- Minnesota Breeding Bird Distribution
- Breeding Habitat
- Population Abundance
- Literature Cited
A regular breeding resident and migrant. The Bay-breasted Warbler was rare during the Minnesota Breeding Bird Atlas (MNBBA).
Found patchily distributed across Canada from the Maritime provinces to British Columbia and in the northern portions of the Upper Midwest and northeastern United States. Highest densities have been observed in eastern Quebec (Figure 1).
Assigned a 10/20 Continental Concern Score by Partners in Flight. Identified as a Species in Greatest Conservation Need by the Minnesota Department of Natural Resources. Designated a Regional Forester Sensitive Species in the Superior National Forest by the U.S. Forest Service.
Long-distance migrant; the species overwinters from Panama to northern South America.
Arthropods, especially caterpillars, all gleaned from trees, although the species will occasionally fly-catch.
Cup nest placed on a horizontal branch of a coniferous tree and highly variable in height.
Roberts (1932) described the Bay-breasted Warbler as “a rare summer resident in the northern evergreen forest” of Minnesota, a status that remains the same today. The species is the rarest of the three spruce budworm specialists, which also include the Tennessee and Cape May Warblers. It was one of the species intensively studied by MacArthur (1958) in his now famous studies on the population ecology of warblers.
Roberts did not report that any nests were found but documented breeding observations of pairs or specimens collected from Cass Lake and Itasca Park, and from Cook, Lake, and Sherburne Counties. The observation from Cass Lake was from July 25, 1916, so it may have been an early fall migrant, but the observations from Sherburne County were from July 9–20, 1925, in a tamarack bog. Regarding the latter observation, Roberts stated, “several seen in a tamarack swamp and both male and female taken in breeding condition.”
More than 40 years later, Green and Janssen (1975) described the Bay-breasted Warbler’s distribution as primarily Cook, Lake, and northern St. Louis Counties. However, they proposed a potential distribution as far west as eastern Roseau and Clearwater Counties and south to northern Hubbard and southern Itasca Counties. They included confirmed nesting only from northern Lake County. A few years later, Janssen (1987) restricted the warbler’s primary potential breeding range to only the northern portions of Beltrami, Cook, Lake, and St. Louis Counties, but he did include all of Koochiching and Lake of the Woods Counties as well as west to eastern Roseau County. Janssen (1987) and Hertzel and Janssen (1998) both identified confirmed nesting in Cook and Lake Counties since 1970.
The Minnesota Biological Survey (MBS) has recorded 16 breeding season locations during their intensive surveys of Minnesota counties. All of their locations were found in Cook, Lake, and St. Louis Counties. At the time of writing, they had not yet completed surveys in northern Beltrami, Koochiching, or Lake of the Woods Counties.
The MNBBA documented 46 breeding records for the Bay-breasted Warbler. These records included breeding evidence from 30 blocks, including 4 confirmed breeding records from northern St. Louis and Lake Counties (Figure 2). Probable nesting was recorded from Cook County, and a possible nesting in northern Aitkin County. These records represented only 0.6% (30/4,733) of the blocks in Minnesota, or 0.8% (19/2,337) of priority blocks covered during the MNBBA (Figures 2 and 3; Table 1). The lack of nesting records in Minnesota reflected a combination of this species’ genuine rarity, the difficulty in its identification, and a possible decline in its population in Minnesota.
With the exception of Roberts’s (1932) reporting of possible Bay-breasted Warblers nesting in Sherburne County, this species’ distribution does not appear to have changed in Minnesota for more than 100 years. Its breeding distribution is closely tied to spruce budworm outbreaks. In their review of the Bay-breasted Warbler in North America, Venier et al. (2011) simply stated that there is “No information” available on historical changes to its distribution.
In Wisconsin, the Bay-breasted Warbler is listed under the title “Seldom Reported Species.” It has never been confirmed as a nesting species in Wisconsin, and the state’s breeding bird atlas from 1995–2000 reported 1 probable nesting in Oneida County in northeastern Wisconsin and possible nesting from 3 other counties. In Michigan, the first confirmed breeding record was discovered in 1933. The species has been most consistently found in the Upper Peninsula, especially in the Keweenaw Peninsula (Chartier et al. 2013). Bay-breasted Warblers were very rare in the northern portions of the Lower Peninsula. The species is widely distributed in the northern portions of Ontario. Cadman et al. (2007) found no differences in Ontario between its first (1981–1985) and second breeding bird atlas (2001–2005).
*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.