- Minnesota Breeding Bird Distribution
- Breeding Habitat
- Population Abundance
- Literature Cited
A regular breeding resident and migrant in Minnesota. Nashville Warblers were an abundant species during the Minnesota Breeding Bird Atlas (MNBBA).
Two subspecies are found in separate distinct breeding populations: one (Oreothlypis ruficapilla ruficapilla) in the northeastern and upper midwestern United States and Canada, and the other in the western United States and Canada (O. r. ridgwayi). Highest densities are found in northeastern Minnesota but also are substantial in the Canadian provinces of Ontario and Quebec (Figure 1).
Assigned a Continental Concern Score of 9/20 by Partners in Flight; identified as a stewardship species by Audubon Minnesota.
Long-distance migrant that overwinters in southern Texas, Mexico, and south as far as Panama; casual winter visitor in the Caribbean.
Gleans insects from foliage.
On the ground in thick foliage or moss hummocks.
The Nashville Warbler is historically described as breeding throughout the forested part of the state, especially in the tamarack swamps and the mixed tamarack and spruce swamps (Roberts 1932). During Roberts’s (1932) time in Minnesota, from the late 1800s to the early 1900s, the species was “sparingly” found in Hennepin and adjacent counties “in and about the numerous large tamarack swamps, but since these have been largely cut off or drained, it has disappeared as a nesting bird.” Roberts then reported the breeding range as confined almost entirely to the Canadian Zone, from northern Isanti County on the south, and as far west as the evergreens extended. He documented breeding activity at seven locations, and all were in tamarack and spruce swamps. They included Aitkin, Becker, and Itasca Counties, plus at Cass Lake, Leech Lake, Itasca Park, and Mille Lacs.
More than 40 years later, Green and Janssen (1975) emphasized that the Nashville Warbler’s primary breeding distribution is in north-central and northeastern Minnesota. They added confirmed nesting records from as far south as Anoka County, and additional confirmed nesting in Clearwater, Crow Wing, Itasca, Lake, and St. Louis counties. They also noted that the Nashville Warbler uses tamarack stands in the southern and western regions of its breeding range. In 1987 Janssen reported confirmed nesting in 12 counties since 1970, of which Beltrami, Cook, Hubbard, Lake of the Woods, and Mille Lacs Counties were new records. Several years later, Hertzel and Janssen (1998) added confirmed nesting in Koochiching County since 1970.
The Minnesota Biological Survey (MBS) recorded 2,078 breeding season locations during their inventories of counties in the state. Their locations also were dominated by those in northeastern and north-central Minnesota but included breeding observation locations throughout the northwestern counties, including Kittson, Mahnomen, Marshall, and Polk Counties. Observations were also made southwest to Douglas and Otter Tail Counties, and south to Anoka, Chisago, Stearns, and Washington Counties.
The MNBBA further documented the extensive distribution and abundance of the Nashville Warbler in the forested regions of Minnesota but also the struggle to confirm nesting activity (Figure 2). The difficulty in finding nests of this species was a point noted also by Roberts (1932) and many regional breeding bird atlases. A total of 4,370 breeding records of this species were gathered during the MNBBA. The ratio of blocks with confirmed nesting to all the other observations was only 7% (104 confirmed blocks to 1,529 blocks where the species was recorded) (Figure 3; Table 1). The MNBBA extended confirmed nesting of the Nashville Warbler to northwest Minnesota, including Marshall, Pennington, and Roseau Counties, and south to Hennepin, Morrison, and Washington Counties. In addition, nesting was confirmed in the east-central region in Carlton and Pine Counties.
It is unknown whether the Nashville Warbler has declined in abundance, but Roberts (1932) noted that its range had contracted from the southern portions of its former range in Minnesota due to the loss of forests, especially large tamarack swamps. In Minnesota the species is found more commonly in coniferous forests than deciduous forests. It also responds positively to successional stages in forests following disturbances such as fire, logging, and wind (Pfannmuller 2012). Coniferous forests and forests where there have been disturbances are found primarily in the northeastern, northern, and north-central portions of the state.
The infrequent encounter of this species by early explorers suggests that its population may have expanded more recently. During the 20th century, Minnesota was increasingly cleared of mature upland forests, especially in the northern portions of the species’ breeding range. Cadman et al. (1987) similarly concluded that the species had benefited in Ontario from extensive clearing and lumbering operations during the past century. In contrast, extensive development of farmland in the southern portions of the province may have led to local extirpation of its breeding population. Lowther and Williams (2011), in their review of the Nashville Warbler in North America, cited evidence that the species became far more widespread in the early and mid-1800s in New England but then declined again in numbers and contracted its range when forests regrew in the late 19th and early 20th centuries.
The documentation of nesting in Hennepin, Morrison, and Washington Counties is likely due to a combination of (1) a more liberal definition of confirmed nesting used in breeding bird atlas accounts compared with those used by the Minnesota Ornithologists’ Union, and (2) the expanded and more intensive coverage of nesting activity by the MNBBA. It is possible that small populations have always existed in these more southerly locations, but they are likely to be more variable and not present every year.
*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.