Take Mr. Long Beach for instance. He appears bigger than the Ring-bills and Herring Gulls and quite assertive when in their midst. I found it humorous to watch him - his lighter coloration setting him out from the other birds. As was pointed out to me by fellow birder, Donna Caporaso, he would call out this squeaky sound and toss his head up as he did so.
They are a great bird to work with in practicing birds in flight imaging as they are fairly easy to focus on and track. As I took pictures, it was amazing to see how they used their wings to maneuver.
"Gulls of the Americas," by Steve NG Howell and Jon Dunn
"Gulls of North America, Europe, and Asia," by Klaus Malling Olsen and Hans Larsson.
Of special note, gulls are a protected group of birds throughout all of the United States. For more information about the Migratory Birds Treaty Act, see the US Fish and Wildlife site: http://www.fws.gov/migratorybirds/RegulationsPolicies/mbta/mbtintro.html