 It is variously treated as a separate species, as a subspecies of the American herring gull ("L. smithsonianus") or included with both the American herring gull and European herring gull in "L. argentatus".The Mongolian gull, "Larus mongolicus", has previously been regarded as a subspecies of the Caspian gull ("L. cachinnans") but is now sometimes lumped with the Vega gull.The Vega gull is similar to the herring gull but is slightly darker grey above.