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Black-and-white Warbler

Mniotilta varia
Overview
Minnesota Seasonal Status:

A regular breeding resident and migrant; the Black-and-white Warbler was a common species during the Minnesota Breeding Bird Atlas (MNBBA).

North American Breeding Distribution and Relative Abundance:

Found throughout the boreal forests of Canada, the Great Lakes region, and New England, south through the Appalachian Mountains and west to central Louisiana and central Texas. Pockets of abundance are scattered throughout its breeding range, but the highest breeding densities within the federal Breeding Bird Survey (BBS) area are found across the Boreal Hardwood Transition Region that stretches from Minnesota, across the Great Lakes, and through New England (Figure 1).

Conservation Concern:
Conservation Status Score 11

Assigned a Continental Concern Score of 11/20 by Partners in Flight.

Life History
Migration:

A long-distance migrant that winters anywhere from the southeastern United States to northern South America.

Food:

Primarily a bark forager consuming a variety of insects, especially caterpillars.

Nest:

An open-cup nest placed on the ground at the base of a shrub or tree; occasionally nests in a stump cavity.

Black-and-white Warbler Black-and-white Warbler. Mniotilta varia
© David Brislance
Figure 1.

Breeding distribution and relative abundance of the Black-and-White Warbler in North America based on the federal Breeding Bird Survey, 2011-2015 (Sauer et al. 2017).

Minnesota Breeding Bird Distribution*

Describing the Black-and-white Warbler’s presence in Minnesota in the late 19th and early 20th centuries, Roberts (1932) reported the species as a rare breeder in southern Minnesota but more common farther north. Regarding its presence in the southeastern woodlands, he wrote:

Formerly the Black and White Warbler was present as a summer resident throughout the southern part of the state wherever there was heavy timber, but in recent years, since the cutting of the deciduous forests, it has become rare as a nesting bird south of the evergreen forests farther north.

His only breeding evidence for this region was a 1904 report of an adult feeding young in Lake City in Wabasha County, and a simple note of a bird or pair “evidently with nest” that he personally observed in Minneapolis in 1875.

He described the species’ abundance in northern Minnesota as “nowhere exactly common,” but he said it was certainly a regular species. Confirmed nesting records (nests with eggs or young) were reported only from the Mille Lacs region and Becker County. Inferred nesting records were reported from Cass Lake (feeding young out of nest), Cook County (young with adults), and Roseau County (pair evidently with nest).

Forty years later, Green and Janssen (1975) described the species as a summer resident primarily in northeastern and north-central Minnesota. Farther south it occurred east of the Mississippi River but was “quite scarce south and west of Mille Lacs Lake.” In addition to Roberts’s confirmed nesting record from Becker County, records were now available from Anoka, Cook, Lake, and St. Louis Counties. No breeding records had been reported from southeastern or northwestern Minnesota in over 50 years.

Several years later, Janssen (1987) reported that recent observations suggested the Black-and-white Warbler’s breeding range might extend as far south as northern Hennepin County in east-central Minnesota and as far west as Roseau and Kittson Counties in northwestern Minnesota. Since 1970 confirmed nesting records were available from 8 counties: Aitkin, Anoka, Cass, Clearwater, Cook, Itasca, Lake, and St. Louis.

The first confirmed breeding record in southeastern Minnesota in more than 75 years occurred in 1990 when a pair was reported breeding in Winona County (Wiens 1991). Nevertheless, despite extensive surveys of remnant forest tracts in this region by the Minnesota Biological Survey (MBS) in the 1990s, no additional observations or breeding records were reported. In northwestern Minnesota, however, the MBS found the species in numerous locations, from Todd and Otter Tail Counties north to Marshall, Kittson, and Roseau Counties (Minnesota Department of Natural Resources 2016).

During the MNBBA, participants reported 3,423 Black-and-white Warbler records in 35.0% (1,662/4,748) of the surveyed atlas blocks and in 41.7% (976/2,338) of the priority blocks. Breeding was confirmed in 83 blocks (Figures 2 and 3; Table 1). The species was reported from 47 of Minnesota’s 87 counties (2 possible records straddled 2 counties each: Nicollet/Brown and Scott/Dakota) and was confirmed breeding in 19 counties. Twelve of the counties were additions to a list of counties with confirmed nesting identified by Hertzel and Janssen in 1998. Most notable were 2 breeding records in southeastern Minnesota: a bird carrying nesting material in southwestern Fillmore County (May 10, 2012), and an adult feeding young in northern Goodhue County (August 20, 2010; photos taken). The overwhelming majority of records, however, were from the Laurentian Mixed Forest Province; the species was also well distributed throughout the Tallgrass Aspen Parklands Province in northwestern Minnesota.

All atlas records south of the species’ primary range were examined carefully. Because the Black-and-white Warbler is a common spring and fall migrant throughout the state, late May and early June records may be observations of late migrants. Indeed, at least 9 “possible” records were invalidated based on the date of the detection, the lack of suitable breeding habitat, and/or insufficient supporting documentation. The “observed” record in Lyon County was retained as it was a report of 2 birds singing the first week of June in suitable habitat in Camden State Park. The Minnesota River “observed” record on the boundary of Brown and Nicollet County was a record from June 22 also in suitable habitat.

The MNBBA predicted breeding distribution map (Figure 4) predicted the Black-and-white Warbler is most abundant in the extensively forested landscape of the Laurentian Mixed Forest Province. It also predicted the species to be common, although at lower densities, throughout the Tallgrass Aspen Parklands Province and the northern reaches of the Eastern Broadleaf Forest Province. Scattered pockets of abundance are present in suitable habitat throughout southern and central Minnesota as well, especially in western Hennepin County and along the Mississippi River south of the Twin Cities.

The Black-and-white Warbler has certainly not returned to the level of abundance in southern Minnesota described by Roberts nearly 100 years ago. Nevertheless, it is more frequently encountered west and south of the primary breeding range that Janssen delineated in 1987. It now is frequently observed, for example, in the northern regions of east-central Minnesota including Sherburne, Isanti, Chisago, and northern Anoka Counties. Perhaps as forest remnants in the region have matured, more suitable habitat is now available.

Elsewhere within its breeding range, there have been few large-scale changes to the warbler’s breeding distribution, particularly in recent decades. One 19th-century ornithologist, Thomas Nuttall, considered the warbler to be common throughout the eastern and central United States, where it is only sparsely distributed today (Kricher 2014). More recently, atlas results in Wisconsin, Ohio, and Ontario have reported few changes other than those predicted by local habitat changes (Cutright et al. 2006; Rodewald et al. 2016; Cadman et al. 2007). Interestingly Michigan’s second atlas documented more records in the southern region of the Lower Peninsula than were reported during their first atlas (Chartier et al. 2013).

*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.

Figure 2.

Breeding distribution of the Black-and-white Warbler in Minnesota based on the Breeding Bird Atlas (2009 – 2013).

Print Map
Figure 3.

Summary statistics of observations by breeding status category for the Black-and-white Warbler in Minnesota based on all blocks (each 5 km x 5 km) surveyed during the Breeding Bird Atlas (2009-2013).

Breeding statusBlocks (%)Priority Blocks (%)
Confirmed83 (1.7%)61 (2.6%)
Probable634 (13.4%)453 (19.4%)
Possible941 (19.8%)462 (19.8%)
Observed4 (0.1%)0 (0.0%)
Total1,662 (35.0%)976 (41.7%)
Table 1.

Summary statistics for the Black-and-white Warbler observations by breeding status category for all blocks and priority blocks (each 5 km x 5 km) surveyed during the Minnesota Breeding Bird Atlas (2009-2013).

Figure 4.

Predicted breeding distribution (pairs per 40 hectares) of the Black-and-white Warbler in Minnesota based on habitat, landscape context, and climate data gathered during the Minnesota Breeding Bird Atlas (2009-2013) using the General Linear Modeling method with an adjustment for detectability.

Breeding Habitat

The Black-and-white Warbler occupies a broad range of deciduous and coniferous forest habitats, from upland maple-basswood and aspen-fir stands to lowland hardwoods and black spruce–tamarack stands (Figure 5; Danz et al. 2007). Although it generally prefers midsuccessional to mature forests with a closed canopy, a well-developed understory, and low shrub density, it can be found in more open, early successional stands (Crawford et al. 1981). Some studies suggest a preference for wetter forest stands (Kricher 2014).

The National Forest Bird (NFB) monitoring program in northern Wisconsin and Minnesota also documented a diverse array of habitats used by the species, including lowland shrubs, wetlands, young regenerating forests, and mature uplands, especially mixed stands of aspen, spruce, and balsam fir (Niemi et al. 2016). Habitat data collected within 200 m of MNBBA point counts where Black-and-white Warblers were detected found the strongest association was with pine forests, followed by northern mixed forest, upland coniferous forests, lowland coniferous forests, and bogs (Figure 6).

Despite its tolerance for a wide range of forest cover types, the Black-and-white Warbler is possibly an area-sensitive species with a low tolerance for habitat fragmentation. Robbins (1979, 1980) suggested a minimum of 300 ha is needed to sustain a viable breeding population. Whitcomb and his colleagues reported the species was absent from stands smaller than 70 ha (Whitcomb et al. 1981). Landscape context may be an important predictor of which stands can be used. One study found Black-and-white Warblers utilizing small tracts if they were close to larger forest tracts (Galli et al. 1976). Danz et al. classified the bird as a forest-interior species (Danz et al. 2007)

Figure 5.

Typical breeding habitat of the Black-and-White Warbler in Minnesota (© Lee A. Pfannmuller).

Figure 6.

Habitat profile for the Black-and-white Warbler based on habitats within 200 m of point counts where the species was present during the Minnesota Breeding Bird Atlas (2009-2013).

Population Abundance

Long-term monitoring data collected by the federal BBS have been used to generate a North American population estimate for the Black-and-white Warbler of 18 million breeding adults (Rosenberg et al. 2016). Several years earlier, in 2013, Partners in Flight estimated that Minnesota supported 3.9% of the continental population (Partners in Flight Science Committee 2013). Applying that percentage to the current continental estimate yields a statewide population of 700,000 breeding adults. When MNBBA point count data are incorporated into the statistical analyses, the statewide estimate is orders of magnitude higher, numbering approximately 5.6 million adults (95% confidence interval of 5.2 to 7.1 million adults).

Minnesota supports the highest breeding density of Black-and-white Warblers among the Great Lakes states. An average of 3 to 4 birds are detected per BBS route in the state each year. Across the entire Boreal Hardwood Transition Region of southern Canada and the United States, an average of 7 warblers are detected per route. This compares to a survey-wide average of only 2 birds per route (Sauer et al. 2017). Local estimates of population densities range from 5 to 29 territories per 40 ha at sites in Alberta, Arkansas, and Maryland. In northern Minnesota, the average density of Black-and-white Warblers across all major habitats in the Chippewa National Forest was 4.4 pairs per 40 ha, and across the Superior National Forest it was 6.6 pairs per 40 ha (Niemi et al. 2016). On the Chippewa the Black-and-white Warbler was most abundant in mature aspen-spruce-fir forest stands (13.4 pairs per 40 ha); on the Superior, it was most abundant in pole-sized jack pine stands (9.6 pairs per 40 ha).

Since the BBS monitoring program began in 1966, the Black-and-white Warbler has experienced a significant annual decline of 0.86% per year through 2015 (Sauer et al. 2017). Overall, biologists estimate the North American population has experienced a cumulative decline of 27% since 1970 (Rosenberg et al. 2016). In Minnesota, however, the statewide population trend has been stable and suggests a slight increase in the past 10 to 15 years (Figure 7). The NFB monitoring program, which covers the core of the species’ breeding range in Minnesota, documents a significant increasing population on both the Superior and Chippewa National Forests from 1995 to 2016 (Figure 8).

Regionally, population trends vary quite widely (Figure 9). Interestingly, BBS data document a declining population from 1966 to 2015 in northern Minnesota but an increasing population within the Eastern Broadleaf Forest, stretching from east-central Minnesota to northwestern Minnesota. Although the more intensive, off-road monitoring conducted by Niemi and his colleagues on the national forests is a better indicator of regional trends in northern Minnesota since 1995, the BBS is clearly detecting an increasing number of reports of Black-and-white Warblers west and south of the species’ primary range, an outcome also noted in the MNBBA. Unfortunately, little is known about the factors influencing local population trends other than local habitat conditions and weather events.

Figure 7.

Breeding population trend for the Black-and-white Warbler in Minnesota from 1967-2015 based on the federal Breeding Bird Survey (Sauer et al. 2017).

Figure 8.

Breeding population trends of the Black-and-white Warbler in the Chippewa ad Superior National Forests of Minnesota from 1995-2016 (Bednar et al. 2016).

Figure 9.

Population trend map for the Black-and-white Warbler in North America for 1966-2015 based on federal Breeding Bird Survey (Sauer et al. 2017).

Conservation

In light of the species’ relative abundance and broad habitat niche, it is not considered a conservation priority at either the state or federal level. Its moderate Continental Concern Score of 11/20 largely reflects concern for its declining population trend (Rosenberg et al. 2016). Nevertheless, the species warrants attention from a forest management perspective based on its area sensitivity. Although few studies have closely examined factors that regulate local populations, collisions with windows and communication towers have been documented as major hazards impacting populations (Kricher 2014; Longcore et al. 2013).

A recent study on the potential impacts of climate change by the National Audubon Society also gives cause for some concern. The Black-and-white Warbler was classified as a “climate threatened” species based on a modeling analysis that predicted the species could lose 83% of its current summer range by the year 2080 (National Audubon Society 2016; Langham et al. 2015). Its plight is not dissimilar to that predicted for many boreal nesting songbirds. Boreal forests in general are likely to be affected more than tropical or temperate forests by warming temperatures due to predicted increases in the frequency of fire and other disturbances (North American Bird Conservation Initiative 2010).

In the short-term, the unmistakable Black-and-white Warbler, foraging like a nuthatch up and down tree trunks, is doing quite well in Minnesota. Populations are stable to increasing, and the warbler appears to be slowly reoccupying portions of its former range to the south and west of the northern mixed forest as local habitat conditions improve.

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