Range Breeds from northern British Columbia, southeastern Yukon, and southwestern Mackenzie across Canada to southern Quebec, southern Newfoundland, and the Maritime provinces; south through mountainous and forested regions to Baja California, New Mexico, Texas, Tennessee, Georgia, and Guatemala. Winters mainly from southern California and the Gulf coast south through Central America to Costa Rica.

Status On the coast, uncommon to fairly common migrant and summer visitant in the Georgia Depression Ecoprovince; rare to uncommon on the southern mainland and very rare on the northern mainland of the Coast and Mountains Ecoprovince; casual on Western Vancouver Island; absent from the Queen Charlotte Islands. In the interior, uncommon to fairly common migrant and summer visitant in the Southern Interior, Southern Interior Mountains, and Central Interior ecoprovinces; rare to uncommon in the Sub-Boreal Interior and Boreal Plains ecoprovinces; rare in the Taiga Plains Ecoprovince and casual in the Northern Boreal Mountains Ecoprovince. Breeds.

Status Change No change.

Nonbreeding The Cassin's Vireo has a widespread distribution across most of southern and central British Columbia. It occurs regularly from southeastern Vancouver Island east to the Rocky Mountain Trench and north to the Cariboo and eastern Chilcotin. North of Williams Lake, the Cassin's Vireo is more sparsely distributed, although it is relatively common in the east-central interior north to the Peace Lowland. It is sparsely distributed in the western Chilcotin and Taiga Plains, and very scarce in the Northern Boreal Mountains. The northernmost records are from near Kwokullie Lake, Coal River, and Dease River. On the central and northern mainland coast, it occurs only in small numbers from the Bella Coola valley north to Stewart, on estuaries and along major river valleys that cut through the mountains from the interior. It is absent from the Queen Charlotte Islands and nearly all other mainland coastal islands north of Vancouver Island. The Cassin's Vireo has been recorded at elevations from near sea level to 500 m on the coast, and from 300 to 1,200 m in the interior. It occurs mainly in valley bottoms and on plateaus and lower mountain slopes. For a detailed discussion of habitat, see BREEDING. Early migrants may arrive in the Georgia Depression as early as the last week of March; however, birds normally arrive around the first week of April, with the main movement between the third week of April and early May. In the Southern Interior and Southern Interior Mountains, early migrants may arrive in early April, but the main movement begins in late April and peaks quickly in early to mid-May. In the Central Interior and southern Sub Boreal Interior, early spring migrants may arrive in late April and the movement peaks in mid-May. In the Boreal Plains and Taiga Plains, spring migrants arrive through Alberta beginning in mid-May; the migration peaks quickly in late May and finishes in early June. In the northeast, autumn migration appears to begin as soon as the young are independent, probably around late July. The main movement occurs during the last 2 weeks of August, with a few stragglers remaining into early September. In the interior south of Prince George, the autumn migration is remarkably synchronous, beginning in August and ending by mid-September. Only in the Southern Interior is there a record of a straggler as late as the second week of October. On the coast, the main autumn movement begins in late August and peaks in the first 2 weeks of September; most birds have gone by the third week of September. On the coast, the Cassin's Vireo has been recorded from 23 March to 5 October; in the interior, it has been recorded from 25 April to 27 September.

Breeding Nests Eggs Nest Success The Cassin's Vireo breeds throughout much of southern British Columbia from southeastern Vancouver Island east to the Rocky Mountains and north to the Cariboo and Chilcotin areas. Further north, breeding records are scarce, but nesting has been confirmed in the Sub-Boreal Interior, in the Peace Lowland, and near the Yukon border. The Cassin's Vireo is likely a more widespread breeder in the northern interior than is currently suggested by the data. On the coast, breeding has not been confirmed on Western Vancouver Island or on the north coast. The Cassin's Vireo reaches its highest numbers in summer in the Southern Interior. An analysis of Breeding Bird Surveys for the period 1968 through 1993 shows that the mean number of birds on interior routes has increased at an average annual rate of 5%; analysis of coastal routes for the same period could not detect a net change in numbers. On the coast, the Cassin's Vireo has been recorded breeding at elevations from near sea level to 500 m; in the interior, it breeds at elevations from 340 to 1,200 m. This vireo inhabits a variety of open forest types. On southeastern Vancouver Island and the Gulf Islands, the Cassin's Vireo is most numerous in mixed forests on dry, rocky sites with open stands of Douglas-fir, Carry oak, and arbutus. Elsewhere on the coast, it is found along the edges of mixed woods where bigleaf maple, red alder, black cottonwood, and western flowering dogwood occur with scattered Douglas-fir and western redcedar. It is also found along powerline rights-of-way, field margins, rural roads, and similar clearings, as well as in middle-aged, regenerating forests on dry sites. In the southern portions of the interior, the Cassin's Vireo, unlike the Warbling and Red-eyed vireos, occurs frequently in both coniferous and deciduous woodlands. It is a characteristic summer bird of western larch, Douglas-fir, ponderosa pine, and lodgepole pine forests on lower mountain slopes. In the Central Interior, Sub-Boreal Interior, and Peace Lowland, it occurs mainly in mixed forests of trembling aspen, willow, lodgepole pine, and spruce. In northern coniferous forests, it occurs more frequently in forest stands near water. Most breeding habitats (n = 30) have been described as mixed (47%), coniferous (30%), or deciduous (10%) forest. Rural, suburban, and other human-influenced habitats were also used (13%). More specifically, the Cassin's Vireo has been found nesting in younger second-growth forest (33%; n = 21), mature forest (24%), and riparian forest (19%); adjacent to roadsides; and in backyard gardens and suburban parks. On the coast, breeding can begin about a month earlier than in the interior. The Cassin's Vireo has been recorded breeding on the coast from 20 April (calculated) to 29 July; in the interior, it has been recorded breeding from 23 May (calculated) to 22 July.

Nests:  Most nests were found in trees (94%; n = 47); 3 nests were in shrubs. Unlike in the case of the Red-eyed and Warbling vireos, conifers were often used for nest sites (43%), including Douglas-fir, western redcedar, grand fir, spruce, and ponderosa pine. Deciduous trees used for nest sites (40%) included red alder, maple, willow, arbutus, birch, Carry oak, and horse-chestnut. One nest was in a dead snag. Almost all nests were placed near the ends of branches, usually suspended from diverging twigs. The nest is cup-shaped and is constructed of grasses, lichens, spider webs, plant fibres, mosses, bark strips, animal hair, and other soft, pliable materials. The heights of 46 nests ranged from 0.8 to 8.0 m, with 63% between 1.8 and 4.5 m. Eggs: Dates for 35 clutches ranged from 27 April to 14 July, with 53% recorded between 3 June and 24 June. Calculated dates indicate that eggs can occur as early as 20 April. Sizes of 32 clutches ranged from 1 to 5 eggs (1E-7, 2E-4, 3E-2, 4E-15, 5E-4), with 59% having 4 or 5 eggs. In Ontario, the incubation period is 13 or 14 days (Peck and James 1987). Nest Success: Of 6 nests found with eggs and followed to a known fate, only 1 produced at least 1 fledgling. Young: Dates for 23 broods ranged from 5 May to 29 July, with 13 dates recorded between 18 June and 17 July. Sizes of 18 broods ranged from 1 to 5 young (1Y-1, 2Y-6, 3Y-5, 4Y-4, 5Y-2), with 11 having 2 or 3 young. The nestling period is poorly known, but is about 14 days (Ehrlich et al. 1988). Brown-headed Cowbird Parasitism: In British Columbia, 9% of 45 nests recorded with eggs or young were parasitized by the cowbird. Parasitism on the coast was 8% (n = 24); in the interior, it was 10% (n = 21). Elsewhere in North America, the rate of parasitism varies greatly (Friedmann et al. 1977). In Ontario it was 4.5% (Peck and James 1987), whereas in Colorado it was 49% (Marvil and Cruz 1989).

Remarks Two subspecies of the Blue-headed Vireo occur in British Columbia: V. s. solitarius occurs northeast of the Rocky Mountains, while V. s. cassinii occurs west of the Rocky Mountains (Munro and Cowan 1947). Recent studies indicate that the 2 subspecies behave as separate species (Johnson et al. 1988). If this change in systematic status is recognized by the American Ornithologists' Union, the name Vireo solitarius (Wilson) will apply to populations east of the Rocky Mountains, and Vireo cassinii (Xantus de Vesey) will apply to populations west of the Rocky Mountains (American Ornithologists' Union 1983; Sibley and Monroe 1990). See James (1978) for additional information on the nesting behaviour of the Cassin's Vireo, and Johnson et al. (1988) for a discussion of relationships within the family Vireonidae.