- Minnesota Breeding Bird Distribution
- Breeding Habitat
- Population Abundance
- Literature Cited
A regular breeding resident and migrant; the Connecticut Warbler was an uncommon species during the Minnesota Breeding Bird Atlas (MNBBA).
The Connecticut Warbler is one of the rarest and most narrowly distributed wood warblers in the northern coniferous forest, second only to the federally endangered Kirtland’s Warbler. Its breeding range stretches across a narrow band of the Canadian boreal forest from western Quebec to eastern British Columbia. South of Canada it is restricted largely to the northern Great Lakes states, specifically, northern Minnesota, northern Wisconsin, and the Upper Peninsula of Michigan. Despite its name, the Connecticut Warbler does not breed in Connecticut, only passing through as an uncommon migrant. The species is sparsely distributed throughout its restricted breeding range, and the densest breeding populations are concentrated in a very small region of southern Ontario and northern Minnesota (Figure 1).
Assigned a Continental Concern Score of 13/20 and designated a Yellow Watch List species by Partners in Flight; designated a Species in Greatest Conservation Need by the Minnesota Department of Natural Resources.
A long-distance migrant that winters in South America.
An insectivore that gleans food from foliage, downed logs, and the ground.
Open-cup nest placed on or near the ground, often in a mossy hummock.
A large portion of Roberts’s (1932) account of the Connecticut Warbler is spent describing the years of ornithological exploration that ensued between the time of the bird’s original discovery in North America in the early 19th century and documentation of its presence as a nesting species in Minnesota. Indeed, over 70 years passed between the year 1812, when the species was first described from a migrant specimen collected in its namesake, Connecticut, to 1883, when the first nest was discovered in Manitoba. Forty-three more years passed before a second nest was found, in Alberta.
Although the Connecticut Warbler was known as an uncommon migrant in Minnesota, summer records were scanty. The first was a bird collected on the state border at Pembina, North Dakota, in 1879. The second was a male collected in 1886 in St. Louis County. Summer reports of singing birds in Aitkin, Isanti, and Marshall Counties and in Itasca State Park soon followed. Adults carrying food were observed in both Isanti County and Itasca State Park, but the discovery of a nest eluded observers until the summer of 1929, when two nests were found in an extensive black spruce–tamarack bog in Aitkin County. Relying on these accounts, Roberts (1932) described the species’ breeding range as stretching from Isanti County in east-central Minnesota to Itasca State Park and eastern Marshall County in northwestern Minnesota. The species, he wrote, “so far as discovered, makes its summer home only in cold tamarack and spruce swamps of typical Canadian Zone character. Such places are numerous and wide-spread in northern Minnesota.”
Forty years later, Green and Janssen (1975) described the warbler as a summer resident throughout northeastern and north-central Minnesota. They considered it rare throughout much of the region with the exception of an area stretching “from Koochiching County northwestward,” where it was “plentiful” in lowland black spruce–tamarack bogs and in upland jack pine and aspen stands. In addition to the confirmed nesting records that Roberts (1932) reported in Aitkin County, records were now available from Hubbard and Lake Counties as well. Farther south, the birds had not been reported from Isanti County since the early 20th century.
When Janssen (1987) provided an updated account several years later, he included a distribution map that delineated 4 counties where nesting had been confirmed since 1970: Aitkin, Beltrami, Hubbard, and Lake. The southern periphery of the species’ Minnesota breeding range was identified as northern Pine County in the east and Hubbard County in the west. Hertzel and Janssen (1998) later added Itasca County to the list of counties with confirmed nest records since 1970.
Fieldwork conducted by the Minnesota Biological Survey (MBS) in northern Minnesota further confirmed the species’ restricted distribution. Although work had yet to be conducted in the far north-central counties of the state, the most southern breeding locations were in northern Pine County and southern Aitkin County. To the west, breeding locations were documented in eastern Kittson County, Roseau County, and northeastern Marshall County (Minnesota Department of Natural Resources 2017).
MNBBA participants tallied a total of 263 Connecticut Warbler detections in 3.4% (161/4,734) of the surveyed atlas blocks and in 3.6% (84/2,337) of the priority blocks. Breeding evidence was documented in only 2 blocks (Figures 2 and 3; Table 1). The birds were reported from 16 of Minnesota’s 87 counties and were confirmed breeding in 2 counties: Aitkin and St. Louis. The Aitkin County record was an adult carrying food; the St. Louis County record was an observation of an adult carrying nesting material.
The predicted breeding distribution map of the Connecticut Warbler, generated using MNBBA data, shows relatively low abundances in much of the southern and eastern portions of the species’ breeding range (Figure 4). However, moderate breeding densities are predicted in portions of southern and central St. Louis County, where extensive peatlands occur, and throughout the entire Northern Minnesota and Ontario Peatlands Section, north and east of Red Lake. The flat, poorly drained landscape in this ecological section is dominated by peatland communities, including black spruce bogs and tamarack swamps, the Connecticut Warbler’s preferred habitat.
Overall, the species’ distribution during the MNBBA was nearly identical to that documented by the MBS, with records as far south as central Pine County and as far west as eastern Kittson County. Other than the loss of the small nesting population in Isanti County, the distribution of this rare little warbler is remarkably unchanged over the past 100 years. The same is true throughout its very narrow and restricted breeding range in the northern Great Lakes and Canada (Pitocchelli et al. 2012). Wisconsin’s first breeding bird atlas confirmed what had been known regarding the species’ distribution for at least the previous few decades (Cutright et al. 2006). Michigan, however, documented a significant decline in the number of records in the northern reaches of the Lower Peninsula between their first atlas, conducted in 1983–1988 and their second atlas, conducted in 2002–2008. There also were fewer detections in the Upper Peninsula in the second atlas (Chartier et al. 2013). By contrast, Ontario documented a 94% increase in the number of detections during their second atlas. The change, they noted, could have been attributed to a real population increase or to an increase in atlas efforts, or it could have simply been “an artifact of a small data set” (Cadman et al. 2007).
*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.