Blue-winged Warbler

A regular breeding species and migrant; the Blue-winged Warbler was uncommon in abundance during the Minnesota Breeding Bird Atlas (MNBBA).

Formerly an inhabitant of the Ozark Mountains east through the open woodlands of Tennessee, Kentucky, northern Alabama, and northern Georgia. Blue-winged Warblers have been expanding northward and now can be found throughout southern New England, southern Ontario, and the southern portions of the Great Lakes states. The species is sparsely distributed throughout its breeding range; some of the highest breeding densities can be found in the Appalachian Mountains, the mid-Atlantic states, and in western Wisconsin and Michigan (Figure 1).

Assigned a Continental Concern Score of 13/20 by Partners in Flight.

A medium-distance migrant that winters in Central America and the Caribbean islands.

Insects and spiders secured by foliage gleaning and probing.

An open-cup nest placed on or near the ground at the base of a shrub or a tussock of herbaceous vegetation.

The Blue-winged Warbler is one of many southern species that has expanded its range north along the Mississippi River floodplain, especially during the 20th century. Roberts (1932) provided a brief history of the warbler’s first appearance in the state in May 1880, when he found a male at Minnehaha Falls in Hennepin County. A few years later, in May 1885, Dr. Hvoslef found the species near the town of Lanesboro in Fillmore County. In 1889 he found a family of 4 in the same area, and by 1891 he had documented the first nest. In the following years, reports were logged from another site in Fillmore County in 1916 (near the town of Preston), again in Hennepin County in 1919, in Goodhue County in 1923, and in Winona County in 1922. Detailing these accounts, Roberts summarized the species’ distribution in the early 1900s as restricted to “a narrow area along the southeastern border of the state.”

When Green and Janssen (1975) provided an updated account of the species’ status, Blue-winged Warblers had been reported as a summer resident as far north as Dakota and Washington Counties. Reports of migrants were available farther north in Anoka County and to the west in Kandiyohi, Nicollet, Sherburne, Stearns, and Wright Counties, suggesting the bird might be even more widely distributed during the breeding season. The authors speculated that most of the northern expansion occurred in the 1940s, although they note that the timing may have coincided with an increase in the number of active observers in the state. Coincident with the expansion was an increase in the number of reports of Brewster’s and Lawrence’s Warblers, both hybrids between the Blue-winged Warbler and Golden-winged Warbler. Most reports of hybrids were from Goodhue, Hennepin, and Winona Counties.

When Janssen provided another update in 1987, he identified 6 counties where nesting had been confirmed since 1970: Hennepin, Houston, Olmsted, Scott, Washington, and Winona. By 1998 nesting had been confirmed in Dakota and Fillmore Counties as well (Hertzel and Janssen 1998). Observations were now being reported farther west along the Minnesota River valley in Scott and Nicollet Counties, and as far north as Stearns and Otter Tail Counties.

Beginning in the late 1980s and early 1990s, as the Minnesota Biological Survey began field work in the southern half of the state, field biologists documented a total of 107 Blue-winged Warbler breeding season locations. Although most records were restricted to the southeastern corner of the state, scattered reports were documented as far north as Morrison County in east-central Minnesota, and as far west as Lincoln County in southwestern Minnesota. The species was well represented along the lower Minnesota River valley from Shakopee to Mankato (Minnesota Department of Natural Resources 2016).

Additional northern records were documented during the MNBBA when participants reported 213 Blue-winged Warbler detections in 2.7% (129/4,737) of the surveyed atlas blocks and in 3.8% (88/2,337) of the priority blocks. The most northerly reports came from southern Cass County and northern Mille Lacs County; the most westerly record was documented near the South Dakota border in Big Stone County. Breeding evidence was documented in only 11 of the surveyed blocks but included 3 records north of the Twin Cities, in Anoka, Sherburne, and southern Pine Counties (Figures 2 and 3; Table 1). The birds were detected in 33 of Minnesota’s 87 counties and were confirmed breeding in 10 counties (2 blocks with confirmed breeding straddled Dakota and Scott Counties and one block straddled Carver and Scott Counties). Six counties were new to the list published by Hertzel and Janssen (1998): Anoka, Carver, Pine, Rice, Sherburne, and Wright.

The Blue-winged Warbler’s gradual range expansion in Minnesota originally was confined largely to the Eastern Broadleaf Forest Province. As the province was settled by European immigrant farmers, the forests were cleared for farmland and timber, soon providing ample early successional habitat favored by the species. The expansion witnessed in Minnesota was simply a continuation of a much broader movement north that had begun in many eastern states in the late 1800s and early 1900s (Gill et al. 2001). In the Upper Midwest, the birds were first reported in Wisconsin in 1867 (Cutright et al. 2006) and in Michigan in 1879 (Brewer et al. 1991).

As reports of Blue-winged Warblers increased in Minnesota and the species continued to expand northward, contacts with the more northerly distributed Golden-winged Warbler increased. This resulted in hybridization between the two species and produced two phenotypically distinct and fertile hybrids noted earlier: Brewster’s Warbler and Lawrence’s Warbler. Indeed a Lawrence’s Warbler hybrid was reported as early as 1945 in the Twin Cities (Eastman 1958), and a Brewster’s Warbler hybrid in 1956 at Whitewater State Park (Theodore 1956).

In their excellent review of the changing distribution of the two species and their hybrids, Svingen and Hertzel (2015) examined how the two species have come into increasingly frequent contact over the years. Their exhaustive review of both published and unpublished records found a total of 125 hybrid records in Minnesota since 1945; 108 of the records were from the southern half of the state. The authors noted the increasing rate at which hybrids were being reported. Although the increasing number of reports may be due to several factors, such as more observers actively searching for the birds, it does not bode well for the future of the Golden-winged Warbler. Another study by Vallender and her colleagues demonstrated that the only genetically “pure” population in the Golden-winged Warbler’s entire breeding range is in southern Manitoba, at the very northwestern edge of its range. Of the 96 Golden-winged Warblers sampled in Minnesota, 1 had genetic material acquired by hybridization with Blue-winged Warblers (Vallender et al. 2009). Although the rate of genetic introgression in the state was small, as the Blue-winged Warbler continues to move northward, this will undoubtedly pose an increasing threat to Minnesota’s Golden-winged Warbler population.

Indeed, prior to the initiation of the MNBBA, Blue-winged Warblers had already been reported during the summer months as far north as Becker, Cass, Clearwater, Hubbard, Lake, Mille Lacs, Morrison, and Todd Counties, all within the Golden-winged Warbler’s breeding range. Scattered summer reports also have been logged in western Minnesota, including in Jackson, Kandiyohi, Lincoln, and Otter Tail Counties (Beneke 1993; Bergman 1977; Millard 1984; Minnesota Ornithologists’ Union 2016).

*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.

Often described as a habitat generalist, the Blue-winged Warbler can be found breeding in a wide range of sites including dry shrubby uplands, weedy fencerows, right-of ways, young successional growth, and forest edges (Figure 4). Most sites are relatively open with scattered thickets of dense shrubs and young trees less than 7 meters in height. Dense ground cover of herbaceous plants also is common (Kaufman 1996; Gill et al. 2001; Potter et al. 2007). Dunn and Garrett (1997) provide one of the most comprehensive overviews of the warbler’s breeding habitat. They note that farther south the birds are frequently found in dry, brushy hillsides; farther north they are more common in mesic sites including streamside thickets and lowland hardwood forests. The species is not considered to be area sensitive (Potter et al. 2007).

Few detailed descriptions of the Blue-winged Warbler’s habitat in Minnesota are available. A significant local population, however, has been present at Murphy-Hanrehan Park Reserve in Scott and Dakota Counties for many years. Many birds in the reserve nest along the edges of trails within mature second-growth hardwood forests. Forest edges bordering abandoned pastures and croplands also are used (Danz et al. 2007).

Although the habitats of the Blue-winged Warbler and Golden-winged Warbler are similar in many respects, the Blue-winged Warbler tends to occupy a wider range of habitats, tolerating older stages of forest succession than those tolerated by the Golden-winged Warbler (Dunn and Garrett 1997). In an area where the species’ ranges overlapped in New York, researchers found Blue-winged Warblers occupied sites with less herbaceous cover and 25% more tree cover than Golden-winged Warblers. At least one study, in West Virginia, found Blue-wings on sites with less tree cover than Golden-wings, but this result is contrary to that found in most other studies (Gill et al. 2001). In Michigan, Will (1986) found no significant difference in habitat within 12 meters of each species’ nest.

Using data gathered by the federal Breeding Bird Survey (BBS), biologists have generated a North American population estimate of only 710,000 breeding adults (Rosenberg et al. 2016). In 2013, when the continental population estimate was 810,000, Minnesota was estimated to support 1.5% of the continental population, or a total of 12,000 individuals (Partners in Flight Science Committee 2013). The current estimate would be approximately 10,700 birds. The species was too uncommon during the MNBBA to develop a more accurate estimate.

Blue-winged Warblers are not an abundant species anywhere within their breeding range (Figure 1). Regionally they reach their greatest abundance within the New England/Mid-Atlantic Coast Region (an average of 2 birds per BBS route per year) and in the Appalachian Mountain Region (an average of 1 bird per BBS route per year). They are rarely detected along BBS routes in Minnesota (Sauer et al. 2017). Few site-level estimates of breeding density are available, but one study in Ohio reported 16 males per 40 ha (Canterbury et al. 1995). Another study, in West Virginia, reported 6 males per 40 ha in an area where their breeding range overlapped with Golden-winged Warblers, and 28 males per 40 ha in an area where populations did not overlap (Canterbury et al. 1996).

Because of the species’ rarity along BBS routes, most assessments of Blue-winged Warbler population trends lack statistical precision. Only the two regions mentioned above supported large enough populations to accurately assess trend lines. In the New England/Mid-Atlantic Coast Region, populations demonstrated a significant long-term decline averaging 2.39% per year since 1966; in the Appalachian Mountain Region, the trend line was stable (Sauer et al. 2017). Biologists estimated the region-wide continental population has experienced a cumulative decline of 22% since 1970 (Rosenberg et al. 2016). In Minnesota, where the species has been slowly expanding its range and numbers, BBS data were meager but suggested an increasing population trend. Despite the limitations of the BBS data in most regions of North America, Figure 5 provides an informative illustration of the status of the species within its limited distribution.

Although populations are increasing to stable in some portions of the Blue-winged Warbler’s range, there are large areas where numbers appear to be significantly declining. Beginning in the late 1800s, when forests throughout the eastern deciduous forest biome were harvested, habitat opportunities for the Blue-winged Warbler were abundant. Today, such wide-scale opportunities are limited. Habitat that was suitable 50 to 100 years ago has often been replaced by development, intensive agricultural operations, or mature forests. The exception may be at the northern edge of the species’ range, where it can continue to exploit sites not previously occupied. Unfortunately this brings it into direct competition with the Golden-winged Warbler.

In light of its restricted breeding range and small population size, the Blue-winged Warbler has been assigned a moderately high Continental Concern Score of 13/20 by Partners in Flight (Rosenberg et al. 2016). In Minnesota it was designated a Species in Greatest Conservation Need by the Department of Natural Resources in 2006 but was removed from the list in 2015 (Minnesota Department of Natural Resources 2006, 2015).

At the regional level, the Upper Mississippi River and Great Lakes Region Joint Venture designated the Blue-winged Warbler a focal species and established a regional population goal of 143,390 birds, which represented an increase of 50% over a 15-year period (Potter et al. 2007). Habitat protection and restoration goals were established for each state, including Minnesota, where the goal is to protect and restore 108 km² of shruland habitat.

Despite the species’ relatively high Continental Concern Score, the conservation focus is less on the Blue-winged Warbler then it is on the impacts of an expanding Blue-winged Warbler population on its near relative the Golden-winged Warbler. Studies have shown that within 50 years of Blue-winged Warblers occupying Golden-winged Warbler habitats where they overlap, the latter species was likely to become locally extinct (Gill 1980). Svingen and Hertzel (2015) discuss the potential for a similar scenario in Minnesota in future years.

Despite its ability to capitalize on new forest openings and edge habitats when they are available, the species may have more pressing challenges in the years ahead as temperatures continue to rise. The 2010 “State of the Birds” report classified the Blue-winged Warbler as having a moderate or medium sensitivity to climate change (North American Bird Conservation Initiative 2010). A more recent analysis by the National Audubon Society classified the warbler as “climate threatened.” Audubon models predicted the species may be forced to accelerate its northward expansion because nearly two-thirds of its current breeding range may no longer be suitable by the year 2080 (National Audubon Society 2016; Langham et al. 2015).

Given its small population size, other threats pose a major risk to the species. For example, it has been identified as 1 of 28 species that is especially vulnerable to collisions with communication towers during migration. Annual mortality from this threat alone is estimated to total nearly 4,000 birds (Longcore et al. 2013).

In the short-term, however, Minnesotans are likely to hear more of the tiny male warbler’s unique bee-bzzz territorial songs during the summer months at even more localities across the state.