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Blue-gray Gnatcatcher

Polioptila caerulea
Overview
Minnesota Seasonal Status:

Regular breeding species and migrant in southern and central Minnesota; the Blue-gray Gnatcatcher was an uncommon species during the Minnesota Breeding Bird Atlas (MNBBA).

North American Breeding Distribution and Relative Abundance:

Broadly distributed across the southern and central United States, north to the Great Lakes and southern New England, the Blue-gray Gnatcatcher is largely absent from the Great Plains, though it may be found locally where suitable woodlands are present. In Canada, it is limited to southeastern Ontario and southwestern Quebec. It is also found year-round in Baja California, Florida, and Central America. The core of the species’ breeding range is in the southeastern United States (Figure 1).

Conservation Concern:
Conservation Status Score 7

Assigned a Continental Concern Score of 7/20 by Partners in Flight.

Life History
Migration:

Breeding birds in the northern regions are long-distance migrants that winter primarily in Central America.

Food:

Insectivorous foliage gleaner that also sallies after flying insects and hovers above the foliage. Gnats are consumed but are not a significant component of their diet.

Nest:

Tightly woven open-cup nest placed in a deciduous tree at variable heights.

Blue-gray Gnatcatcher Blue-gray Gnatcatcher. Polioptila caerulea
© Mike Lentz
Figure 1.

Breeding distribution and relative abundance of the Blue-gray Gnatcatcher in North America based on the federal Breeding Bird Survey, 2011–2015 (Sauer et al. 2017).

Minnesota Breeding Bird Distribution*

Roberts (1932) gave only a brief account of the Blue-gray Gnatcatcher’s presence in Minnesota. He described it as a “southern species which reaches Minnesota in only very limited numbers.” Although a specimen had been collected as far west as Lake Shetek in Murray County in 1900, the bird was considered to be restricted to the Mississippi floodplain and its larger tributaries. Apparently the northward extension of the Gnatcatcher’s range has followed the Mississippi River bottom-land, the Murray County bird probably veering off westward up the Des Moines River which begins in Lake Shetek.” Roberts himself had encountered the species only once, when he observed a pair near Minneapolis in 1931.

Although many accounts of the Blue-gray Gnatcatcher’s northward range expansion state that its first appearance in Minnesota occurred in the 1930s (e.g., Chartier et al. 2013; Kershner and Ellison 2012), the first documented record in Minnesota was a specimen collected in 1877 in Minneapolis (Roberts 1932). Then, in 1895, an account was published in The Auk by a local student, Walton Mitchell, describing 6 Blue-gray Gnatcatcher nests found in the general vicinity of St. Paul (5 specifically from West St. Paul in Dakota County) from 1892 to 1894! Nearly 30 years would pass until summer observations, although still rare, at least became more regular with each passing year (Roberts 1932). Other breeding evidence would come from Frontenac in Goodhue County, where a family party was observed for many years beginning in 1924, and from northern Washington County, where a brood of 3 young birds were being fed by an adult (1930).

The absence of Minnesota records for nearly 30 years, between the 1890s and the 1920s, is similar to the species’ pattern of range expansion in New England. The Blue-gray Gnatcatcher became a regular breeding species in eastern Pennsylvania and northern New Jersey following a major spring flight that was unprecedented in its size and scope in 1947 (Ellison 1993). That same year the species bred for the first time as far north as Connecticut (Saunders 1950). It is not unusual during such flights for some birds to overshoot their normal spring destination points. Often the birds make adjustments and return southward. But if conditions are suitable, a few may remain and breed far north of their normal range, as they did in Connecticut. Twenty years and many more major spring flights passed before the species became established as a regular breeding species in New England in the 1970s.

By 1975, Green and Janssen depicted the Blue-gray Gnatcatcher’s breeding distribution as stretching from those counties bordering the Mississippi River north to southeastern Stearns County in east-central Minnesota and along the St. Croix River in Washington County. Spring migration and summer observations beyond these counties suggested that the breeding range might extend even farther north and west. Breeding had been confirmed in 8 counties: Carver, Dakota, Hennepin, Houston, Fillmore, Sherburne, Stearns, and Winona.

Janssen (1987) described the species’ continued range expansion west along the Minnesota River valley and northwest along the Mississippi River valley. The species was now well represented in Nicollet County and as far west as Lyon County (Garvin Park).” Confirmed nesting records were even reported from Rock County in extreme southwestern Minnesota (1981) and from Otter Tail County in west-central Minnesota (1979). In addition to these 2 counties, nesting had been confirmed in 15 other counties since 1970: Anoka, Brown, Dakota, Goodhue, Hennepin, Houston, Morrison, Nicollet, Olmsted, Ramsey, Renville, Scott, Sherburne, Wabasha, and Washington. Hertzel and Janssen (1998) later added 7 more counties to the list: Blue Earth, Clay, Hubbard, Kandiyohi, Mower, Stevens, and Winona.

Beginning in the late 1980s through 2014, the Minnesota Biological Survey (MBS) documented 362 Blue-gray Gnatcatcher breeding season locations, further expanding the species’ statewide distribution. Records were most abundant in southeastern Minnesota and along the Minnesota River valley as far as the upper reaches of the river in Lac qui Parle and Swift Counties. Records were common also north through Otter Tail County in the west and southern Aitkin County in the east. Scattered records from Mahnomen, Marshall, and the border of Pennington and Red Lake Counties documented the species’ occasional presence in northwestern Minnesota (Minnesota Department of Natural Resources 2017).

MNBBA participants reported 432 Blue-gray Gnatcatcher records from 6.3% (297/4,737) of the surveyed blocks and from 8.6% (200/2,337) of the priority blocks. Breeding was confirmed in 57 blocks (Figures 2 and 3; Table 1). The birds were reported in 54 of Minnesota’s 87 counties and were confirmed breeding in 28 counties. Twelve of the counties were additions to the list published by Hertzel and Janssen (1998): Benton, Carver, Cass, Crow Wing, Fillmore, Isanti, Jackson, Kanabec, McLeod, Pope, Rice, and Stearns. Gnatcatchers were firmly established from southern Cass and Crow Wing Counties south through east-central and southeastern Minnesota. They also occurred throughout the Minnesota River valley as far west as Yellow Medicine and Renville Counties. Scattered records were reported in south-central, southwestern, and west-central Minnesota, where they were particularly common along the prairie-forest border. The most northern record was a pair of birds sighted in East Grand Forks in western Polk County in late May 2013.

The MNBBA predicted distribution map (Figure 4) predicts the highest abundance of Blue-gray Gnatcatchers along the river valleys of southeastern Minnesota with scattered pockets of moderate abundance in counties just north of the Twin Cities and along the Minnesota River valley. Elsewhere very low densities are predicted to occur throughout the Eastern Broadleaf Forest Province as far north as Mahnomen County. Forest lands along floodplains in south-central Minnesota also provide suitable habitat, including the floodplains of the Blue Earth and Des Moines Rivers.

As noted earlier, the Blue-gray Gnatcatcher has been gradually expanding its range northward throughout the 20th century but most notably since the 1930s. Kershner and Ellison (2012) noted that even since the mid-1980s, its range has expanded nearly 314 km to the north. They attribute the change to a warming climate.

Wisconsin’s first breeding bird atlas, conducted from 1995 to 2000, documented a northward range extension of at least 50 km since the early 1990s. A few confirmed breeding records were found in scattered locations even farther north of its primary range including one in Douglas County which borders Lake Superior (Cutright et al. 2006). Michigan also documented a gradual expansion north into the northern Lower Peninsula (LP) and into the southern Upper Peninsula (UP). They noted that in the LP some of the range extension appears to follow the shorelines of Lake Michigan and Lake Huron, while farther inland it follows lakefront and riparian habitats. Expansion in the UP, however, appears to originate from populations in northeastern Wisconsin (Chartier et al. 2013). In South Dakota, Blue-gray Gnatcatchers were first reported breeding in 2004 in the southeastern corner of the state. The state’s second atlas (2009-2012) documented breeding populations in all corners of the state, including numerous records in the Black Hills (Drilling et al. 2016). Nearby in Canada, Blue-gray Gnatcatchers have not yet expanded north into Manitoba (Bird Studies Canada 2017), but they are well established in Ontario’s southern shield and are continuing to expand northward in that province (Cadman et al. 2007).

*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 Blue-gray Gnatcatcher in Minnesota based on the Breeding Bird Atlas (2009 – 2013).

Print Map
Figure 3.

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

Breeding statusBlocks (%)Priority Blocks (%)
Confirmed57 (1.2%)32 (1.4%)
Probable70 (1.5%)51 (2.2%)
Possible168 (3.5%)116 (5.0%)
Observed2 (0.0%)1 (0.0%)
Total297 (6.3%)200 (8.6%)
Table 1.

Summary statistics for the Blue-gray Gnatcatcher 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 Blue-gray Gnatcatcher 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 Blue-gray Gnatcatcher inhabits a wide range of wooded habitats, from oak woodlands and chaparral in the southwestern United States to riparian forests and deciduous uplands in the East (Figure 5). Along the northern periphery of its breeding range, the species is frequently associated with riparian and lakeshore habitats (Kershner and Ellison 2012). In New England, the birds frequent floodplain forests of green ash and silver maple (Kershner and Ellison 2012). Mossman (1991) documented the species’ preference for floodplain forests of silver maple–black ash and swamp white oak–black ash along the St. Croix River in Wisconsin and Minnesota. In both states the species prefers to utilize the tops of tall trees in mature deciduous forests (Niemi and Hanowski 1992).

Open understories and large trees are characteristic features of the Blue-gray Gnatcatcher’s habitat and may be more important than the composition of tree species. The documentation of 2 nests in southern Cass County in 1999 found the birds nesting in a “band of sparse jack pine located between the edge of a deciduous forest and an open grassland” (North 2002). Habitat data collected within 200 m of MNBBA point counts where the species was detected demonstrate the species’ strong association with oak forests (Figure 6).

Although the evidence is mixed, some studies have suggested that the Blue-gray Gnatcatcher is area sensitive, becoming more abundant in larger forest tracts (Niemi and Hanowski 1992). In particular, a study conducted in the mid-Atlantic states of Delaware, Maryland, West Virginia, and Virginia reported that 40 acres was the minimum area required for breeding; the birds were most abundant in sites larger than 3,000 ha (Robbins et al. 1989). Others, however, have suggested that features at the microhabitat level, including the presence of stream valleys and forest gaps, may be more limiting than the size of the forest stand (Kershner and Ellison 2012).

Figure 5.

Typical breeding habitat of the Blue-gray Gnatcatcher in Minnesota (© Lee A. Pfannmuller).

Figure 6.

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

Population Abundance

Data from the federal Breeding Bird Survey (BBS) have been used to generate an estimate of the size of the North American population at 210 million birds (Rosenberg et al. 2016). In 2013, Minnesota was estimated to support only 50,000 breeding adults, when the North American population was estimated at 110 million birds (Partners in Flight Science Committee 2013). Now the statewide estimate may number at least 100,000 adults. Numbers were too low to generate a reliable state estimate with MNBBA data.

Although the population has certainly continued to expand northward in Minnesota, the state still lies on the northern periphery of the species’ breeding range, and breeding densities are fairly low compared to those in states farther south (Figure 1). The average number of Blue-gray Gnatcatchers detected each year on BBS routes in the state is less than 1 (0.11); range-wide the average is 3 birds per BBS route. The birds are most abundant in the south-central region of the United States, in a band stretching from Arkansas and Missouri east to Virginia and the Carolinas. In this region, the average number of Blue-gray Gnatcatchers on BBS routes ranges from 8 to 18 (Sauer et al. 2017).

Despite its range expansions, since 1966, the Blue-gray Gnatcatcher has demonstrated a relatively stable population trend, with a small but significant average annual increase of 0.38% per year throughout North America (Figure 7). Data from the past 20 years suggest an even greater increase. Because the species is so rarely encountered on BBS routes in Minnesota, the BBS data are not statistically robust. Nevertheless, the trend line is definitely positive (Sauer et al. 2017).

Figure 7.

Breeding population trend for the Blue-gray Gnatcatcher in North America for 1966­2015 based on the federal Breeding Bird Survey (Sauer et al. 2017).

Conservation

The Blue-gray Gnatcatcher is not the focus of any major conservation efforts. Partners in Flight has assigned it a relatively low Continental Concern Score of 7/20 based on its stable and abundant population, its broad distribution, and low threats to its breeding and wintering habitats (Rosenberg et al. 2016).

Given the species’ vulnerability to nest parasitism by Brown-headed Cowbirds and its potential status as an area-sensitive species, protection and restoration of large forest tracts may be an important conservation measure. In particular, forest floodplain restoration efforts may be important to the species.

Although climate change is a potential threat to many of North America’s songbirds, the Blue-gray Gnatcatcher is one species that has benefited from a warming climate. In future years it is likely to become an even more familiar bird to Minnesotans, adding its high and wispy song to the morning chorus of songbirds throughout the state’s deciduous forest region.

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  • Cadman, Michael D., Donald A. Sutherland, Gregor G. Beck, Denis Lepage, and Andrew R. Couturier, eds. 2007. The Atlas of the Breeding Birds of Ontario, 2001–2005. Toronto: Bird Studies Canada, Environment Canada, Ontario Field Ornithologists, Ontario Ministry of Natural Resources, and Ontario Nature.

  • Chartier, Allen T., Jennifer J. Baldy, and John M. Brenneman, eds. 2013. Michigan Breeding Bird Atlas II. Kalamazoo, MI: Kalamazoo Nature Center.

  • Cutright, Noel, Bettie R. Harriman, and Robert W. Howe, eds. 2006. Atlas of the Breeding Birds of Wisconsin. Waukesha: Wisconsin Society of Ornithology, Inc.

  • Drilling, Nancy E., Robert A. Sparks, Brittany J. Woiderski, and Jason P. Beason. 2016. South Dakota Breeding Bird Atlas II: Final Report. Technical Report M-SDBBA2-07. Brighton, CO: Rocky Mountain Bird Observatory. http://gfp.sd.gov/images/WebMaps/Viewer/WAP/Website/SWGSummaries/SDBBA2 Final Report T-41-R.pdf

  • Ellison, Walter G. 1993. “Historical Patterns of Vagrancy by Blue-gray Gnatcatchers in New England.” Journal of Field Ornithology 64: 358–366.

  • Green, Janet C., and Robert B. Janssen. 1975. Minnesota Birds: Where, When and How Many. Minneapolis: University of Minnesota Press.

  • Hertzel, Anthony X., and Robert B. Janssen. 1998. County Nesting Records of Minnesota Birds. Minnesota Ornithologists’ Union Occasional Papers, no 2. Minneapolis: The Minnesota Ornithologists’ Union.

  • Janssen, Robert B. 1987. Birds in Minnesota. Minneapolis: University of Minnesota Press.

  • Kershner, Eric L., and Walter G. Ellison. 2012. “Blue-gray Gnatcatcher (Polioptila caerulea).” The Birds of North America, edited by Paul G. Rodewald. Ithaca: Cornell Lab of Ornithology. Retrieved from the Birds of North America: https://birdsna.org/Species-Account/bna/species/buggna doi: 10.2173/bna.23

  • Minnesota Department of Natural Resources. 2017. “Blue-gray Gnatcatcher (Polioptila caerulea).” Minnesota Biological Survey: Breeding Bird Locations. http://files.dnr.state.mn.us/eco/mcbs/birdmaps/blue_gray_gnatcatcher_map.pdf

  • Mitchell, Walton. 1895. “Breeding of the Blue-gray Gnatcatcher in Minnesota.” Auk 12: 185.

  • Mossman, Michael J. 1991. “Breeding Birds of the St. Croix River, Wisconsin and Minnesota.” Passenger Pigeon 53: 39–77.

  • Niemi, Gerald J., and JoAnn M. Hanowski. 1992. “Detailed Species Descriptions – Forest Birds.” In Forest Wildlife: A Technical Paper Prepared for a Generic Environmental Impact Statement on Timber Harvesting and Forest Management in Minnesota, compiled by Jaakko Pöyry Consulting, Inc. St. Paul, MN: Jaakko Pöyry Consulting, Inc.

  • North, Michael R. 2002. “Nesting Habits of Blue-gray Gnatcatchers in Cass County.” Loon 74: 229–233.

  • Partners in Flight Science Committee. 2013. Population Estimates Database. Version 2013. http://rmbo.org/pifpopestimates

  • Robbins, Chandler S., Deanna K. Dawson, and Barbara A. Dowell. 1989. “Habitat Area Requirements of Breeding Forest Birds of the Middle Atlantic States.” Wildlife Monographs 103: 3–34.

  • Roberts, Thomas S. 1932. The Birds of Minnesota. 2 vols. Minneapolis: University of Minnesota Press.

  • Rosenberg, Kenneth V., Judith A. Kennedy, Randy Dettmers, Robert P. Ford, Debra Reynolds, John D. Alexander, Carol J. Beardmore, Peter J. Blancher, Roxanne E. Bogart, Gregory S. Butcher, Alaine F. Camfield, Andrew Couturier, Dean W. Demarest, Wendy E. Easton, Jim J. Giocomo, Rebecca Hylton Keller, Anne E. Mini, Arvind O. Panjabi, David N. Pashley, Terrell D. Rich, Janet M. Ruth, Henning Stabins, Jessica Stanton, and Tom Will. 2016. Partners in Flight Landbird Conservation Plan: 2016 Revision for Canada and Continental United States. Partners in Flight Science Committee. http://www.partnersinflight.org/wp-content/uploads/2016/08/pif-continental-plan-final-spread-single.pdf

  • Sauer, John R., Daniel K. Niven, James E. Hines, David J. Ziolkowski Jr., Keith L. Pardieck, Jane E. Fallon, and William A. Link. 2017. The North American Breeding Bird Survey, Results and Analysis 1966–2015. Version 12.23.2015. Laurel, MD: U.S. Geological Survey Patuxent Wildlife Research Center. https://www.mbr-pwrc.usgs.gov/bbs/

  • Saunders, Aretas A. 1950. “Changes in Status of Connecticut Birds.” Auk 67: 253–255.