SPECIES

Canada Warbler Cardellina canadensis

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

Conservation and Management

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Conservation Status

BirdLife International considers the Canada Warbler to be of Least Concern (217) owing to its large range and global population size (~3,000,000 individuals; 1); despite the decreasing population trend, the decline was not deemed sufficiently rapid to approach thresholds for Vulnerable status.

The Canada Warbler was listed as Threatened in Canada in 2008 (218) and appended to Schedule 1 of the Species at Risk Act (SARA) in 2010 as a Threatened species because most (80%) of the breeding range occurs in Canada, it has experienced a long-term decline that is particularly evident within Canada. The Partners in Flight Landbird Conservation Plan listed Canada Warbler as a Yellow Watch List species (1). Northeast Endangered Species and Wildlife Diversity Technical Committee recognized the species as one of the region's highest priorities for conservation and research (219). Considered a high priority for conservation owing to population declines and rapid winter habitat loss in northern Andes region (220, 221). Considered a species of high conservation priority in many states in the United States (e.g., Maine, New York, New Jersey).

Effects of Human Activity

Shooting and Trapping

No information; probably not a concern within breeding range.

Pesticides and Other Contaminants/Toxics

No information.

Ingestion of Plastics, Lead, etc.

No information.

Collisions With Stationary/Moving Structure or Objects

Six mortalities from collisions with television towers reported in Michigan in early September (222) and 2 reported in late September in eastern Kansas after a day of low clouds (223). Thirty-four mortalities reported during 5 yr in a 7-year study a television tower in Ontario (148). Forty-seven of 66 Canada Warbler specimens in Illinois State Museum bird collection were from mortalities from collisions with television towers, including 27 individuals on the night 3 September 1981 (138). Collisions with a large chimney killed 325 birds in Ontario 1972–1982, including 131 killed on the night of 5 September 1981 alone (134). Mortalities from flying into house windows, tall buildings, and lighthouses during migration in museum collections.

Degradation of Habitat

Overwintering

Considered vulnerable to human disturbance of mature forest on overwintering grounds in Andes, but will use sites with moderate disturbance (224). Common in northern Andes region where human population pressure is great and habitat loss is proceeding rapidly (221). In Ecuador, occur in forested and partially forested habitats with varying degrees of disturbance (P. Benham, MTH, LR, personal observations). Often dense along forest/pasture edges of eastern Andes in Ecuador where naranjilla agriculture is principal cause of deforestation followed by cattle pasturing.

Breeding

Some evidence that habitat degradation/loss has negatively affected populations. More abundant in non-thinned compared to thinned mature stands of northeastern oak in central Massachusetts (214). Rate of habitat change considered moderate to rapid in British Columbia owing to logging of aspen forests for pulpwood and brush control; increased nest parasitism and nest predation resulting from habitat fragmentation considered short-term population threat, but much habitat will probably remain unharvested due to steepness of slopes (126). Management practices aimed at reducing the broad-leafed component in forests of Maritimes Provinces would likely have a negative effect if continued over wide areas (183). Considered sensitive to forest fragmentation (51, 225); only 4 of 75 forest species surveyed showed a stronger positive relationship between abundance and forest area (112). Probability of occurrence in the Allegheny Mountains of western Maryland and northwestern West Virginia was greatest in contiguous forests > 3,000 ha, and probability of occurrence was reduced 50% in forests of 400 ha, and 0% in forests < 187 ha (112).

In contrast, occupies young, disturbed forest in northern Wisconsin (102). Abundance highest in areas heavily logged 5–15 yr prior (compared to less-heavily logged and unlogged areas), based on point-count surveys of northern hardwood (maple–birch–beech) forests in New York (226). Present in 10- and 20-yr-old clearcuts and selectively-cut areas but not in recent clear-cuts or uncut, mature forest areas in maple-oak-beech forests in West Virginia (207). Minor to moderate disturbance may increase habitat suitability in some mature, closed canopy forests; common in hurricane-wrecked maple swamps in New England (2), and in forested areas with evidence of tree fall disturbance in British Columbia (99). Abundance increased in years following storm-induced blow downs of canopy trees which created canopy openings (and probably understory vegetation in gaps) but returned to original numbers as the openings closed in a virgin red spruce (Picea rubens)–northern hardwood (birch-beech-maple) forest in West Virginia (227).

Appears sensitive to reduction of understory vegetation by forest ungulates. In central Massachusetts, DeGraaf et al. (214) observed 80 individuals in mature stands of northeastern oak with few deer (1–3 deer/km2), but only one individual in mature stands with many deer (13–23 deer/km2). Increased density as a result of increased food availability associated with spruce budworm (Choristoneura fumiferana) outbreak (228).

In general, probably responds favorably to habitat changes that increase density of understory and shrub-layer vegetation within forests, especially in situations where some trees remain emerging from the canopy (104), but negatively to changes that decrease forest understory or severely reduce forest canopy. Known to decline with forest maturation in mixed forests of west-central New Hampshire, especially beyond 25 years after extensive harvest.

Disturbance at Nest and Roost Sites

One incubating female flushed from nest when human approached within 1 m but another allowed observer within 0.6 m for some time before flushing (95). Tend not to abandon even when nestling growth is monitored over alternating days of the nestling phase (M. Goodnow et al., unpublished data). Earlier in the nesting cycle, rarely abandons nest once egg-laying begins as long as visits to nest are infrequent and disturbance to nest microsite is minimal (MG, LR, personal observations).

Human/Research Impacts

See above. Little information, but considered not particularly sensitive to minimal disturbance (126). Young fledged from nest in park with bicycle and hiking paths within 14 m (164).

Management

Conservation Status

Partners in Flight includes the Canada Warbler on its Yellow Watch List, given the strong population declines noted over the past 5 decades (1; for details, see Population Status. It is listed as a species of Least Concern by BirdLife International owing to its large range and population sizes, and although the population trend appears to be decreasing, the decline is not rapid enough to approach the thresholds for Vulnerable status (229).

Conservation Measures and Habitat Management

Several areas with potential Canada Warbler habitat have been proposed as protected areas in British Columbia (126). State wetland regulations protect wooded swamp nesting habitats in most states, but not all forested wetlands are well delineated and thus many remain unprotected. Populations are probably not monitored effectively by common monitoring programs because of dense, wet, inaccessible breeding habitats. This also makes detailed studies difficult (but not impossible), as evidenced by the longer-term population study in New Hampshire. Recent work has attempted to put forth guidelines for managing forests in the northeastern U.S. and mid-Atlantic region to optimize recruitment and persistence of Canada warblers. Particularly across large tracts of industrial forest (230). Modeling of optimal management opportunities within the southeastern Canadian portion of BCR 14 has also been undertaken (231).

The Northeast Association of Fish and Wildlife Agencies has sponsored development of guidelines for managing forests in the northeastern United States and mid-Atlantic regions to maintain or enhance conditions for Canada warblers (230). A related effort, supported by Environment and Climate Change Canada, has produced recommendations for planning timber harvests and delineating ecological reserves in the southeastern Canadian portion of BCR 14 (232). A spatial prioritization of management and conservation opportunities has also been undertaken in this region (231). The Canada Warbler International Conservation Initiative (CWICI) has provided a framework for coordinating ongoing research to build range-wide knowledge of habitat requirements and conservation approaches that could be applied at stand and landscape levels (see the CWICI website on Nature Canada: Canada Warbler International Conservation Initiative.

Effectiveness of Measures

No direct information, but recent reductions in forested wetland losses have apparently not halted population declines. Indeed, in New Hampshire, despite high pairing and fledging success, the local population monitored since 2003 has declined by as much as 50% in specific areas with entire neighborhoods disappearing almost certainly due to reduced understory and shrub-layer densities as the forest matures.

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