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Chroicocephalus maculipennis
Both Gull and Seagull are generic terms for all species of Gull. It is OK to use them as Common Names if that is all you know but in cases such as this they are non-specific.
Identification is achieved by using the scientific name which consist of Genus + species only, all higher ranking names being non-specific.
For more information on scientific naming there is an easy to understand article here http://animaldiversity.org/animal_names/...
This bird is in winter plumage and the second picture helped to clinch the ID.