Selected Birds of the Boreal Forest of North America



Canada Warbler, male
© Harold Lindstrom



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Arrows represent general migration routes.
The arrows do not necessarily link specific
breeding and wintering grounds.
Canada Warbler
Wilsonia canadensis 

This brightly-colored warbler is sometimes called the Necklaced Warbler because of the band of black streaks across the male’s breast.  It catches insects on the wing more than most warblers, earning it the now obsolete names of Canadian Flycatcher and Canadian Flycatching Warbler.  This species has yet to receive intensive study and much of its biology remains unknown.  On the breeding grounds it tends to be associated with mixed deciduous-coniferous woodlands .  Steep declines in recent decades have made this species a high conservation priority.

Habitat
At all times of year, the Canada Warbler is a bird of moist thickets.  It nests in riparian thickets, brushy ravines, forest bogs, etc. at a wide range of elevations and across a variety of forest types.  In the northwestern parts of its range it frequents aspen forests; in the center of the range, it is found in forested wetlands and swamps; and in the south it occupies montane rhododendron thickets.  It is somewhat more catholic during migration, opportunistically utilizing brushy areas in a wide variety of moist woodlands, parks, swamps, scrub, and forests.  In winter it tends to prefer mid- and upper-elevation habitats and has been found in coffee plantations and hedgerows.  There is some evidence of altitudinal segregation by the sexes on the winter grounds.

Diet/feeding behavior
Like other warblers, the Canada Warbler eats insects for the most part, supplemented by spiders, snails, worms, and, at least seasonally, fruit.  Flying insects, especially flies, make up the bulk of the diet.  It employs a wide variety of foraging tactics, often flushing insects from foliage and catching them on the wing.  It also gleans from branches and occasionally on the ground and hover-gleans.  As with many other warbler species, males tend to forage higher than females on the breeding grounds.  It often joins mixed-species flocks in winter.

Breeding
The Canada Warbler breeds across the southern boreal region, from extreme southeastern Yukon to Nova Scotia, and south to the Great Lakes region, New England, and higher elevations of the Appalachians to Georgia.  An estimated 64% of the population nests in Canada’s boreal region.  It is at least seasonally monogamous; pairs have been observed together on migration in Central America and it has been suggested that pair bonds may be permanent, contradicting the evidence that the sexes winter at different elevations.  Nests, built by the females, are placed on or very close to the ground in dense cover.  Nest sites include root masses, hummocks, stumps, stream banks, mossy logs, and less often in leaf litter, grass clumps, rock cavities, and brush piles.  Moss is a frequent element.  Clutch size is 2-6, usually 4-5.  The incubation period is about 12 days; female apparently does all incubation .  The male often follows the female during foraging bouts.  Both parents feed the young, which fledge at about 8-10 days, still unable to fly.  Nest parasitism by cowbirds is frequent where cowbirds occur, in the southern parts of the range.  As with other long-distance migrants, there is only one brood per year.

Migration/Winter range
The Canada Warbler spends less time on the breeding grounds than most species, being one of the last to arrive and first to depart; the total time on the breeding grounds may be no more than two months.  It migrates at night, rapidly and synchronously, seldom being found anywhere for more than three weeks.  Despite observations of pairs traveling together, males generally arrive slightly ahead of females.  The migration path is generally south and west to the Texas coast, then along the Gulf Coast to southern Mexico.  From there birds move inland to the mountains (mostly adults?) or cross over to the Pacific (mostly young?) and continue south to the winter grounds in northwestern South America, arriving in late September to early October.  The winter range extends around the northern and western side of the Andean crest from western Guyana to northwestern Bolivia.  Spring migration is also late and compressed, beginning late March to mid-April.  Peak passage as far south as Panamá may be as late as early May.  Populations in the western part of the winter range may cross the Pacific to southern Mexico in spring and most birds seem to cross North American farther east in spring than in fall.

Conservation Issues and Status
Habitat changes that increase understory density tend to increase Canada Warbler density; conversely, changes that decrease understory have the opposite effect.  This principle has resulted in fluctuations in the Canada Warbler’s population trend over time.  Numbers on the southern part of the breeding grounds (especially northeast U.S.) probably decreased prior to 1900 as a result of forest clearance and draining of forested wetlands, then surpassed original levels during the early to mid 1900s as farmland began to revert to brushy, secondary forest (tempered somewhat by increased cowbird parasitism and nest predation).  However, further maturation of these forests in the latter half of the 20th Century combined with over-browsing by deer reduced understory and caused another decline in Canada Warblers.  The Breeding Bird Survey shows a 1.9% annual decline from 1966-2002 and 3.2% annually from 1980 onward.  The strongest declines have been in the Adirondack Mountains, Northern New England, and Northern Spruce-Hardwoods Regions.  Migration data from Ontario show a 3.7% annual decline from 1961-1988.  Favored foothill habitats on the wintering grounds are among the habitats being cleared most rapidly for agriculture and may limit available wintering habitat for the species.

 

References

Blancher, P.  2003.  The importance of Canada’s boreal forest to landbirds.  Canadian Boreal Initiative [Online].

Conway, C.  1999.  Canada Warbler (Wilsonia canadensis).  In The Birds of North America, No. 421 (A. Poole and F. Gill, eds.).  The Birds of North America, Inc., Philadelphia, PA.

Francis, C. M., and F. Cooke.  1986.  Differential timing of spring migration in wood-warblers (Parulinae).  Auk 103:548-556.

Krause, H.  1965.  Nesting of a pair of Canada warblers.  Living Bird 4:5-11.

Sauer, J. R., J. E. Hines, and J. Fallon. 2004. The North American Breeding Bird Survey, Results and Analysis 1966 - 2003. Version 2004.1. USGS Patuxent Wildlife Research Center, Laurel, MD.

Sodhi, N. S., and C. A. Paszkowski.  1995.  Habitat use and foraging behavior of four Parulid warblers in a second-growth forest.  J. Field Ornithol. 66:277-288.

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