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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.
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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.
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Status
Change
No change.
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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.
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Breeding
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.
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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).
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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.
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Located
at:
675 Belleville Street,
Victoria, British Columbia,
CANADA
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Copyright © Royal BC Museum
All rights reserved
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Nests