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Alder Flycatcher

Empidonax alnorum
Overview
Minnesota Seasonal Status:

A regular breeding species and migrant. The Alder Flycatcher was a common species during the Minnesota Breeding Bird Atlas (MNBBA).

North American Breeding Distribution and Relative Abundance:

Broadly distributed across Alaska and boreal Canada, the Alder Flycatcher’s range extends south across the Great Lakes region and New England. Pockets of high breeding densities can be found in several areas, including New Brunswick, Nova Scotia, Prince Edward Island, Ontario, British Columbia, and northern Minnesota (Figure 1).

Conservation Concern:
Conservation Status Score 9

Assigned a Continental Concern Score of 9/20 by Partners in Flight.

Life History
Migration:

Long-distance migrant that winters in South America.

Food:

During the summer months the species feeds almost entirely on insects captured by fly-catching and foliage gleaning.

Nest:

An open-cup nest usually placed in shrubs approximately 0.3 to 1.0 m from the ground.

Alder Flycatcher Alder Flycatcher. Empidonax alnorum
© Michael Furtman
Figure 1.

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

Minnesota Breeding Bird Distribution*

Minnesota is on the southern periphery of the Alder Flycatcher’s range. The species’ range is broadly understood today, but the task of summarizing its relative abundance and distribution in Minnesota since the late 1800s is akin to unraveling a mystery in species identification. When Roberts first published his account of the species in 1932, the bird was commonly known as Traill’s Flycatcher (Empidonax traillii), embracing what is now recognized as two different species: the Alder Flycatcher (E. alnorum) and the Willow Flycatcher (E. traillii). Further complicating the mystery is the fact that the two species are nearly indistinguishable to the field observer. The best clues to their identification are their song and call notes (Eckert 2008). And although they occupy slightly different habitats from one another in different regions of the state, their distributions overlap. They have even been observed within hearing distance of one another on the same site.

Thinking there was only one species, Roberts (1932) described the flycatcher as a common breeding resident throughout the state, particularly in northern Minnesota. Yet for years, many field biologists recognized that the songs were different. One was usually described as a three-syllable fee-bee-o or a two-syllable fee-BEER song heard in the northern portion of the species’ range; the second was frequently a two-syllable FITZ-bew heard further south. Roberts’s numerous depictions of the flycatcher’s song were most similar to the song heard in northern Minnesota (Zink and Fall 1981).

For years, biologists believed the two song types simply represented regional differences in a widely distributed and relatively common bird. Variations in the song, however, were not the only difference. Field observers also began to note subtle differences in their nest structure, habitat, and morphology (Aldrich 1953; Snyder 1953). The observations eventually prompted more researchers to closely examine this rather nondescript and cryptic little flycatcher.

At the Cornell Laboratory of Ornithology, Stein (1958, 1963) led a series of extensive field studies that included observations across the continent, ranging from upstate New York to British Columbia. Although the northern and southern birds he examined looked nearly identical, even when in the hand, he found their vocalizations and general biology quite distinct. By 1973, his carefully documented evidence led the American Ornithologists’ Union, the official arbiter of bird nomenclature, to declare that the Traill’s Flycatcher was indeed two different species. Stein (1963) included a general range map of the two species, depicting the Alder Flycatcher as a species of the northern boreal forest and the Willow Flycatcher as a species of the Great Plains. This left state biologists and birders to sort out the finer details of each species’ distribution, particularly in areas like Minnesota, where it was apparent that both species occurred.

In Minnesota, Green and Janssen (1975) described the Alder Flycatcher as a resident in the state’s northern regions and the Willow Flycatcher as a resident in the western and southern regions. They noted additional field observations were necessary before their respective ranges could be clearly delineated. As Zink and Fall (1991) noted, it was generally assumed the ranges of the two species did not overlap.

Meanwhile, Robbins (1974) used his extensive field data in Wisconsin to carefully delineate the range of the two species with more precision. The outcome was the delineation of a broad region of overlap in the state where the species co-occurred. Extending from the southeast corner of the state northwest to Polk County, which borders east-central Minnesota, the area of overlap encompassed more than 20 of Wisconsin’s 72 counties.

A few years later, Zink and Fall (1981) followed Robbins’s (1974) lead and made the first effort to delineate the range of the two flycatchers in Minnesota. Using their own field data and records compiled from several other sources, they identified a similar zone of sympatry, where the species’ ranges overlapped. Roughly contiguous to that described by Robbins, the zone stretched from the east-central counties of Anoka and Ramsey, west to Pope County in west-central Minnesota and north along the Glacial Lake Agassiz beach ridges to Canada. Included in their assessment was the identification of 11 sites where the species co-occurred. Their paper, along with Eckert’s (2008), are good resources for Minnesota birders interested in learning more about the characteristics that distinguish the two species in the field.

Although the Alder Flycatcher is still best described as a species of Minnesota’s northern forests, there are occasional summer reports further south. During the course of their extensive field studies, biologists with the Minnesota Biological Survey have reported a total of 1,314 breeding season locations for the Alder Flycatcher. These include four localities as far south as Brown, Dodge, Freeborn, and Mower Counties (Minnesota Department of Natural Resources 2016). Yet, despite its broad distribution, by 1998 Hertzel and Janssen had identified only 7 counties, all north of the Minnesota River, where nesting had been confirmed: Aitkin, Beltrami, Cook, Lake, Ramsey, St. Louis, and Stearns.

During the MNBBA, observers reported 2,502 Alder Flycatcher records in 29.0% (1,374/4,744) of the surveyed atlas blocks and in 37.3% (871/2,337) of the priority blocks. Breeding evidence was gathered in 26 blocks (Figures 2 and 3; Table 1). The birds were reported in 56 of Minnesota’s 87 counties; 22 of the 31 counties where they did not occur were located south of the Minnesota River. Breeding evidence was gathered from 12 counties, stretching from Hennepin County north to the Canadian border. Of these, 9 were additions to the list published by Hertzel and Janssen (1998): Becker, Carlton, Hennepin, Hubbard, Koochiching, Lake of the Woods, Marshall, Morrison, and Roseau.

The Alder Flycatcher was recorded in 13 blocks located south of the Minnesota River. With one exception in Lincoln County, all the records were in southeastern and south-central Minnesota. Because flycatchers are late migrants and are difficult to identify in the field if a full song is not heard, all atlas reports of Alder Flycatchers in the southern half of the state were carefully reviewed. At least nine reports were invalidated because they occurred in early June and were considered to be late migrants and not breeding residents at the site where they were detected.

Compared with the flycatcher’s distribution delineated by Zink and Fall in 1981, the MNBBA showed that the species’ breeding range appears to extend further south, with records in several counties of the Twin Cities metropolitan region and in the southeast. It is not clear if the species has actually extended its range southward or if it was simply overlooked by earlier observers. Regardless, its true breeding status in southern Minnesota remains uncertain. As noted above, reports from this region are often assumed to be late spring migrants or early fall migrants. Yet all of the southern MNBBA reports that were accepted either were reported during the second or third week of June or were accompanied by notes that supported their retention. Further survey work is certainly warranted.

Interestingly, observers for Wisconsin’s first atlas (1995-2000) also documented more observations in southern Wisconsin than Robbins (1991) originally described. They attributed the difference to better coverage during the atlas than to any recent range expansion (Cutright et al. 2006). Another notable difference in the Alder Flycatcher’s Minnesota breeding distribution is that the range of overlap with the Willow Flycatcher appears to be considerably wider than originally proposed by Zink and Fall (1981).

Atlas data were combined with data on habitat availability, landscape context, climate, and detectability to develop a model to predict the breeding density of Alder Flycatchers at sites throughout the state (Figure 4). Low breeding densities are predicted throughout much of the state’s Eastern Broadleaf Forest Province. Although moderate to high densities are predicted throughout the Tallgrass Aspen Parklands and Laurentian Mixed Forest Provinces, the largest region with the highest predicted abundance is the Agassiz Lowlands Subsection of north-central Minnesota.

*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 Alder 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 Alder 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 (%)
Confirmed26 (0.5%)23 (1.0%)
Probable517 (10.9%)410 (17.5%)
Possible827 (17.4%)437 (18.7%)
Observed4 (0.1%)1 (0.0%)
Total1,374 (29.0%)871 (37.3%)
Table 1.

Summary statistics for the Alder 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 (pairs per 40 hectares) of the Alder 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 with an adjustment for detectability.

Breeding Habitat

Generally described as a species of wet shrubby thickets, the Alder Flycatcher is frequently encountered in dense, damp lowlands located in or adjacent to streams and lakes (Figure 5). Alder, willow, bog birch and red osier dogwood are common species within its territories (Bent 1942; Lowther 1999). Not restricted entirely to wet habitats, it can be found less commonly in dense upland shrubby habitat, especially in stands of young trees and shrubs that characterize regenerating areas following harvest or other disturbances (Lowther 1999).

In Minnesota, Zink and Fall (1981) described the species’ typical habitat as “dense, boggy alder (Alnus spp.) and willow (Salix spp.) thickets, often near streams.” Willow Flycatchers, in contrast, were more likely to be found in scattered patches of small trees and tall shrubs embedded in open grassland habitats, as well as in low swales and along woodland edges. When both species were found on the same site they noted that the Alders were invariably 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 in Minnesota’s two national forests—the heart of the Alder Flycatcher’s range—demonstrated its strong association with lowland shrubs as well as wet sedge meadows that are often interspersed with dense shrub thickets (Niemi et al. 2016). Its occasional use of young, regenerating stands was also apparent. Statewide data collected by the MNBBA illustrated the flycatcher’s strong association with shrub wetlands as well as several other northern wetland communities (Figure 6).

Figure 5.

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

Figure 6.

Habitat profile for the Alder 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

Biologists have been using the federal Breeding Bird Survey (BBS) data, which has been collected since 1966, to predict overall abundance of bird species. The most recent North American population estimate for the Alder Flycatcher is 130 million breeding adults (Rosenberg et al. 2016). In 2013, Minnesota was estimated to support approximately 0.8% of the North American population (Partners in Flight Science Committee 2013). Applying that percentage to the 2016 population estimate generated a statewide estimate of 1 million adults. MNBBA point count data were used to test the BBS-derived estimates and generated a slightly higher estimate of 1.4 million birds (95% confidence interval of 1.2 to 2.0 million).

Although Minnesota is located on the southern periphery of the flycatcher’s breeding range, within the BBS’s survey area of the United States and southern Canada, Minnesota supports pockets of very high breeding densities (Figure 1). An average of 6 birds are reported on BBS routes in the state per year. Minnesota and Maine support populations much smaller than those found in many Canadian provinces, where mean numbers ranged from a low of 11 birds per route per year in British Columbia to a high of 38 birds per route per year in Nova Scotia (Sauer et al. 2017).

Local breeding densities at various sites throughout Canada range from as low as 2.6 pairs/40 ha in Quebec to as high as 53 to 82 pairs/40 ha on sites in Ontario that were 10–19 years post logging (Lowther 1999). In the Chippewa National Forest in Minnesota, an average of 1.0 pairs/40 ha were detected; on the Superior National Forest an average of 1.4 pairs were detected (Niemi et al. 2016) Significantly higher breeding densities were found in recently harvested lowland conifer stands in the Agassiz Lowlands Ecological Subsection, where an average of 8.7 Alder Flycatcher pairs were detected per 40 ha (Bednar et al. 2016).

Across its breeding range, the Alder Flycatcher has experienced a long-term population decline that averages just under 1% per year since 1966 (Sauer et al. 2017). Range-wide populations have experienced a cumulative loss of 37% since 1970 (Rosenberg et al. 2016). The decline is not apparent in Minnesota, where populations are relatively stable or even slightly increasing (Figure 7). In Minnesota’s two national forests, the population trend from 1995 through 2016 has been nonsignificant, although decreasing slightly in the Chippewa National Forest and increasing slightly in the Superior National Forest. When data from the two forests were pooled, the species shows a steady, nearly linear trend line (Figure 8). Aside from changes in the availability of suitable habitat, the factors influencing these population numbers are poorly understood and have received little attention.

Figure 7.

Breeding population trend for the Alder Flycatcher in Minnesota for 1967–2015 based on the federal Breeding Bird Survey (Sauer et al. 2017).

Figure 8.

8. Breeding population trends of the Alder Flycatcher in the Chippewa and Superior National Forests and the combined regional trend, 1995–2016 (Bednar et al. 2016).

Conservation

Widely distributed across boreal Canada and the northern United States and with a relatively large population, the Alder Flycatcher has been assigned a moderate Continental Concern Score of 9/20 primarily because of its declining population trends (Rosenberg et al. 2016). It is not considered a priority species at either the state or federal level. Although not examined in depth, its vulnerability to climate change is considered low (North American Bird Conservation Initiative 2010). This signature species of Minnesota’s northern shrub swamps likely will remain an important and abundant member of the avifauna for years to come.

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