Lesser Black-backed Gull immature (first year) was the opinion of the folks at the Tropical Audubon birdboard. Alex Harper elaborated as follows (thank you!):
Indeed a Lesser Black-backed Gull. It looks to be a first cycle bird more: fine brown streaking on the forehead, some white mottling on back, no defined mantle color, pink legs and a black bill. The somewhat rounded head helps separate it from the "blockier headed" Herring and the slim bill and body helps dismiss the much bulkier Great Black-backed Gull.
Africa (entire continent rather than south of Sahara)
AN
Antarctica
AO
Atlantic Ocean
AU
Australasia (Wallacea (Indonesian islands east of Wallace's line), New Guinea and its islands, Australia, New Zealand and its subantarctic islands, the Solomons, New Caledonia, and Vanuatu)
PAL
Eurasia (Europe, Asia from the Middle East through central Asia north of the Himalayas, Siberia and northern China to Japan)
IO
Indian Ocean
LA
Latin America (Middle and South America)
MA
Middle America (Mexico through Panama)
NA
North America (includes the Caribbean)
NO
Northern oceans
OR
Oriental Region (South Asia from Pakistan to Taiwan, plus Southeast Asia, the Philippines, and Greater Sundas)