SPECIES

Canada Warbler Cardellina canadensis

Len R. Reitsma, Michael T. Hallworth, Marissa McMahon, and Courtney J. Conway
Version: 2.0 — Published May 7, 2020

Demography and Populations

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Measures of Breeding Activity

Age at First Breeding; Intervals Between Breeding

Many first year males and females successfully breed in New Hampshire. No significant difference between younger, first-time male breeders (second-year) and older (after-second-year) males in ability to fledge at least 1 young, although second-year birds had larger territories, suggesting poorer quality habitat (98, 104, 198).

Clutch

Usually 4 or 5 eggs per clutch. Mean 4.13 eggs (range 2–6, n = 23 nests) in Ontario; most (17 of 23) contained 4 or 5 eggs (87). Mean 4.36 eggs (range 3–5, n = 11 nests) in Michigan (90). Mean 4.45 eggs ± 0.70 SD (range 3–5, n = 75 nests) based on non-parasitized nests in 5 nest record schemes (NRS) and egg collections (CNRCP, Maritimes NRS, Quebec NRS, Royal Ontario Museum NRS, WFVZ egg collection); most (66 of 75) contained 4 or 5 eggs. Four-egg and 5-egg clutches laid with equal frequency in Vermont (97). One nest with 6 nestlings found in Ontario (87). Mean 4.8 eggs (range 4–6, n = 54 nests) in New Hampshire; most (42 of 54) contained 5 eggs (L. Reitsma et al., unpublished data).

No evidence of double-clutching in New Hampshire but second nest attempts initiated up to late June if first nest fails (L. Reitsma et al., unpublished data). See Number of Broods Normally Reared per Season, below.

Annual and Lifetime Reproductive Success

Fledging brood size 3.8 young ± 1.03 SD (range 2–6, n = 26; range wide, 4 nest record schemes). Daily nest survival 0.9555 for 37 nests (10 failures) from 5 nest record schemes/egg collections that reported > 1 nest visit.

In New Hampshire, reproductive success generally high among years, with relatively low nest loss to predation (although 2009 had unusually high losses to predators, 9 of 20; previously recorded highs were 3 of 27 and 3 of 18 in 2008 and 2007, respectively).

Number of Broods Normally Reared per Season

Evidence strongly suggests only 1 brood per season (L. Reitsma et al., unpublished data; 12). Brauning (92) suggested that pairs may occasionally raise 2 broods/yr, based on wide range of dates (31 May–26 July) that birds were observed feeding young in Pennsylvania. Peterjohn and Rice (182) and Reitsma et al. (unpublished data), however, suggested that renesting attempts are responsible for fledged young seen with adults through the end of July, and that the record for fledged young on 31 May in Pennsylvania is questionable.

Proportion of Total Females That Rear at Least One Brood to Nest-Leaving or Independence

No information.

Life Span and Survivorship

Maximum reported life span 7 yr, 11 mo (199; R. Pantle, personal communication). No reliable estimates of survivorship available. Of 46,973 banded birds reported to the U.S.G.S. Bird Banding Lab, only 20 individuals were subsequently recovered (199).

Breeding population study initiated in 2003 in New Hampshire has documented 4 individuals at least 6 yr old; one male caught in 2003 as an adult (after-second-year) may have reached 7 yr old. Several more have reached 5 yr old and 4-yr-old birds are common (L. Reitsma et al., unpublished data).

Disease and Body Parasites

No information.

Causes of Mortality

No information.

Population Spatial Metrics

Individual Distance

Nests are often within 30 m of each other. Three pairs nested within 30 m of each other along stream in West Virginia and 5 nests were found along 46 m of stream in Vermont (Cornell Nest Record Card Program). In one case in New Hampshire, 2 nests were 32 m apart, both within the same male's territory, suggesting a polygamous male (L. Reitsma et al., unpublished data). Two pairs feeding newly fledged young just out of nest only 60–90 m apart (191).

Territory Size

Considerable recent characterization of territory sizes in different habitats as well as male age-class differences (98, 104, 198). In New Hampshire, 16 years of continuous territory mapping documented mean territory sizes of older males ranging from 0.67–1.16 ha and for younger males from 1.16–1.36 ha. Territory sizes depended upon predominant habitat type. Four year averages were 0.88 ha for red maple (Acer rubrum) swamp compared to 1.5 ha for predominantly early-mid succession mixed forest, but neither age class or habitat type affected ability to fledge young—suggesting behavior can adequately compensate for habitat and differences in age-related experience (98, 104, 198). In Ontario, average territory size 0.2 ha in Algonquin Provincial Park (200); 1 territory in Quebec 0.4 ha (A. Cyr in 155). Two paired males apparently defended areas of 0.8 and 1.2 ha in New York (176).

Home Range Size

No information.

Population Status

Numbers

Using data from the North American Breeding Bird Survey (BBS), the mean annual population for Canada Warbler was estimated at 3,000,000 individuals for the United States and Canada for the period between 2005 and 2014 (1). Point-count survey data collected by state and provincial breeding bird atlas projects yielded estimates of annual population size of 900,000 individuals for Ontario, 2001–2005 (42) and 27,000 singing males (95% CI: 21,500 to 38,000) for Pennsylvania, 2004–2009 (48). Breeding population estimated at 58,000 ± 15,000 pairs in Maritimes Provinces (183).

Not abundant within most of its breeding distribution (95, 86; Figure 6). Within suitable habitat, breeding density typically 1–5 pairs/10 ha (range 0.25–13) but varies by location and forest type; average of 0.25 pairs/10 ha in mature maple-beech-birch-hemlock forest in New York (201) and in mature white pine–red oak (PinusQuercus) forest in coastal Maine (202); 0.3 males/10 ha within maple stands in Nova Scotia and Ontario (203); 0.4 pairs/10 ha in aspen-fir-birch-spruce forests of Saskatchewan (204); 1.2–1.7 territorial males/10 ha within stands of aspen, birch and fir in New Brunswick and Ontario (203); 1.7 and 2.5 pairs/10 ha based on spot mapping within mid- and late-seral stages of previously clearcut spruce–fir forests with dense deciduous understory, but absent from late-seral stage stands with open forest floor in Maine (105); 2.2 pairs/10 ha in eastern mixed-deciduous forest of New York (205); 1.9–3.5 birds/10 ha in coniferous, deciduous, and mixed forest habitats adjacent to clearcuts in north-central Maine (206).

Estimates can be difficult owing to patchiness of suitable habitat; e.g., 5 pairs/10 ha in red maple swamps in New Hampshire including uninhabited upland forest interspersed with the swamp. Species tends to occur in clusters or “neighborhoods,” often at densities up to 5 pairs/5 ha (98, 104, 198).

More than 3 singing males/10 ha in maple-oak-beech forest in West Virginia (207); 1.7–8.6 pairs/10 ha in poplar/aspen forests with dense understory and 3.8–4.1 pairs/10 ha in balsam fir/white birch forest of western Manitoba (204); 2.1–10 pairs/10 ha in southern Quebec (155). Four to five singing males/10 ha in hemlock-cedar forests in Algonquin Provincial Park in Ontario (200); 5.6 territorial males/10 ha within good habitat in northern Alberta (126); 6.0 birds/10 ha in subalpine valleys of New Hampshire (157), 6.2 singing males/10 ha on Black Mountain in Kentucky (55). Up to 13 pairs/10 ha in mixed-wood cut-over forest in northeastern Ontario (208). Where present in Great Smoky Mountains of Tennessee and North Carolina, 1.2–5.4 breeding pairs/10 ha in moist stream valley deciduous forests with dense Rhododendron thickets, 6.7–13 breeding pairs/10 ha in eastern hemlock-deciduous forests with extensive birch and Rhododendron understory, and 1.0–1.3 breeding pairs/10 ha in beech–oak forests (209).

Trends

See Figure 6. Based on BBS data (1966–2017), populations declined –1.9%/yr (95% CI: –2.6 to –1.2; n = 820 routes) throughout the breeding range (210). Within Bird Conservation Regions (BCR), strongest declines were –3.7%/yr in the Atlantic Northern Forest (95% CI: –4.4 to –3.0; n = 264), –2.1%/yr in the Boreal Hardwood Transition (95% CI: –3.1 to –1.2; n = 230), and –1.3%/yr in the Appalachian Mountains (95% CI: –2.1 to –0.6; n = 104); no BCR showed a population increase (210). Over a 45-year period (1970–2014), BBS data indicated that the range-wide population has decreased by an estimated 62% (1). See also Figure 6: according to those data, population gains appear only in the Appalachians, northern Maine, and the central Great Lakes region.

In Ontario, populations declined 3.7%/yr (P < 0.01) based on migration counts from 1961 to 1988; declines greatest in the latter part of the period (211). In contrast, captures during migration at banding stations in Massachusetts and Pennsylvania showed no obvious trends between 1970 and 1988 (212). Note that in the 4 areas with the best data (Minnesota, Ontario, Adirondacks New York, northern spruce-hardwood forest), annual declines 1980–2007 ranged from –1.8% to –4.8%.

Local and range-wide population declines probably a result of change in forest structure over the past century, combined with loss of forested wetlands. Forest regeneration of previously farmed lands in the Northeast provided optimal habitat (forest with dense understory) in early and mid-1900s, but continued forest maturation (along with increasing deer browse) likely eliminated understory and, hence, suitability to Canada Warbler. In addition, significant portions of the forested wetlands in the northeastern North America were drained, filled, and developed between 1950 and 1980 (213). At one aspen/poplar forest site in Manitoba, population density apparently increased 400% from 1972 to 1992 due to blow downs, creating more dense understory shrubs and more open canopy (204). Common (282 pairs) in early 1930s, but rare (7 pairs) in early 1980s at a study area in New York, probably owing to loss of young forest seres coupled with a poorly developed understory (201). Became locally extinct at Sapsucker Woods, Ithaca, New York, between 1950 and 1980 (8.7 pairs/40 ha to completely absent; 205)

Population Regulation

Populations respond to processes that alter forest understory: regeneration, forest succession, storm-induced tree blow downs, fire, logging, and grazing. Processes that increase forest understory vegetation usually increase abundance, while processes that decrease forest understory decrease abundance. In New England, Canada Warbler is uncommon in oak forests with many deer (214), owing to elimination of understory vegetation. In Ontario, numbers declined after burn in mixed-conifer forest (215). In New York, more abundant in old, regenerating burns than in unburned forest (11, 94). Thus, populations appear to decrease initially after fire due to loss of understory vegetation, but eventually benefit from fire following regeneration of dense understory vegetation in burned areas. In many areas, populations respond to forest succession; more common in young and mid-successional forest with well-established understory vegetation than in late successional forest lacking understory. Residual tree retention that provides emergent song perch trees may be an important component to timber harvest in promoting good breeding habitat for this species. Trees emerging from low canopies occur naturally in red maple swamps, considered among the best breeding habitats for the species, but must be an intentional part of timber harvest practices to maximally benefit the species (104). A forest in New Hampshire that was heavily harvested in the early 1980s experienced a > 50% decline in breeding territories from 2005 to 2019 due to successional changes that reduced the density of shrub and understory stems (L. Reitsma, unpublished data).

See Conservation and Management: Effects of Human Activity.

In Canada, abundance was positively related to spruce budworm (Choristoneura fumiferana) abundance the previous year (216). Canada Warblers either respond to increased prey availability during outbreaks or to changes in forest structure and composition following budworm outbreaks (216).

Recommended Citation

Reitsma, L. R., M. T. Hallworth, M. McMahon, and C. J. Conway (2020). Canada Warbler (Cardellina canadensis), version 2.0. In Birds of the World (P. G. Rodewald and B. K. Keeney, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.canwar.02