In addition, the photographs in The Audubon Society Master Guide to Birding , although not showing all plumages, provide a good supplement to the NGS guide. This is an excellent attribute for assigning a perched bird to a size category.
Larger gulls have relatively broader wings than smaller ones, apparent as they fly overhead.
As in most groups of birds, the larger species have slower wing beats than the smaller ones, again possibly of value when attempting to place a distant bird in a size category. Flight may be quite different under different wind conditions, so comparisons are safer than single birds. The evenly colored upperparts in a gull, including the back and most of the wing surface, is called the mantle. Backlighting will make any underwing look darker. Birds with darker underwings seem to show a more prominent white rear edge on the wing, as darker underwing colors contrast more than lighter ones with the white tips to the secondaries that all species have.