Range Bonaparte's Gull breeds from western and central Alaska east to James Bay and south to south-central British Columbia, central Alberta, Saskatchewan, and central Ontario. Nonbreeding birds occur in summer along the Pacific coast south to California, the Atlantic coast south to New England, and on the Great Lakes. Winters on the Pacific coast from southern British Columbia to Mexico and in the east from the Great Lakes south through the Mississippi River to the Gulf of Mexico and from New England to Greater Antilles.

Status On the coast, very abundant spring and autumn migrant, rare to uncommon in summer. In winter, casual on the northern mainland coast and rare on the west coast of Vancouver Island; rare to very common, at times very abundant, in the southern Strait of Georgia and Juan de Fuca Strait. In the interior, common to abundant spring and autumn migrant; accidental in winter. Common summer visitant to the central and northern interior, where it is a widespread breeder.

Status Change No change.

Nonbreeding The Bonaparte's Gull occurs throughout the province from sea level to at least 1,700 m elevation. On the coast, it frequents bays, harbours, lagoons, estuaries, areas of tidal convergence and upwelling, passages and narrows, as well as large rivers and sewage lagoons. Wherever food is abundant, concentrations of tens of thousands can occur. Kelp beds, offshore islets, and log booms are favourite roosting sites. In the interior, large lakes and rivers, estuaries, marshes, and ponds are preferred habitats. In spring, the Bonaparte's Gull is most numerous on the coast, especially in the vicinity of Pacific herring spawning areas in the Juan de Fuca Strait and Strait of Georgia, where flocks commonly number in the thousands. The northward movement usually begins in late March and early April. Numbers build throughout the month, peak in late April and early May, and begin to dwindle by mid-May. In late May, non-breeders begin to appear in numbers. The latter movement carries on into June. Small but variable numbers remain through the summer. The autumn movement begins in late July and continues well into November, when spectacular numbers accumulate on the south coast in areas such as Discovery Passage, Active Pass, Haro Strait, and Juan de Fuca Strait. Again, adults precede young and subadults. In winter, flocks of up to 6,000 gulls occur in inner coastal areas of the Strait of Georgia, especially the southern Gulf Islands, and Juan de Fuca Strait. There is a single winter record from the northern mainland coast near Kitimat. In the interior, spring migrants appear during the third week of April in southern areas and numbers peak there in very late April and early May. In south-central areas (e.g. Williams Lake), migrants appear consistently during the first week of May. In northern areas (e.g. Fort Nelson) migrants appear late in the first week of May and peak during the second and third weeks of May. Nonbreeding birds are widely scattered in summer, and numbers may reach 300 birds locally. Autumn migration is evident after mid-July and continues into early November, although most birds have departed by mid-October. Adults precede immatures. There is 1 winter record from Sorrento. During the warm days of summer and early autumn, flocks of hundreds of Bonaparte's Gulls can be seen capturing large flying insects (e.g. termites, field ants) in the air over forests, beaches, and large lakes. Extreme dates for the interior are 6 April and 17 November. Regional dates are: Fort St. John, 2 May and 31 October; Williams Lake, 29 April and 15 October; Okanagan, 24 April and 11 November.

Breeding Nests Eggs Young The Bonaparte's Gull breeds in the interior from Bridge Lake through the Fraser Plateau and Fraser Basin regions, to the southern Peace Lowlands and across far northern British Columbia. There are no records for the north-central interior, but, the inaccessibility of much of the area prevents a clearer picture of the Bonaparte's Gull's breeding distribution there. For Vancouver Island, there is one unconfirmed breeding record: an -I adult was seen "probably feeding young" at a nest on Pye Lake, 8 July 1977. Nest contents were not actually observed. The Bonaparte's Gull breeds in the vicinity of lakes, ponds, muskegs, and alpine marshes in coniferous woodland from 305 to 1,318 m elevation. It prefers small wooded islands for nesting. It nests singly or in loose colonies. The number of nesting birds in the province is not known. Bridge Lake, in the Cariboo Plateau, is the most accessible breeding locality and accounts for 27% of all the breeding records.

Nests:  Most nests (62%; n=52) were situated on small, forested islands. Others were found on lake margins, alpine ponds, and marshes in fresh water lakes. One nest was located 60 m from water. Ninety-two percent of nests were in trees, including spruces (81%), Douglas-fir (6%) and western hemlock (3%). Nest heights (n=41) ranged from 1.5 to 17 m, with 58% between 4 and 6 m. Tree nests were positioned on branches up to 2 m from the trunk. Four ground nests were found on top of mounds of marsh vegetation. Nests were composed of small twigs, mosses, lichens, grasses, sedges, and other marsh vegetation. They were loose to compact structures. Three nests ranged from 23 to 33 cm in outside diameter and 8 to 13 cm in height. Eggs: Dates for 34 clutches ranged from 20 May to 4 July with 53% recorded between 2 and 19 June. Clutch size ranged from 1 to 3 eggs (1E-1, 2E-12, 3E-21), with 62% having 3 eggs. Incubation period is 23 to 24 days (Godfrey 1986). Young: Dates for 59 broods ranged from 15 June to 2 August, with 54% recorded between 26 June and 12 July. Calculated dates indicate that nestlings could still be found in mid-August. Fledged young were recorded as early as 26 June at Bridge Lake in the Cariboo. Brood size ranged from 1 to 3 young (1Y-6, 2Y-49, 3Y-4), with 83% having 2 young.

Remarks The feeding activity of fish-eating birds and their impact on salmonid fry released from a hatchery on eastern Vancouver Island was studied between 1979 and 1981 (Mace 1983). Significant differences were noted between 2 major predators—the Bonaparte's and Glaucous-winged gulls. The migratory Bonaparte's Gull occurred in tight feeding aggregations with peak numbers corresponding closely to fish density, while the resident Glaucous-winged Gull was distributed in loose flocks and their numbers did not seem to bear any direct relationship to numbers of fishes. In addition the Bonaparte's Gull only fed below tidal heights of about 3 m while there was no detectable correlation between feeding activities and tidal height in the Glaucous-winged Gull. In 1980, 8 species of piscivorous birds captured an estimated 300,900 to 354,200 chinook fry which ranged between 10.4 to 12.2% of the total release. The most efficient predator was the Bonaparte's Gull which accounted for 8.3 to 9.9% of the total release.