Ring-billed Gulls breed in two more or less distinct populations: a western group centred on the prairies and breeding south to Colorado and Wyoming, and an eastern one concentrated in the Great Lakes but extending down the St. Lawrence River to the Gulf of St. Lawrence, with outliers in James Bay, northeastern Newfoundland, and southern Labrador (Blokpoel and Tessier 1986). The western population shares some of the Pacific coast wintering range of California Gulls, but also winters inland in the Mississippi valley and northern Mexico. Eastern breeders winter on the Atlantic coast, especially in Florida and on the coast of the Gulf of Mexico, straggling to the West Indies. Until recently, these two populations were separated by a clear "migratory divide" west of Lake Superior, but new colonies have been discovered at Lake of the Woods in the centre of this divide (Blokpoel and Tessier 1986).
Most banding operations have concentrated on chicks. In the eastern population, 70% of all bands have been put on in Ontario. Encounters have been analyzed by Southern (1974), Blokpoel and Haymes (1979), and Blokpoel and Tessier (1986), who also documented the dramatic increase in this species since the late 1940s. Encounters of western birds were analyzed by Vermeer (1970a). Blokpoel and Haymes (1979; see also Blokpoel and Courtney 1982) used band encounters to determine the origins of birds establishing a new colony near Toronto and developed a "contribution index," which showed that the largest and closest colonies contributed most recruits to the new colony.
From May to September, most encounters reported for eastern birds were from the Great Lakes themselves. Dispersal begins in July and is widespread through September, with a number of encounters along the St. Lawrence seaway to the Gulf of St. Lawrence and a few on the U.S. Atlantic coast. There are a few xceptional encounters in Florida in July and August, but the main movement south is from October to December, chiefly along a corridor from lakes Erie and Ontario southeast to the Atlantic coast around Chesapeake Bay. In January and February, over half the encounters are in Florida (record 1), where many birds evidently winter. Northward migration begins in late February, retracing the southward route; by May, very few remain in Florida, and three-quarters of encounters are within the Great Lakes region. A decline in the proportion of encounters from Florida in November–February since the 1960s suggests that the centre of gravity of the wintering area may have shifted northwards. The distribution of 1-year-old birds during the breeding season is unclear; they are encountered within the Great Lakes region then, but are rarely seen at the breeding colonies.
Although most of the population breeding in eastern Canada winters no farther south than Florida, there are significant numbers of encounters in the Caribbean, as well as one in Brazil (record 2). The latter is by far the most southerly record of the species (Brewer and Salvadori 1978); at that time, Ring-billed Gulls had not been recorded anywhere in continental South America (Meyer de Schauensee 1970). Thirty-two birds banded as nestlings in Ontario (mainly) and Quebec were encountered as follows: Bahamas, 11; Cuba, 7; Hispaniola, 6 (record 3); Jamaica, 3: Puerto Rico, 1; Guadeloupe, 2 (record 4); Antigua, 1; St. Lucia, 1 (record 5). The species is considered a "vagrant" in the Lesser Antilles (Bond 1985). Small numbers of eastern birds were encountered in Mexico, mostly on the Caribbean coast and in Yucatán (record 6); however, there are single encounters on the Pacific coast of Mexico (record 7) and in El Salvador (record 8).
The western population of Ring-billed Gulls winters especially in California (record 9). In contrast to the eastern population, birds banded in the Prairie provinces showed no encounters in the Caribbean basin; although many were encountered in Mexico, they were all north of 18°N and were predominantly from Pacific coast states, with about 30 encounters inland (records 10–12), including one in the Federal District. In spring, some birds take a direct, inland route to the breeding grounds, rather than retracing the ll migration, as California Gulls do (Vermeer 1970a). There is apparently very little interchange between western and eastern populations (Ryder et al. 1983), although a few encounters suggest dispersal between them (records 13–15).
Ring-billed Gulls occur regularly in Europe, with records from Spitsbergen, Norway, to Morocco and the Canary Islands; they occur annually in Britain (Perrins and Snow 1998). These records have become much more numerous in recent decades, perhaps because of the enormous increase in the North American population (Blokpoel and Tessier 1986); however, record 16 predates this increase and suggests that transatlantic stragglers may not be a new phenomenon. Record 17, which was fully authenticated, was at the time only the second record of the Ring-billed Gull in Spain and in mainland Europe. There has now been a second encounter in Iberia (record 18).
An Ontario-banded bird (record 19) encountered in 1981 at Presqu'île Provincial Park in Ontario holds the longevity record for this species of 25 years and 1 month (Clapp et al. 1982). The encounter code of record 20, encountered 29 years after banding, does not preclude the bird having been long dead and so does not supersede this record. There are several other >25-year-old encounters, but none was recently dead when found.