The 'Common Gull' or 'Mew Gull' ('''Larus canus''') is a medium-sized
gull which breeds in northern
Asia, northern
Europe and northwestern
North America. It
migrates further south in winter.
[del Hoyo, J., et al., eds. (1998). ''Handbook of the Birds of the World'' 3: 621. Lynx Edicions ISBN 84-87334-20-2.]
Adults are 40-46 cm long, obviously smaller than the
Herring Gull, and slightly smaller than the
Ring-billed Gull, also differing from this in its shorter, more tapered bill with a more greenish shade of yellow, as well as being unmarked during the breeding season. The body is grey above and white below. The legs are greenish-yellow. In winter, the head is streaked grey, and the bill often has a poorly-defined blackish band near the tip (sometimes sufficiently obvious to cause confusion with Ring-billed Gull). They have black wingtips with large white "mirrors". Young birds have scaly black-brown upperparts and a neat wing pattern, and grey legs. They take two to three years to reach maturity. The call is a high-pitched "laughing" cry.
[Snow, D. W. & Perrins, C. M. (1998). ''The Birds of the Western Palearctic'' Concise Edition. OUP ISBN 0-19-854099-X.]
Taxonomy
There are four subspecies, two of them considered distinct species by some authorities:[Olsen, K. M., & Larsson, H. (2004). ''Gulls of Europe, Asia and North America''. Helm ISBN 0-7136-7087-8.]
★ '''Larus canus canus''' Linnaeus, 1758. 'Common Gull'. Europe and western Asia. Small; mantle medium grey (palest subspecies); wingtips with extensive black; iris dark. Wingspan 110-125 cm; weight 290-480 g.
★ '''Larus canus heinei''' Homeyer, 1853. Central northern Asia. Medium size; mantle dark grey (darkest subspecies); wingtips with extensive black; iris dark. Weight 315-550 g.
★ '''Larus canus kamschatschensis''' (Bonaparte, 1857); syn. ''L. kamschatschensis''. "Kamchatka Gull". Northeastern Asia. Large; mantle medium-dark grey; wingtips with extensive black; iris pale. Weight 394-586 g.
★ '''Larus canus brachyrhynchus''' Richardson, 1831; syn. ''L. brachyrhynchus''. 'Mew Gull' or "Short-billed Gull". Alaska and western Canada. Small; mantle medium-dark grey; wingtips with little black and much white; iris pale. Wingspan 96-102 cm; weight 320-550 g.
Ecology
Both Common and Mew Gulls breed colonially near water or in marshes, making a lined nest on the ground or in a small tree; colony size varies from two to 320 or more pairs. Usually three eggs are laid (sometimes just one or two); they hatch after 24-26 days, with the chicks fledging after a further 30-35 days. Like most gulls, they are omnivores and will scavenge as well as hunt small prey. The global population is estimated to be about one million pairs; they are most numerous in Europe, with over half (possibly as much as 80-90%) of the world population.[Hagemeijer, W. J. M., & Blair, M. J., eds. (1997). ''The EBCC Atlas of European Breeding Birds''. Poyser, London ISBN 0-85661-091-7.] By contrast, the Alaskan population is only about 10,000 pairs.
Vagrancy
The Common Gull occurs as a scarce winter visitor to coastal eastern Canada and as a vagrant to the northeastern USA,[1] and there is one recent record of Mew Gull in Europe on the Azores.[2]

Adult Mew Gull. Anchorage Coastal Wildlife Refuge, Alaska.

Common Gull in winter plumage. Tallinn, Estonia.
References
1. Sibley, D. (2000). ''The Sibley Guide to Birds''. ISBN 0-679-45122-6.
2. Alfrey, P., & Ahmad, M. (2007). Short-billed Gull on Terceira, Azores, in February-March 2003 and identification of the 'Mew Gull complex'. ''Dutch Birding'' 29 (4): 201-212.
★ Database entry includes justification for why this species is of least concern
★ "National Geographic" ''Field Guide to the Birds of North'' ''America'' ISBN 0-7922-6877-6
★ '' Seabirds, an Identification Guide'' by Peter Harrison, (1983) ISBN 0-7470-1410-8