Golden-winged Warbler

Regular breeding resident in northern Minnesota and migrant; the Golden-winged Warbler was a common species during the Minnesota Breeding Bird Atlas (MNBBA).

Formerly occurred from southern Manitoba south through the Great Lakes Region, southern New England, the north-central states, and the Appalachian Mountains. Today two primary population centers are recognized: (1) a Great Lakes population that stretches from central Manitoba across the northern Great Lakes states to southern Quebec, and (2) an Appalachian Mountain population that stretches from southeastern New York south to northeastern Georgia. The core of the species’ breeding population occurs in northern Minnesota (Figure 1).

Assigned a Continental Concern Score of 16/20 and designated a Red Watch List species by Partners in Flight; designated a Species in Greatest Conservation Need by the Minnesota Department of Natural Resources and a Stewardship Species by Audubon Minnesota.

A long-distance migrant that winters in southern Central America and northern South America. Minnesota breeding birds are now known to spend the winter in Central America, from Guatemala and Honduras through western Panama, with major concentrations in Honduras, Nicaragua, and Costa Rica (Kramer et al. 2017).

An insectivorous foliage gleaner.

Open-cup nest usually located on or slightly above the ground, supported by the stalks of surrounding vegetation.

The Golden-winged Warbler was first documented nesting in Minnesota in 1878 (Svingen and Hertzel 2015). Roberts (1932) considered it a regular breeding species in the early 1900s from Lake Itasca east to Lake Mille Lacs and south throughout the Big Woods region of Hennepin, Wright, and Stearns Counties. He also reported the species was seen, “though rarely,” as far south as Fillmore County. When he was a young man in the late 1800s, he shared his fond memories of encounters with the species near the Twin Cities:

There was always, every springtime, a pair of these Warblers, close by and almost within reach of, the spray of the Laughing Waters [Minnehaha Falls], and several other pairs made their summer homes in the heavy timber south of Lake Calhoun.

These common haunts of the Golden-winged Warbler were long gone by the early 1900s as the Twin Cities grew into a bustling metropolitan area. Roberts wrote, “It is now apparently a rare summer resident in what is left of the hardwood timber in the southern part of the state. All recent records in the nesting-season are from the coniferous forests or close to their southern border.” At the time of his writing, confirmed nesting records (nests with eggs) were available only from Hennepin and Becker Counties, and inferred nesting records (broods of young out of the nest) from Sherburne and Mille Lacs Counties.  Svingen and Hertzel (2015) provided additional details regarding the species’ early presence in the state in the late 19th and early 20th centuries.

Many years later, Green and Janssen (1975) discussed the species’ distribution at length. “It is difficult,” they wrote, “to document any change in the range of the Golden-winged Warbler because ornithological exploration has been uneven.” In particular, they called into question Roberts’s assertion that the species may have been a rare breeding resident south of the Twin Cities. His sole evidence was based on a bird J. C. Hvoslef shot in Lanesboro in mid-June 1888. Regardless, they noted additional, albeit poorly documented, evidence of the species nesting in southeastern Minnesota in Winona County in 1934, and in Olmsted County in 1954.

Green and Janssen (1975) noted also that most of Roberts’s original account of the Golden-winged Warbler was based on his personal experience with the bird, especially in Hennepin and Stearns Counties, as well as from accounts by others in Otter Tail, Mille Lacs, Aitkin, and Sherburne Counties. Other than brief references to its presence at Cass Lake in 1927, and in or near Itasca State Park from 1919 to 1929, there is little documentation from northern Minnesota in the early 1900s. Yet, by the mid-1900s, Golden-winged Warblers were frequently reported from localities in north-central Minnesota. Indeed, in the 20 years prior to Green and Janssen’s 1975 publication, all but one specimen deposited in the collections at the Bell Museum of Natural History were from northern counties. As a result, Green and Janssen concluded that the species might actually be expanding its range northward.

Several years later, Janssen (1987) described the species’ primary breeding range as extending from “Chisago, Pine, and Carlton Counties in the east, westward through northern Anoka, Isanti, Kanabec, Aitkin, Cass, and Crow Wing Counties and as far west as Hubbard and Clearwater Counties.” It was just beginning to expand northeast into the Arrowhead region and farther north into Koochiching, Beltrami, and Lake of the Woods Counties. Since 1970, nesting had been confirmed in 7 counties: Aitkin, Anoka, Clearwater, Crow Wing, Hubbard, Mille Lacs, and St. Louis. Hertzel and Janssen (1998) later added Becker County to the list.

Beginning in the late 1980s, field staff with the Minnesota Biological Survey documented 547 breeding season locations for the Golden-winged Warbler. None of the locations were south of a line from southern Chisago and northern Anoka Counties west to southeastern Stearns Counties. The warbler was still uncommon in the Arrowhead region. The majority of locations were in a region stretching from Lake Mille Lacs northwest to southern Beltrami and Clearwater Counties (Minnesota Department of Natural Resources 2016).

Records collected by MNBBA participants documented the species’ broad distribution throughout the Laurentian Mixed Forest Province (except Cook County), the northern reaches of the Eastern Broadleaf Forest Province, and the far eastern region of the Tallgrass Aspen Parklands Province. Observers reported a total of 1,329 Golden-winged Warbler detections in 16.9% (805/4,759) of the surveyed atlas blocks and in 23.1% (540/2,337) of the priority blocks. Breeding evidence was documented in 37 blocks (Figures 2 and 3; Table 1). The species was reported in 31 of Minnesota’s 87 counties and was confirmed breeding in 14 counties. Eight of the counties were additions to Hertzel and Janssen’s list: Beltrami, Cass, Itasca, Kanabec, Lake, Morrison, Pine, and Sherburne.

MNBBA data were used to generate a predicted breeding distribution map for the species across the state (Figure 4). Highest breeding densities were predicted in the southern and western reaches of the Laurentian Mixed Forest Province. In the far northeastern counties of Lake and Cook, the highest probability of occurrence was along the North Shore of Lake Superior and in the young growth of the 2007 Ham Lake fire. Very low breeding densities were predicted in much of the Eastern Broadleaf Forest Province where suitable habitat may be present but likely occupied by its near relative, the Blue-winged Warbler.

The Golden-winged Warbler’s northward range expansion and southern range contraction in Minnesota since 1880 are well-documented by a series of maps presented by Svingen and Hertzel (2015).  Similar changes have been witnessed throughout the species’ breeding range in the past century. Wide-scale clearing of the eastern deciduous forest in the late 1800s and early 1900s likely created an abundance of new shrubland habitat for this original inhabitant of the Ohio River valley, southern New England, and Appalachian Mountains. Populations gradually pushed farther north into New England, a movement facilitated in later years by the abandonment of farmlands, and farther north in the midwestern states. The warbler first appeared in southern Ontario in the 1930s and was widespread throughout New England by the mid-1900s (Confer et al. 2011).

When much of the landscape in southern New England and the southern Great Lakes region was converted to croplands and urban development, the warbler’s range began to contract along its southern periphery. Before the end of the 20th century, Golden-winged Warblers were largely gone from southern Wisconsin, Illinois, Indiana, Ohio, and southern New England. In the next 10 years the species’ range further contracted in central Michigan, southern Ontario, New York, and throughout the Appalachian Mountains (Golden-winged Warbler Working Group 2013). Although habitat changes certainly propelled many of the range contractions, competition and hybridization with the Blue-winged Warbler also played a major role (Confer et al. 2011; Roth et al. 2012).

*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.

Originally dubbed the “golden-winged swamp warbler” by John James Audubon (Helmberger 2012), the species was long considered an exclusive inhabitant of shrubby wetlands. But young, open upland stands can also provide suitable habitat. Indeed, the species’ use of early successional stands, particularly young aspen in the Great Lakes region, made it a poster child for protection and maintenance of young forest habitat beginning in the 1990s, overshadowing the warbler’s equally important dependence on wetlands (Hanowski 2002).

Many studies have demonstrated that Golden-winged Warblers utilize a variety of upland and lowland open habitats that are characterized by patches of forbs, shrubs, and scattered trees, often located adjacent to a forest edge. Patchiness and structural complexity of forest stands are particularly important (Golden-winged Warbler Working Group 2013). In areas where Blue-winged Warblers and Golden-winged Warblers co-occur, the Golden-wings show a definite preference for wetter habitats (Will 1986).

In the core of the species’ breeding range in northern Minnesota and Wisconsin, the National Forest Bird (NFB) Monitoring Program demonstrated a strong association with lowland shrubs and sedge wetlands followed by open wetlands and regenerating stands (Niemi et al. 2016). Habitat data collected within 200 m of MNBBA point counts where Golden-winged Warblers were detected illustrate the species’ strong association with shrub wetlands followed by northern hardwoods and a wide variety of other forest and wetland habitats (Figure 6).

Research on Golden-winged Warblers, which has greatly increased in recent years, has demonstrated that habitat selection has additional dimensions throughout the nesting season. Until recently, assertions regarding habitat use were based largely on the location of singing males. When Streby and his colleagues (2012, 2015) used tiny radios to follow both adults and young, they discovered that the birds were also using mature forests. Females often nested in older forests, and the young, once they fledged, spent significant time in these mature stands, demonstrating that a matrix of forest types and ages, including shrubby wetland openings, are important elements of suitable habitat.

Federal BBS data were used to generate a U.S. and Canadian population estimate of 400,000 breeding adults (Rosenberg et al. 2016). In 2013, Minnesota was estimated to support approximately 47% of the continental population, or 190,000 adults (Partners in Flight Science Committee 2013). However, when MNBBA point count data were used in statistical analyses to estimate population size, the result predicted a significantly larger state population, numbering 1.2 million breeding adults; the estimate, however, had a large confidence interval. Nevertheless, the results suggest that the national population may also be significantly larger. Regardless of its ultimate population size, Minnesota is clearly the core of the species’ North American population. As a result, Audubon Minnesota designated it a Minnesota Stewardship Species (Pfannmuller 2012).

The high relative abundance of Golden-winged Warblers in Minnesota is also demonstrated by their frequency along BBS routes. Minnesota is the only state where the average number of birds detected per route is slightly greater than 1 (Sauer et al. 2017).  Although Golden-winged Warblers reach their highest breeding densities in North America in north-central Minnesota, they are still a relatively uncommon species. They were not among the 20 most common bird species reported in any habitat on either the Superior or Chippewa National Forest. Local breeding densities measured by the NFB Monitoring Program ranged from a high of 1.4 pairs per 40 ha on the Chippewa to 0.4 pairs per 40 ha on the Superior (Niemi et al. 2016).  By comparison, the most common warbler on both national forests was the Ovenbird with a breeding density of 15.1 pairs per 40 ha on the Chippewa and 17.7 pairs per 40 ha on the Superior.

Minnesota also is the only location where BBS data are robust enough to generate a reliable population trend. Since 1967, the population has remained fairly stable, perhaps even increasing in recent years, although the trend line is not statistically significant (Figure 7). Although the data lack precision, the population trend across the BBS survey region revealed a significant decline, averaging 2.28% per year (Sauer et al. 2017). Even with imprecise data, the regional map of population trends generated with BBS data clearly demonstrates the cause for concern regarding the species’ status. There are only a few small areas within the warbler’s restricted breeding range where populations appear to be increasing (Figure 8). Overall, biologists estimate that the population has declined more than 60% since 1970, one of the steepest declines of any North American songbird (Rosenberg et al. 2016).

Given the importance of Minnesota to the overall status and health of the Golden-winged Warbler population, the NFB Monitoring Program on the Chippewa and Superior National Forests may be the single most important source of population data on the species in North America. On both forests, the species has demonstrated a stable population from 1995 through 2016, with a non-significant decrease on the Chippewa and a non-significant increase on the Superior (Figure 9).

Factors responsible for the species’ range-wide population decline are many. Although competition and hybridization with the Blue-winged Warbler has added to the conservation challenges, widespread loss of both breeding and wintering habitat are considered the primary contributors to the decline.

When Partners in Flight began focusing conservation efforts on Neotropical songbirds in the early 1990s, the Golden-winged Warbler became one of the initiative’s early mascots. The Golden-winged Warbler Working Group was established in 1999 and launched the Golden-winged Warbler Atlas Project. Led by the Cornell Lab of Ornithology, the atlas was designed to assess the distribution and habitat requirements of Golden-winged Warblers, Blue-Winged Warblers, and their hybrids. This and other research studies became the foundation for a major Golden-Winged Warbler Status Assessment and Conservation Plan published in 2012. The detailed plan established a population goal of 621,000 breeding individuals by 2050, a goal that will require approximately 1 million acres of additional habitat. Partners in Flight has assigned the warbler a very high Continental Concern Score of 16/20 and placed it on their Red Watch List, a group of 19 species with restricted distributions and small, declining populations (Rosenberg et al. 2016).

Guidelines for managing breeding habitat also have been developed by the Golden-winged Warbler Working Group (2013). Management guidelines for the Great Lakes region identified 16 focal areas where management efforts should be concentrated: 1 includes much of north-central Minnesota and northwestern Wisconsin; the second includes the aspen parklands of northwestern Minnesota and southeastern Manitoba. The guidelines emphasize the importance of establishing patches of habitat that have interspersed clumps of shrubs and/or saplings, an open canopy, and adjacency to mature forest stands.

Concern regarding declining populations led conservation organizations to petition the U.S. Fish and Wildlife Service in 2010 to list the species as endangered or threatened under the federal Endangered Species Act. Federal biologists are conducting a more thorough review to assess if listing is warranted (U.S. Fish and Wildlife Service 2011). The species is also a focal species for the Upper Mississippi River and Great Lakes Region Joint Venture, an area that covers the densest U.S. populations in central and eastern Minnesota, Wisconsin, and Michigan (Potter et al. 2007). Because of its relative abundance in Minnesota, the warbler is not a state-listed species but has been designated a Species in Greatest Conservation Need (Minnesota Department of Natural Resources 2015).

Perhaps the most interesting, albeit challenging, aspect of the Golden-winged Warbler’s life history and conservation needs is its relationship with its congener, the Blue-winged Warbler, a species found south of the Golden-winged Warbler’s breeding range. Scientists believe the two species originated from a geographic split of a single population during the most recent glaciation nearly 10,000 years ago. Their time apart, however, was not long enough to allow sufficient differentiation that would prevent successful interbreeding. So, when the Blue-winged Warbler began expanding north in the 1800s, fertile, first-generation hybrids were produced. Known as Brewster’s Warblers, the hybrids are encountered regularly in the area of overlap (Will 2009). Another variant, Lawrence’s Warbler, resulted when the Brewster’s Warbler mated with either a Golden-winged Warbler or Blue-winged Warbler. Volumes have been written over the decades about these hybrids and their parents. Indeed, Morse (1989) wrote that “they are probably the most intensively studied pair of hybridizing bird species in North America.”

Many scientists consider the Blue-winged Warbler a primary threat to the future sustainability of Golden-winged Warbler populations. Because the two species utilize similar habitats, they likely compete for available resources in areas of overlap. There is evidence that Golden-wings are often extirpated from an area within 50 years of when Blue-wings first appear (Gill 1980). If Golden-winged Warblers keep retreating northward, this would not be a problem, but the amount of suitable habitat north of southern Manitoba seems limited at present.

Minnesota provides an excellent example of the interactions between the two species as the Blue-winged Warbler continues to expand its range northward since it was first documented in southeastern Minnesota in the late 1800s (Roberts 1932). Svingen and Hertzel (2015) provided an excellent review of the knowledge of the two species in Minnesota, how their breeding ranges have changed over time, and a detailed account of known records of hybrids. Their review documented an increasing rate of the number of hybrids reported in recent years. Although they note there are many variables that influence the number of reports received, their work strongly suggests an increased rate of hybridization is occurring in the state.

An equally important concern of some is the “genetic dilution” that results from hybridization (Confer et al. 2011). Even today, birds that appear to be “pure” representatives of either species carry genetic material from the other. In Minnesota, for example, one of the 96 phenotypically “pure” Golden-winged Warblers that were sampled showed evidence of genetic interbreeding with Blue-winged Warblers (Vallender et al. 2009). The study’s authors cautioned that it was only a matter of time before a larger percentage of the state’s Golden-winged Warblers become less “genetically pure.” Indeed, recent genetic analysis suggests that the two species, if they can even be considered two separate species, have had a long history of successive episodes of isolation and sympatry. There appear to be only a half-dozen nuclear genes that clearly distinguish the two forms from one another. (Toews et al. 2016).

Unfortunately, another challenge that awaits the Golden-winged Warbler is climate change. Recently classified as a “climate threatened” species by the National Audubon Society, models predicted that by the year 2080 the species’ current summer range will be entirely replaced by a breeding range located farther north. As dire as this projection seems, their models also predicted that more habitat may be available in Canada (Langham et al. 2015; National Audubon Society 2016). Unfortunately, their optimism was based on the assumption that the species specializes in second-growth habitats, which ignores their dependence on shrub wetlands.

The Golden-winged Warbler certainly presents resource managers with interesting conservation challenges. No matter how the future plays out, protecting the matrix of different forest cover types currently used by the majority of the species’ population is critical to ensuring that Minnesota continues to provide suitable habitat for this stunning little warbler.