christie, D.A. & Garcia, E.F.J. (2018). Black-headed Gull (Larus ridibundus). 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/53995 on 18 September 2018).
Howell, S.N.G. and Dunn, J. (2007) A Reference Guide to the Gulls of the Americas. Peterson Field Guides, New York.