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Willow Flycatcher

Empidonax traillii
Overview
Minnesota Seasonal Status:

A regular breeding species and migrant; the Willow Flycatcher was an uncommon species during the Minnesota Breeding Bird Atlas (MNBBA).

North American Breeding Distribution and Relative Abundance:

With two rather distinct cores to its breeding distribution, the Willow Flycatcher can be found throughout the northeastern and north-central states as well as in the Pacific Northwest, stretching southeast through Wyoming, Utah, Colorado, Arizona, and New Mexico. Its highest breeding densities occur in the Pacific Northwest (Figure 1).

Conservation Concern:
Conservation Status Score 10

Assigned a Continental Concern Score of 10/20 by Partners In Flight; the subspecies E. t. extimus, which is restricted to the southwestern United States, is federally listed as an Endangered Species.

Life History
Migration:

A long-distance migrant that winters in Central America and northern South America.

Food:

Feeds almost entirely on insects during the summer months, secured by fly-catching and foliage gleaning.

Nest:

An open cup placed in a deciduous tree or shrub; often located within 2 m of the ground but may be much higher.

Willow Flycatcher Willow Flycatcher. Empidonax traillii
© D & D Sudbury
Figure 1.

Breeding distribution and relative abundance of the Willow Flycatcher in North America based on the federal Breeding Bird Survey from 2011 to 2015 (Sauer et al. 2017).

Minnesota Breeding Bird Distribution*

There is virtually no historical record of the Willow Flycatcher’s distribution and relative abundance in Minnesota before it was recognized as a distinct species in 1973 (American Ornithologists’ Union 1973). Roberts’s (1932) treatise on Minnesota birds only included an account for Empidonax traillii, known at the time as the Traill’s Flycatcher, which was originally named by Audubon himself in honor of his English friend, Dr. Thomas S. Traill (Bent 1942). Today the Willow Flycatcher is recognized as a separate species from the Alder Flycatcher (E. alnorum).

Commenting on the distribution and relative abundance of the Traill’s Flycatcher, Roberts wrote:

If looked for in the right situations, it will be found fairly common throughout the southern part of the state, even in the bushy lowlands of the prairies, and northward it becomes increasingly abundant until in the upper half of the state its harsh cry comes from every alder thicket and wooded swamp.

He went to great length to describe the various renditions of the bird’s song and, as noted by Zink and Fall (1981), his descriptions largely depict those of what is recognized today as the Alder Flycatcher, although at least one account is likely that of the Willow. Most written accounts of the Willow Flycatcher’s song describe it as a two-syllable FITZ-bew, with the accent on the first syllable. Sometimes, this is preceded by an audible creet. By contrast, the Alder Flycatcher’s song is described as a three-syllable fee-BEE-o, with the accent on the second syllable, though many observers find it difficult to distinguish all three notes. Because both species give song variations that sound like two-syllable songs and three-syllable songs, this only adds to the challenge of distinguishing what is now recognized as two different species (Zink and Fall 1981).

Although he did not know it at the time, Roberts’s account of the Traill’s Flycatcher actually included information on both species. Biologists simply considered the different song types to be examples of regional variation in a widely dispersed species. As noted in the MNBBA’s Alder Flycatcher account, it wasn’t until observers began to report hearing both songs at the same site that biologists began to take a closer look at this enigmatic little flycatcher.

Indeed, it was the careful field studies of Dr. Robert Stein at the Cornell Lab of Ornithology in the 1950s and 1960s that led to the formal delineation of two different species, the more northern boreal Alder Flycatcher and the more southern and western Willow Flycatcher (Stein 1958, 1963). Even though the two birds appear nearly identical, significant differences in vocalizations, habitat, distribution, and breeding biology were sufficient to identify two different species.

Minnesota biologists and birding enthusiasts were left with the task of carefully documenting each species’ relative abundance and distribution. As early as 1975, Green and Janssen broadly described the Willow Flycatcher as occupying the southern and western regions of the state, while the Alder Flycatcher was a denizen of the northern region. Given that observers had simply reported on Traill’s Flycatcher for years, however, a great deal more work was needed to delineate the distribution and abundance of this new member of Minnesota’s avifauna.

One of the most important assessments was conducted by Zink and Fall in 1981. Supplementing their own field observations with reports from others who had worked across the state, they documented differences in the two species’ songs and calls, their habitats, and their breeding ranges. The Willow Flycatcher not only occurred in southern and western Minnesota, it had summer records as far north as Pennington and Marshall Counties in northwestern Minnesota and Aitkin County in north-central Minnesota. The authors also documented a broad zone of overlap where both species occurred that stretched from east-central Minnesota west to Pope County and north along the prairies of the Red River valley to the Canadian border (Zink and Fall 1981). This zone was contiguous with one described several years earlier by Robbins (1974) in Wisconsin.

Field studies conducted by the Minnesota Biological Survey provided additional details on the Willow Flycatcher’s statewide distribution. A total of 438 breeding season locations documented the species’ occurrence across the southern half of the state and along the ancient beach ridges of Glacial Lake Agassiz in northwestern Minnesota. Records were documented as far north as Kittson County in northwest Minnesota and Morrison County in east-central Minnesota (Minnesota Department of Natural Resources 2016). Despite the species’ wide distribution, by 1998 nesting had only been confirmed in 7 counties in widely scattered locations: Anoka, Clay, Dakota, Hennepin, Hubbard, Murray, and Wilkin (Hertzel and Janssen 1998).

During the MNBBA, observers reported a total of 592 Willow Flycatcher records in 8.3% (392/4,740) of the surveyed atlas blocks and in 9.8% (229/2,337) of the priority blocks. Breeding evidence was confirmed in 11 blocks (Figures 2 and 3; Table 1). The birds were reported in 65 of Minnesota’s 87 counties and were confirmed breeding in 11 counties; only one breeding location (Polk County) was in the northern half of the state. Eight of the 11 counties where breeding was confirmed were additions to the list published by Hertzel and Janssen (1998): Big Stone, Kandiyohi, Lyon, Polk, Rice, Scott, Steele, and Wabasha.

Although the Willow Flycatcher was documented in 6 more counties than the Alder Flycatcher, it is more sparsely distributed, reported in only 5.6% of the surveyed atlas blocks compared to 19.7% for the Alder Flycatcher. The MNBBA data documented a very similar distribution for the Willow Flycatcher to that delineated by the Minnesota Biological Survey, with two exceptions: 1) more birds were found further north in east-central Minnesota, with records as far north as central Pine County and southern Aitkin County; and 2) in far west-central Minnesota, the birds were not restricted to the upper Minnesota River valley but were located in numerous blocks outside of the river’s floodplain. The zone of overlap between the breeding distribution of the Willow Flycatcher and the Alder Flycatcher also appeared to be much broader than originally proposed by Zink and Fall (1981).

The MNBBA predicted breeding distribution map predicts low breeding densities throughout most of the southern half of Minnesota, with abundance decreasing north along the Red River valley (Figure 4). Although extremely rare, the model suggests that Willow Flycatchers might even be encountered as far north as Beltrami, Koochiching and Lake of the Woods Counties.

To the east, Wisconsin’s first breeding bird atlas (1995-2000) also documented more northern records for the Willow Flycatcher than previously described by Robbins (1991). Three possible explanations were given for the discrepancy: 1) the species’ range expanded northward, 2) better coverage of the state by atlas observers, and 3) possible confusion with the more northern Alder Flycatcher. Elsewhere within its breeding range, eastern populations of the Willow Flycatcher appear to be expanding both to the south (e.g., in Ohio, Pennsylvania, and West Virginia) and to the north (e.g., in Ontario, New York, and Vermont). Although the factors responsible for these movements are not always clear, habitat changes at the landscape level are occasionally noted (Sedgwick 2000).

*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 Willow Flycatcher in Minnesota based on the Breeding Bird Atlas (2009 – 2013).

Print Map
Figure 3.

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

Breeding statusBlocks (%)Priority Blocks (%)
Confirmed11 (0.2%)8 (0.3%)
Probable89 (1.9%)67 (2.9%)
Possible290 (6.1%)152 (6.5%)
Observed2 (0.0%)2 (0.1%)
Total392 (8.3%)229 (9.8%)
Table 1.

Summary statistics for the Willow Flycatcher 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 (birds detected per 10-minute point count) of the Willow Flycatcher 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 without an adjustment for detectability.

Breeding Habitat

The Willow Flycatcher is a shrub specialist that is frequently associated with riparian habitats. Like the Alder Flycatcher, it can be found in dry, shrubby uplands, including young regenerating forest stands where the growth is quite dense (Sedgwick 2000). Although, as its name implies, it often is closely associated with willow (Salix spp.) thickets, a wide variety of other shrubs are used. In the northern Great Plains, the Willow Flycatcher has been described as an edge-adapted species found in riparian woodlands, low-lying prairie sites, and woodland edges. Further east it is commonly associated with wet willow thickets, upland shrubs, and shrubby roadsides (Johnsgard 1986; Graber et al. 1974).

In Minnesota, Zink and Fall (1981) described the species’ typical habitat as scattered patches of small trees and tall shrubs embedded in open grassland habitats (Figure 5), as well as low swales and woodland edges. They made special note of the different niches occupied by the Alder and the Willow Flycatcher when both species were present on the same site. Invariably, the Alders would be found in “denser and taller vegetation with a wet boggy substrate, and Willows were either in more open vegetation (if very wet) or drier areas (if dense vegetation).” Other accounts have noted a similar separation of habitat preferences (Rodewald et al. 2016; Cutright et al. 2006).

Data collected by the MNBBA illustrated the Willow Flycatcher’s strong association with marshes and wet meadows followed by its occasional presence in oak woodlands (likely forest edges) and upland grasslands, the latter with shrubby thickets (Figure 6).

Figure 5.

Typical breeding habitat of the Willow Flycatcher in Minnesota (© Lee A. Pfannmuller).

Figure 6.

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

Population Abundance

Breeding Bird Survey (BBS) data were used to generate a North American population estimate of 9.4 million breeding adults (Rosenberg et al. 2016). Earlier accounts estimated that Minnesota supported approximately 0.70% of the North American population (Partners in Flight Science Committee 2013), which resulted in a statewide population of the Willow Flycatcher of approximately 66,000. The species was too uncommon to develop a reliable statewide estimate using MNBBA data.

The Willow Flycatcher is not a common species in Minnesota. During the atlas, the total number of records ranked it as an uncommon species. Far higher breeding densities are reached in the Pacific Northwest, particularly in western Washington and Oregon (Figure 1). BBS routes in Minnesota averaged less than 1 bird per route per year compared to nearly 11 in Washington. Even further to the southeast, in Ohio, Pennsylvania, and New York, the number of birds observed on BBS routes averaged 2 to 3 per route per year (Sauer et al. 2017). Site-level densities of breeding pairs ranged quite widely from a low of 7.1 breeding pairs per km2 in Michigan to a high of 111.1 per km2 in Connecticut. In the Upper Mississippi River & Great Lakes Region Joint Venture, which includes eastern Minnesota, breeding densities ranged from 7 to 11 territories per km2 with an average of 8.5 (Potter et al. 2007).

Across its breeding range, the Willow Flycatcher has shown a significant long-term population decline averaging 1.48% per year since 1966 (Figure 7). The cumulative population decline since 1970 is approximately 46% (Rosenberg et al. 2016). In Minnesota, the species’ relatively low abundance results in a trend line that lacks statistical precision. Available data suggest a stable population (Sauer et al. 2017).

The loss of wetland habitats could be the most important factor impacting the Willow Flycatcher’s long-term population decline. Grazing, stream channelization, water diversions, invasive species, residential development, and more intensive agricultural practices all place these wetlands at risk. Other factors frequently cited as negatively impacting reproductive success are the high rates of nest predation and nest parasitism by Brown-headed Cowbirds (Sedgwick 2000). All these factors are responsible for the listing of the southwestern subspecies of the Willow Flycatcher (E. t. extimus) as federally endangered (U.S. Fish and Wildlife Service 2002).

Figure 7.

Population trend map for the Willow Flycatcher in North America from 1966-2015 based on the federal Breeding Bird Survey (Sauer et al. 2017).

Conservation

Given its declining population trend, its relatively small population size, and the potential threats to its breeding habitat, the Willow Flycatcher has been assigned a Continental Concern Score of 11/20 (Rosenberg et al. 2016). It was designated a Species of Continental Importance by Partners in Flight (Rich et al. 2004), a Bird of Conservation Concern by the U.S. Fish and Wildlife Service (U.S. Fish and Wildlife Service 2008), and a focal species by the Upper Mississippi River & Great Lakes Region Joint Venture, which includes eastern Minnesota (Potter et al. 2007).

Partners in Flight established a general management goal to increase the Willow Flycatcher’s population by 50% (Rich et al. 2004). To achieve this, habitat protection and restoration goals were established for each state in the Upper Mississippi River & Great Lakes Region Joint Venture. In Minnesota, the specific target is to protect an additional 217 km2 of wetland habitat and to restore an additional 111 km2 (Potter et al. 2007). Monitoring habitat changes at the landscape level to assess if these habitat and population goals are being achieved is a priority of the Joint Venture.

In addition to threats to its wetland habitat, warming temperatures pose another challenge for the species. The 2010 State of the Birds report on climate change assessed the flycatcher’s vulnerability as relatively low (North American Bird Conservation Initiative 2010), but a recent analysis by the National Audubon Society predicted the species could lose 84% of its current summer breeding range by the year 2080 (Langham et al. 2015National Audubon Society 2016).  The species was classified as “climate threatened”.

Like all wetland- and grassland-dependent birds, intensive habitat protection and restoration initiatives are critical for sustaining viable populations. Recent interagency efforts to collaborate in protecting and restoring wetland and grassland habitats throughout western Minnesota by the Minnesota Prairie Plan Working Group (2011), coupled with efforts by the Upper Mississippi River & Great Lakes Region Joint Venture, should help secure the future of the Willow Flycatcher and its wetland associates.

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