Canada Warbler

A regular breeding resident and migrant; the Canada Warbler was common during the Minnesota Breeding Bird Atlas (MNBBA).

Restricted to the Canadian boreal forest, from British Columbia east to the Maritime Provinces, and to the northern forests of the Great Lakes, New England, and the Appalachian Mountains. The core of the species’ breeding range is in northeastern Minnesota and southwestern Ontario (Figure 1).

Assigned a Continental Concern Score of 14/20 and designated a Yellow Watch List species by Partners in Flight.

A long-distance migrant; winters are spent in northern South America.

Insects captured by fly-catching, foliage gleaning, hover gleaning, and ground gleaning.

An open-cup nest located on or near the ground in dense undergrowth.

The Canada Warbler is an inhabitant of Minnesota’s northern forests. Roberts (1932) described the species’ summer distribution as stretching as far south as northeastern Pine and southern Mille Lacs Counties and as far west as Itasca State Park and Lake of the Woods County. Roberts and other contributors described the species as relatively common in a number of localities, including the North Shore of Lake Superior, at Lake Vermilion, along the Iron Range in northern St. Louis County, and at Leech Lake and Cass Lake. It was described as regular but not common north of Deer River in Itasca County. It was, Roberts wrote, “most numerous in the far-northern part of the state.” One report noted that the Canada Warbler was second only to the Mourning Warbler in abundance in Cook County. The species’ abundance declined, however, as one moved farther west. Although Roberts himself spent several summers working at Itasca State Park, he found only 1 male, in June 1917. The warbler was abundant in some localities, but breeding was confirmed only in Cass County (nest with eggs), Lake of the Woods County (feeding young), and at Mille Lacs Lake (feeding young) and Cass Lake (feeding young).

Roberts devoted much of the Canada Warbler account to his personal lament regarding the species’ name:

This charming little bird, with its beautiful necklace of black pendants, is surely deserving of a less prosaic name than that with which it has been inflicted. Canada Warbler, it is true, indicates plainly enough that the summer home of this bird is chiefly to the north of the United States, but this applies with no more force to this species than to many others of its kind. . . . Let us hope there may be evolved appropriate common names, descriptive enough to really mean something distinctive.

He gave as an example a name others had proposed for the species, “Necklaced Warbler.” Needless to say, the name Canada Warbler has remained.

In 1975 Green and Janssen added confirmed nesting reports from Beltrami, Cook, Lake, and St. Louis Counties and an inferred nesting report in Clearwater County. A few years later Janssen (1987) noted 2 summer reports of Canada Warblers at the Cedar Creek Ecosystem Science Reserve in Anoka County from June 1978 and June 1980. Although the species is one of the latest spring migrants, arriving from early May to mid-June (Janssen 1987), each of these observations was in late June: June 23 and 30, 1978, and June 20, 1980 (Bardon 1998). The species’ status in the county, however, would remain unclear for nearly 20 years. Janssen (1987) also included a state distribution map that identified 4 counties where nesting had been confirmed since 1970: Clearwater, Cook, Hubbard, and Lake. Hertzel and Janssen’s updated map published in 1998 still identified only 4 counties with confirmed nesting.

In the interim, the Minnesota Biological Survey reported 447 breeding season locations documented during the course of fieldwork. The overwhelming majority of records were from eastern St. Louis County east through Cook County. To the south, survey staff also observed the species in east-central Pine County and in northern Mille Lacs County; to the west they observed it in southern Clearwater, western Mahnomen, and western Becker Counties (Minnesota Department of Natural Resources 2016).

Then, in June 1997 and 1998, further evidence of the species’ status in east-central Minnesota was gathered. In both years, Karl Bardon documented 2 males on territory at the Boot Lake Scientific and Natural Area in Anoka County. Although he repeatedly saw 1 male carrying food in 1997, he could not locate a female or a nest. In 1998, however, he was successful in locating a pair of Canada Warblers and a nest with 5 young on the Falls Creek Scientific and Natural Area in northeastern Washington County. Both of these sites, he noted, as well as the Cedar Creek Ecosystem Science Reserve, represent outposts of the northern boreal forest ecosystem and support a suite of other northern forest species, including Winter Wren, Nashville Warbler, and Common Raven (Bardon 1998). Several Canada Warblers continued to be found on territory in Anoka and Washington Counties during the 1999, 2000, and 2003 summer breeding seasons (Minnesota Ornithologists’ Union 2017).

The MNBBA further confirmed the species’ known breeding range in the state, with a few new reports outside of the traditional northern forest region. Participants reported 1,059 Canada Warbler detections in 10.3% (486/4,736) of the surveyed atlas blocks and in 11.5% (268/2,337) of the priority blocks. Breeding was confirmed in 33 blocks (Figures 2 and 3; Table 1). Once again, the majority of reports were located from eastern St. Louis County and farther east through Cook County. The MNBBA documented possible records outside of the species’ traditional breeding range, in Marshall, Pennington, and Roseau Counties, and an observed record in Murphy-Hanrehan Park Reserve in Scott and Dakota Counties. The latter record, submitted by Bruce Fall, was a singing male observed on June 14, 2009; the bird repeatedly sung for more than 30 minutes. Unfortunately, there were no reports during the atlas from either Anoka or Washington Counties. Canada Warblers were reported from 18 of Minnesota’s 87 counties (1 record straddled 2 counties: Scott and Dakota) and were confirmed breeding in 6 counties: Beltrami, Cass, Cook, Itasca, Lake, and St. Louis. All but Cook and Lake were additions to the list published by Hertzel and Janssen (1998).

The MNBBA predicted distribution map predicts the highest breeding densities are primarily limited to a narrow band that stretches across northern St. Louis, Lake, and Cook Counties; moderate breeding densities occur as far south as Pine and northwestern Morrison Counties and as far west as eastern Becker, southeastern Mahnomen, and southern Clearwater Counties. Breeding densities decline rapidly beyond the boundaries of the Laurentian Mixed Forest Province (Figure 4).

Overall, the Canada Warbler’s breeding distribution has changed very little in the past 100 years. Reports outside of the Laurentian Mixed Forest Province indicated that birds occasionally find appropriate habitat in sites farther south and in the far northwest, but such occurrences are not common. The core of the species’ range is in the extensive forest landscape of northeastern Minnesota.

Elsewhere there have been few wide-scale changes in the Canada Warbler’s breeding range in the past 100 years. Forest clearing and development have resulted in some range retractions, such as in southern Michigan and eastern Pennsylvania. Meanwhile, in areas of New England, where forest cover is increasing, populations appear to be more widely distributed (Reitsma et al. 2009).

*Note that the definition of confirmed nesting of a species is different for Breeding Bird Atlas projects, including the definition used by the Minnesota Breeding Bird Atlas, compared with a more restrictive definition used by the Minnesota Ornithologists’ Union. For details see the Data Methods Section.

Across its breeding range, the Canada Warbler occupies a wide range of deciduous and coniferous forest habitats. In western Canada it is most prominent in mature aspen forests, in the Appalachian Mountains it is common in rhododendron thickets in hemlock ravines, and in Ontario it is common in white cedar swamps and alder thickets (Phinney 2015; Dunn and Garrett 1997; Cadman et al. 2007). On the Chippewa and Superior National Forests in northern Minnesota, the species was most strongly associated with upland mixed forests followed by lowland conifers (Figure 5; Grinde and Niemi 2016). Habitats within 200 m of MNBBA point counts where Canada Warblers were detected were dominated by upland coniferous stands (boreal jack pine/black spruce and white spruce/fir) followed by northern mixed forest (Figure 6).

The common theme in all sites is the availability of shrubs in moist habitats (Grinde and Niemi 2016; Reitsma et al. 2009). Many sites not only are mesic or moist but are located adjacent to wetlands, small creeks, and streams. Most stands also are mature with gaps in the canopy that promote a dense growth of shrubs and young saplings in the understory. Such gaps are created by selective timber harvest or by natural disturbances such as windstorms, tree falls, or insect outbreaks (Grinde and Niemi 2016).

Microhabitat features also contribute to a site’s suitability. Not only is the Canada Warbler a ground-nesting species, it also spends most of its time below the forest canopy. In addition to a well-developed shrub layer, a rich, uneven ground layer, covered with mossy hummocks, ferns, root masses, and woody debris, is an important habitat component (Goodnow and Reitsma 2011; Lambert and Faccio 2005).

In the northeastern United States, breeding densities were highest in lowland and riparian forests with well-developed shrub layers; densities were lower in mature upland forests where disturbance created canopy gaps that encouraged shrub growth. Even-aged forests were not utilized. Removing shrubs and saplings by thinning or deer browsing reduces a stand’s suitability (Lambert and Faccio 2005).

The species’ strong preference for older stands (> 80 years) was noted by Zlonis and Niemi (2014). Their work demonstrated the species’ higher abundance in unmanaged forests of the Boundary Waters Canoe Area Wilderness compared with managed stands on the Superior National Forest. Unmanaged stands were older and contained more structure and age diversity than the managed stands; the diversity was due to natural disturbances that created small canopy gaps. Grinde and Niemi (2016) also pointed out that before the canopy closes, young stands with a complex structure on the forest floor and a dense shrub layer can provide suitable Canada Warbler habitat.

Finally, the Canada Warbler is considered sensitive to forest fragmentation (Reitsma et al. 2009). Grinde and Niemi’s work in northern Minnesota supports that designation. They found that a greater percentage of forest cover within 500 m of a forest stand positively affected the site’s suitability.

Using data gathered from the federal Breeding Bird Survey (BBS), biologists with the Partners in Fight Initiative have estimated a North American population of 3 million breeding adults (Rosenberg et al. 2016). In 2013 Minnesota was estimated to support approximately 5% of the continental population, which would place the current statewide estimate at 150,000 individuals (Partners in Flight Science Committee 2013). The statewide estimate using MNBBA data is nearly 1.1 million adults (95% confidence interval of 866,000 to 1.9 million).

In the United States, Minnesota is one of only four states where the average number of Canada Warblers observed per BBS route in the state is equal to or greater than 1. In Minnesota, Vermont, and New Hampshire, the average is 1 bird per route each year; in Maine the average is 2 birds per route. Given the size of Minnesota and the small portion that actually supports the most suitable forest habitat, it is instructive to examine the five BBS routes that occur within Lake and Cook Counties. Here the average number of Canada Warblers per route is 7.25, ranging from a low of 1.5 along the Hovland Route (#31) in eastern Cook County to a high of 12 on the Lockport Route (#32) in western Cook County.

Local breeding densities generally range from 4 to 20 pairs per 40 ha (Reitsma et al. 2009). On the Chippewa National Forest the long-term National Forest Bird (NFB) Monitoring Program detected an average of 0.9 pairs per 40 ha; on the Superior National Forest an average of 4.4 pairs per 40 ha were detected. On the Superior, Canada Warblers reached their highest abundance in mature aspen-spruce-fir forests, where breeding densities averaged 6.1 pairs per 40 ha (Niemi et al. 2016)

Since the federal BBS began in 1966, the Canada Warbler has shown a statistically significant decline across its range of 2.28% per year (Sauer et al. 2017). The species has experienced a cumulative decline of 62% since 1970 (Rosenberg et al. 2016). Factors responsible for the decline are largely unknown but likely include the loss and degradation of winter habitat in South America and breeding habitat in North America, plus collisions with buildings and communication towers (Reitsma et al. 2009). Grinde and Niemi (2016) suggest that long-term declines may also be the result of forest management practices that alter natural disturbance regimes. Harvesting upland deciduous stands before they have reached full maturity (> 80 years) stops natural processes that improve the stands’ suitability for Canada Warblers at the mature and old-growth stages.

Contrary to the national trend, the Canada Warbler population in Minnesota has been relatively stable since 1967 (Figure 7). On Minnesota’s two national forests, the NFB Monitoring Program shows that the species has had a significant increase on the Chippewa National Forest from 1995 to 2016 (4.13% per year). Populations remained stable on the Superior National Forest and demonstrated a significant increasing trend at the regional level (1.05% per year; Figure 8).

Partners in Flight has assigned the Canada Warbler a Continental Concern Score of 14/20, a relatively high score among Minnesota’s warblers, exceeded only by a score of 16/20 for the Golden-winged Warbler and 15/20 for the Cerulean Warbler (Rosenberg et al. 2016). Partners in Flight also added the species to its Yellow Watch List, a group of 12 species that warrant constant care to prevent further decline. Threats to the Canada Warbler’s wintering habitat and concern regarding its widespread decline are the primary factors that elevated its score.

Canada has classified the species as a federally threatened species in light of its rather precipitous population decline from 1968 to 2007. BBS data demonstrate a cumulate decline of 85% during this time period, a trend that is particularly acute in Ontario, Quebec, and the Maritime Provinces, where the majority of the population nests (Committee on the Status of Endangered Wildlife in Canada 2017).

Although its declining population has garnered the attention of many conservation organizations, the Canada Warbler is not officially listed in Minnesota or at the federal level. Regionally it is a focal species for the Upper Mississippi River & Great Lakes Region Joint Venture. In Minnesota it was designated a Species in Greatest Conservation Need in 2006 but was dropped from the list in 2015 (Minnesota Department of Natural Resources 2015).

In addition to concerns about the species’ breeding and wintering habitat, climate change also poses a significant threat in the future. Recent work to assess the impact of warming temperatures on North American birds by the National Audubon Society classified the species as “climate threatened” and predicted that the Canada Warbler could lose all of its current summer habitat by the year 2080 (Langham et al. 2015; National Audubon Society 2017).  Like so many boreal-nesting species, warming temperatures may significantly reduce the availability of habitat further north.

In the meantime, specific guidelines to protect and improve habitat for the species have been outlined in detail by Lambert and Faccio (2005). Recommendations focus largely on protecting and managing large, mesic, mixed forest stands with a dense understory, a structurally complex forest floor, and a semiopen canopy. Overall, the species responds well to forest management that maintains or improves the shrub layer within a few years following harvest.

In the future, forest management plans and timber harvest activities in the core of the species’ North American breeding range in Cook and Lake Counties need to be sensitive to the habitat requirements of the Canada Warbler. Minnesota plays a vital role in the future of the “Necklaced Warbler.”