Swainson's Thrush Catharus ustulatus This information was scanned from The Birds of British Columbia (Campbell et al.), Volume III, pages 394-401. 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 western and central Alaska, central Yukon, and western and southern Mackenzie across the Prairie provinces north of the grasslands, and east into Labrador and Newfoundland; in the west, south to southern California; in the east, to southern New York, New Hampshire, and West Virginia. Winters primarily from southern Mexico south to Brazil, Peru, and Argentina.

Status On the coast, a fairly common to common migrant and summer visitant in the Georgia Depression Ecoprovince; uncommon to fairly common on Western Vancouver Island, the mainland coast, and the Queen Charlotte Islands of the Coast and Mountains Ecoprovince. Casual in winter in the Georgia Depression. In the interior, a fairly common to common migrant and summer visitant in the Southern Interior and Southern Interior Mountains ecoprovinces, becoming uncommon to fairly common further north. Accidental in winter in the Southern Interior. Breeds.

Status Change No change.

Nonbreeding The Swainson's Thrush is widely distributed throughout the province, including offshore islands. Information is lacking regarding this thrush's occurrence in the mountainous areas of the Southern Mainland Coast, the western Chilcotin Plateau, the Blackwater region, the lower Nechako and McGregor river basins, and the Rocky Mountain Trench northward between Mackenzie and Lower Post. While some of these gaps may reflect the lack of suitable habitat, others suggest the need for further biological exploration. In migration along the coast, the Swainson's Thrush has been reported from sea level to about 800 m elevation. In the southern portions of the interior, it usually occupies elevations between 450 and 1,200 m; further north the species is rarely found above 800 m. Since the Swainson's Thrush is a characteristic summer bird in much of the province, there is considerable overlap between the wide variety of habitats it occupies during the nonbreeding and breeding periods. In the Coast and Mountains, it is most numerous along marine and freshwater shorelines where dense thickets of salal, salmonberry, or devil's club produce a supply of fruit close to the abundant invertebrates of the beaches. It frequently forages along the foreshore close to cover; in riparian forests of alder, willow, or cottonwood; in mixed stands of western redcedar, thimbleberry, and crab apple that surround lakes or beaver ponds; as well as the edges of forest openings, fields, sewage ponds, creeksides, roadsides, and muskeg edges. On the west coast of Vancouver Island, where the Swainson's Thrush is particularly widespread, it prefers 30- to 35- and 50- to 60 year-old forests of western hemlock, amabilis fir, Douglas-fir, western redcedar, and Sitka spruce, with red alder in the wet areas (Bryant et al. 1993). In wetter parts of the interior, the Swainson's Thrush occurs in the widespread coniferous forests of lodgepole pine, Douglas-fir, Engelmann spruce, white spruce, western larch, and subalpine fir. In drier areas, it occupies deciduous woodland with scattered conifers, as well as thickets of black hawthorn, choke cherry, and saskatoon that occur in the swales and gullies of grassland slopes. It coexists with the Veery in riparian cottonwood stands with a dense shrub understorey. In the north, the Swainson's Thrush occurs in mixed deciduous woodlands, in forests of white spruce mixed with aspen, birch, or willow, or where black spruce and tamarack predominate, and where berry-producing shrubs are widespread.  The Swainson's Thrush coexists with other closely related thrushes in British Columbia. In the southern interior, where it shares habitat with the Veery, the Swainson's Thrush forages higher above the ground, and prefers habitat with less undergrowth than that preferred by the Leery On the Queen Charlotte Islands, where it shares habitat and perhaps the same food sources with the Hermit Thrush, the spring migration of the Swainson's Thrush reaches the islands later than that of the Hermit Thrush. Thus, although the same resource base may be used by the 2 species, the periods of peak demand are separated (Sealy 1974). On the south coast, the spring migration begins in April, or occasionally in very late March. In early May, numbers increase tenfold. Migration in the Georgia Depression reaches a peak towards the end of May. This peak is a little later further north, reaching the northern mainland Coast and Mountains and Queen Charlotte Islands in early June. In the southern portions of the interior, the first birds may arrive as early as the third week of April in the east and west Kootenays, but not until early May in the Okanagan; however, the main movement does not occur until late May or early June. The timetable for spring arrival is similar further north, and indicates the rapidity of the northward movement.  The southward migration is more protracted than the spring movement. In the northern ecoprovinces, south to the Central Interior, an autumn migration is difficult to discern. In the Southern Interior and Southern Interior Mountains, the autumn migration begins in early August, with a notable movement occurring between the third week of August and the second week of September. Most birds have left the north by the end of August and the southern regions by the end of September. On the coast, the autumn departure from the Queen Charlotte Islands and the Northern Mainland Coast is completed about a month before it is on the Southern Main land Coast and 2 months before the Georgia Depression. Throughout much of the coast, migration begins in late July, is at its height in August, and is completed in September. Exceptions are the Queen Charlotte Islands, where there are no records after the end of August, and the Georgia Depression, where there are a few late autumn records. Rarely are birds recorded in winter. In the interior, the Swainson's Thrush occurs regularly from 18 April to 1 October; on the coast, it occurs from 23 March to 12 October.

Breeding Nests Eggs Nest Success The Swainson's Thrush undoubtedly breeds throughout its summer range in the province. On the coast, however, there are no records of nesting from northern Vancouver Island, from the south Moresby Island archipelago, or along the entire mainland coast between Queen Charlotte Strait and the mouth of the Skeena River. In northern British Columbia, nesting has been documented in the Tatshenshini River valley, Atlin, the Rabbit River valley near Coal River, and the vicinity of Fort Nelson. Elsewhere across this vast area, nesting has been recorded only at Junction and Telegraph creeks, near the south end of Dease Lake (Swarth 1922), and at a number of locations in the Peace Lowland. Much remains to be learned about the nesting distribution of the Swainson's Thrush in northern British Columbia.  The highest numbers for the Swainson's Thrush in summer occur in the southern Coast and Mountains and SubBoreal Interior. An analysis of Breeding Bird Surveys for the Swainson's Thrush in British Columbia for the period 1968 through 1993 could not detect a net change in numbers on either coastal or interior routes. On the coast, the Swainson's Thrush nests from sea level to about 930 m elevation. In summer, Weber (1975) found the Swainson's Thrush to be the most abundant songbird on Mount Seymour, north of Vancouver, between 90 and 370 m, while the Hermit Thrush was the most abundant species between 740 and 1,050 m elevation (Weber 1975). In the interior, the Swainson's Thrush ranges from valley bottoms to 1,850 m, but in the Southern Interior, few Swainson's Thrushes are found below 500 m elevation (Cannings et al. 1987). Along the coast, this thrush is a characteristic nesting species in second-growth forests of Douglas-fir, Sitka spruce, western redcedar, western hemlock, and grand fir where there is a dense undergrowth of salal, thimbleberry, huckleberry, salmonberry, and other fruiting shrubs. Similar habitats are used in the Southern Interior, where nesting habitat is usually in relatively undisturbed Douglas-fir and lodgepole pine forests mixed with trembling aspen, spruce, and willow. In the northern interior, the Swainson's Thrush frequents mixed spruce, paper birch, and trembling aspen forests. Mixed forest was the most frequently reported nesting habitat class (39%; n = 146), followed by deciduous stands (17%), coniferous stands (17%), rural and suburban areas (12%), and shrublands (9%). Most nests (84%; n = 244) were associated with undisturbed forest, followed by disturbed forest and shrubland. Nearly 40% (n = 106) of nests were reported from riparian situations regardless of the general forest type. In New York, Dilger (1956) found that, of the 5 species of North American forest thrushes studied, the Swainson's Thrush was the most closely associated with undisturbed coniferous growth. The Swainson's Thrush has been recorded breeding in the interior between 31 May and 6 August; on the coast it has been recorded from 30 May to 27 August.

Nests:  Most nests were placed in trees (60%; n = 340), including deciduous trees (30%), conifers (24%), and snags. Shrub and brush tangles (38%) accounted for most of the other general nest locations. Nests were built among branches, saddled on a branch, or lodged in the fork or crotch of a branch (96%; n = 288). A few nests were found among tree roots or in shrubs, and 1 nest was found on a post. The heights for 329 nests ranged from ground level to 24 m, with 60% between 1 and 2 m. Nests were cups of grass, moss, leaves, fine twigs, plant fibres, and rootlets. Eggs: Dates for 328 clutches ranged from 30 May to 7 August, with 51% recorded between 14 and 30 June. Sizes of 313 clutches ranged from 1 to 5 eggs (1E-30,2E-25,3E-72,4E-172, 5E-14), with 54% having 4 eggs. The incubation period in British Columbia ranged from 11 to 14 days (n = 4). Harrison (1979) gives the incubation period as 10 to 13 days. Nest Success: Of 31 nests found with eggs and followed to a known fate, 5 produced at least 1 fledgling, for a nest success rate of 16%. Young: Dates for 115 broods ranged from 13 June to 27 August, with 52% recorded between 29 June and 15 July. Sizes of 74 broods ranged from 1 to 5 young (1Y-9, 2Y-23,3Y-18,4Y-23,5Y-1), with 86% having 2 to 4 young. The nestling period in British Columbia is 11 to 14 days (n = 4). Populations breeding in the Georgia Depression may have 2 broods a year. Ehrlich et al. (1988) state that the number of broods produced each year is uncertain. Brown-headed Cowbird Parasitism: In British Columbia, 8% of 395 nests found with eggs or young were parasitized by the cowbird. There was an additional record of adults feeding a fledged cowbird. The parasitism rate both on the coast (n = 250) and in the interior (n = 145) was 8%. Friedmann et al. (1977) report this species as an infrequent host for the cowbird.

Remarks Two subspecies of Swainson's Thrush are found in British Columbia. The "Olive-hacked" Thrush (C. u. almae) occurs east of the Coast Mountains and Cascade Mountains, while the "Russet-hacked" Thrush (C. Il. ustulatus) inhabits coastal forests (Bond 1963). There are 2 banding reports of the Swainson's Thrush from British Columbia: a bird banded at Madison, Wisconsin, on 11 September 1961 was recovered at Okanagan Landing on 8 April 1963; 1 banded near Hays, Kansas, on 24 May 1973 was recovered near Grand Forks on 21 August 1973. There are a number of reports of the Swainson's Thrush before 23 March and after 12 October, including sightings from Christmas Bird Count locations where the very similar Hermit Thrush is known to winter. These include Vancouver 27 December 1955-1 (Weber and Weber 1975) and Pender Islands 28 December 1965-5 (Stevens and Stevens 1966). See also the following Christmas Bird Counts: Victoria 23 Dec 1961-1 (Stirling 1962a; Anderson 1976k),1 Jan 1969-1 (Tatum 1970; Anderson 1976~); Pender Islands 23 Dec 1978-1 (Anderson 1979; McLardy 1979); Duncan 15 Dec 1979-1 (Comer 1980); White Rock 28 Dec 1975-1 (Anderson 1976a; Schouten 1974); Vancouver 26 Dec 1975-1 (Anderson 1976a;Kautesk 1976); Nanaimo 31 Dec 1977-1 (van Kerkoerle 1978). All but 3 records, however, lack convincing details and have been excluded from the account.