w_vireo

English

Warbling Vireo
Vireo gilvus
This information was scanned from The Birds of British Columbia (Campbell et al.), Volume III, pages 516-521. Volumes I, II and III of The Birds of British Columbia, can be ordered electronically at: orders@ubcpress.ubc.ca from UBC Press in Vancouver, British Columbia.

Range
Breeds from southeastern Alaska, southeastern Yukon, northern British Columbia, west-central and southwestern Mackenzie, and northern and western Alberta southeast across Saskatchewan, southern Manitoba, western and southern Ontario, southern Quebec, and New Brunswick; south in the west to southern Baja California and in the east to Louisiana and Tennessee. Winters in the mountains from southern Sonora south to Guatemala and El Salvador.

Status
On the coast, a fairly common migrant and summer visitant in the Georgia Depression Ecoprovince; uncommon to locally fairly common in the southern and northern mainland of the Coast and Mountains Ecoprovince; rare on Western Vancouver Island; absent from the Queen Charlotte Islands.

In the interior, an uncommon to fairly common (occasionally common) migrant and summer visitant in the Southern Interior, Southern Interior Mountains, Central Interior, Sub-Boreal Interior, and Boreal Plains ecoprovinces; uncommon in the Northern Boreal Mountains and Taiga Plains ecoprovinces.

Breeds.

Status Change
No change.

Nonbreeding
The Warbling Vireo has a widespread distribution across most of British Columbia, but is absent from the Queen Charlotte Islands. It is relatively abundant from eastern Vancouver Island east across the southern portions of the province to the Rocky Mountain Trench and north through the interior to the Peace Lowland; further north, it becomes more thinly distributed. It is also sparsely distributed along the outer coast, including Western Vancouver Island, and on islands and the mainland of the central and northern coast. On the mainland north of the Georgia Depression, it has been recorded mainly from the heads of fiords where rivers cut through the mountains from the interior.

The Warbling Vireo has been reported at elevations from sea level to about 300 m on the coast, and from 300 to 1,700 m in the interior. On the coast, it avoids continuous coniferous forests and occurs only in deciduous or mixed forests, which are often restricted to river valleys, wetlands, or estuaries. In mountainous interior areas, it occurs mainly in valleys, along lower-elevation passes, and on the edges of slides and avalanche chutes where patches of deciduous forest occur. It seldom occurs in conifer-dominated forests found on most mountain slopes and plateaus. Spring migration begins in the Georgia Depression in early April; numbers increase during late April but the movement does not peak until mid-May. In the southern and central portions of the interior, the first spring migrants arrive in late April and early May, with the movement peaking in mid May. On the north coast and in the northern interior, spring migration probably begins in early to mid-May and peaks in late May.

In northern British Columbia, the autumn movement begins shortly after the young are independent, probably by late July, and all birds are gone by the end of August. Near Vanderhoof, Munro (1949) noted that the main movement occurred between mid-August and early September. In southern portions of the interior, autumn migration occurs from mid-August to mid-September. Along the south coast, the autumn migration also begins about mid-August and peaks during the first 2 weeks of September, but most birds have left by the third week of September. There have been only 5 records of the Warbling Vireo in the province in October, all from the southern coast (Vancouver, Saanich, Victoria).

On the coast, the Warbling Vireo has been recorded from 1 April to 10 October; in the interior, it has been recorded from 21 April to 19 September.

Breeding

Nests

Eggs

Nest Success

The Warbling Vireo has a widespread breeding distribution throughout most of British Columbia from the Peace Lowland southward. It has not been reported from the

Queen Charlotte Islands. Breeding has been confirmed in al most all areas of its summer range except the west coast of Vancouver Island and most of the central and northern main land coast. In the interior, there are no confirmed breeding records north of Fort St. John. However, an individual was observed carrying nesting material at Liard River, and the Warbling Vireo probably breeds throughout the northern interior.

This vireo reaches its highest numbers in summer in the Sub-Boreal Interior. Relatively high numbers also occur on southeastern Vancouver Island, the Fraser Lowland, and the valleys of the Kootenay and Columbia rivers.

Populations become less abundant north of the Sub-Boreal Interior. An analysis of Breeding Bird Surveys in British Columbia shows that the mean number of birds on interior routes increased at an average annual rate of 3% over the period 1968 through 1993; analysis of coastal routes for the same period could not detect a net change in numbers. On the coast, the Warbling Vireo has been recorded breeding at elevations from near sea level to 300 m; in the interior, from 330 to 1,450 m. It is a conspicuous bird where it occurs during spring and summer, because males are easily detected by their melodious song while they forage in thick foliage in the crowns of trees. It breeds mainly in open deciduous wood lands, but also in mixed woodland with some conifers. In general, it prefers taller and larger trees in riparian habitats at lower elevations, especially around ponds, sloughs, wet meadows, and lagoons, and at the edges of forest clearings provided by farms, powerlines, and roads. In suburban areas it breeds in parks, along treed boulevards, in well-treed gardens, and in orchards, where a variety of non native tree species are used.

In the Bulkley valley, Polar (1995) found the Warbling Vireo in stands of variable age, including sapling, mature, and old-growth trembling aspen and mixed conifer-aspen stands, but mature and old-growth aspen stands had the highest density of singing males.

Most nesting habitats have been described as forests (60%; n = 75), including mixed (33%), deciduous (15%), and coniferous (11%) forests. Another 33% of nests were in rural, suburban, or agricultural settings. Most nest sites were near a forest-clearing edge or in relatively open forest parkland.

On the coast, the Warbling Vireo has been recorded breeding from 25 May (calculated) to 28 July; in the interior, it has keen recorded from 31 May (calculated) to 3 August (Cannings et al. 1987).

Nests:

Most nests were found in deciduous trees (76%; n = 118), including trembling aspen (27%), poplar (10%), birch (10%), red alder (8%) black cottonwood (7%), and vine maple; or deciduous shrubs (14%) such as willow, red-osier dogwood, and elderberry. Nests are cup-shaped pendants, generally placed in a fork between 2 twigs or attached to adjacent twigs. Nest materials included grass (65%; n = 118), plant fibres (28%), plant down (28%), lichens (22%), bark strips (21%), hair (20%), moss (17%), spider webs (13%), and several other materials.

The heights for 114 nests ranged from 1 to 16 m, with 61% between 2 and 6 m. Higher nests are probably largely undetected by observers; for example, in Manitoba, the Warbling Vireo nested mainly in maples at a mean nest height of 12.6 m (MacKenzie et al. 1982).

Eggs:

Dates for 84 clutches ranged from 27 May to 19 July, with 52% recorded between 12 June and 27 June. Calculated dates indicate that eggs can occur as early as 25 May. Sizes of 75 clutches ranged from 1 to 5 eggs (lE-10, 2E-12, 3E-19, 4E30,5E-4), with 65% having 3 or 4 eggs. The incubation period has been reported as 12 days (Ehrlich et al. 1988) or 13 to 14 days (Harrison 1979).

Nest Success:

Of 23 nests found with eggs and followed to a known fate, 10 produced at least 1 fledgling.

Young: Dates for 38 broods ranged from 20 June to 30 July, with 57% recorded between 30 June and 14 July. Calculated dates indicate that young can be found as early as 10 June. Sizes of 31 broods ranged from 1 to 5 young (1Y-6, 2Y-6, 3Y10, 4Y-8, 5Y-1), with 58% having 3 or 4 young. The nestling period of 2 nests in British Columbia was about 13 days. In southern Ontario, the average nestling period was 12 days, with a maximum of 13 days (Howes-Jones 1985).

Brown-headed Cowbird Parasitism: In British Columbia, 49% of 98 nests found with eggs or young were parasitized by the cowbird. Parasitism on the coast was 40% (n = 20); parasitism in the interior was 51% (n = 78). In 37 parasitized nests with eggs, the number of cowbird eggs ranged from 1 to 6 eggs (lE-18, 2E-15, 3E-2, 4E-1, 6E-1). Adult vireos feeding fledged cowbird young were also reported from the interior (n = 10) and coast (n = 1).

In Ontario, the parasitism rate was 11% (Peck and James 1987). The Warbling Vireo is known as a frequent victim of the Brown-headed Cowbird (Friedmann et al. 1977) and appears to be one of the most frequently parasitized species in British Columbia.

Remarks
One subspecies, V.g. swainsoni, occurs in British Columbia (American Ornithologists' Union 1957); however, see Browning (1974) for a discussion of the possible presence of a second subspecies.