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Henslow’s Sparrow

Ammodramus henslowii
Overview
Minnesota Seasonal Status:

A regular breeding resident and migrant in Minnesota; an uncommon species during the Minnesota Breeding Bird Atlas (MNBBA).

North American Breeding Distribution and Relative Abundance:

The Henslow’s Sparrow has a very restricted breeding range that spans the grasslands of Minnesota south to eastern Kansas, east across the Ohio River valley and Great Lakes to southern Ontario, western New York, and Pennsylvania. Small populations occur along the mid-Atlantic coast from Maryland south to Virginia. Even within this limited distribution, populations may be localized in occurrence. The species is sparsely distributed throughout its breeding range; the highest breeding densities occur in north-central Missouri (Figure 1).

Conservation Concern:
Conservation Status Score 14

Assigned a Continental Concern Score of 14/20 and designated a Yellow Watch List species by Partners in Flight. Officially listed as a state Endangered Species in Minnesota and designated a Species in Greatest Conservation Need by the Minnesota Department of Natural Resources.

Life History
Migration:

A short-distance migrant that winters in the southeastern United States.

Food:

An omnivorous ground feeder that consumes insects, particularly grasshoppers, crickets, and beetles, and seeds.

Nest:

An open-cup nest located on or near the ground; some of the surrounding ground litter may form a partial dome over the top.

Henslow’s Sparrow Henslow’s Sparrow. Ammodramus henslowii
© Mike Lentz
Figure 1.

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

Minnesota Breeding Bird Distribution*

Audubon (1841) described the Henslow’s Sparrow as a “mouse with wings”, a reference to the small bird’s habit of evading intruders by running like a mouse through the grasses rather than taking flight.  Only a handful of records of this rare sparrow were available when Roberts published his 1932 treatise on Minnesota birds. He described it as “a generally distributed summer resident throughout the southern half of Minnesota as far north as Grant and Isanti Counties, where it breeds.” There were 2 isolated records from the northwestern region: (1) a June 1898 record from Hallock, Kittson County, and (2) a July 1879 record from Pembina, North Dakota, “just across the river from Minnesota.” The records were so rare that he detailed every report from 1880 to 1928. The records covered all corners of the state’s grassland landscape, including Goodhue, Grant, Hennepin, Isanti, Jackson, Kittson, and Pipestone Counties. Only 1 nesting record was available from Jackson County, in 1902. The most detailed observation was from June 1928, when several were found “in the meadows along the south side of the Minnesota River Valley above Fort Snelling.” Roberts concluded his account of the species by noting “the bird is probably much more common and more widely distributed than these observations indicate, as its secretive habits and weak song make it difficult to find.” When he published the second edition of his book in 1936, nesting had also been confirmed in Stearns County near St. Cloud (Roberts 1936).

Despite Roberts’s optimism about the species’ abundance, breeding season reports only trickled in during the next several decades. Through the 1960s, documented reports included a nest in Lac qui Parle County (Chambers 1947), a report of “several” birds in Mahnomen County (Janssen 1959), and 1 adult with 1 young in Clay County (Huber 1961). With one exception, 1 or more pairs would appear at a site one year only to disappear in subsequent years.

The exception was in Winona County. Several grassland sites with Henslow’s Sparrows were frequently visited by Brother Theodore Voelker in the 1950s. Voelker first reported 3 nests in 1953 (Herz 1954) and 6 nests that raised 20 young in 1955 (Guttman 1956). It is unclear if the site or sites he visited were in the same area that became O. L. Kipp State Park in 1976, and was renamed to Great River Bluffs State Park in 1997. Unfortunately most of the early accounts simply refer to the “usual Winona County areas” in reports submitted to the Minnesota Ornithologists’ Union (MOU). Summer seasonal reports compiled by MOU imply that there were several sites where the birds could be found, but once the state park was established, nearly all Henslow’s Sparrow records in Winona County make sole reference to the state park (Minnesota Ornithologists’ Union 2016). If Warner’s map of all known Henslow’s Sparrow records up until 1959 is accurate, it suggests that Brother Voelker’s original site(s) was northwest of the state park (Warner 1959).

The majority of reports in subsequent years, from the mid-1970s through the early 1990s, continued to come from the southeastern corner of the state, especially from O. L. Kipp State Park. This was the one site in the state where birders usually could be assured of spotting a Henslow’s Sparrow. Gradually, however, reports began to accumulate from other sites in southern and western Minnesota. Eckert (1974) documented the species’ presence at Blue Mounds State Park in Rock County, and Fall and Eliason (1982) found a nest in Hennepin County. Other summer reports were submitted from several east-central and southeastern counties, including Dakota, Fillmore, Houston, Ramsey, Sherburne, Steele, and Wabasha (Minnesota Ornithologists’ Union 2016). Farther west, reports came from Grant (Igl 1996), Norman (Johnson 1986), and Wilkin Counties (Millard 1995). Hanowski and Niemi (1983) even documented the presence of several birds in the peatlands north of Waskish in Beltrami County!

During this period, an intensive study of the species was conducted by graduate student Lynelle Hanson, who studied the breeding population at O. L. Kipp State Park from 1987 through 1989 (Hanson 1994). Included was an assessment of 23 sites throughout the state where the birds had been reported since 1968. In all three years of the study, the overwhelming majority of birds reported in the state were located at O. L. Kipp State Park, where she documented 23 adults in 1987, 19 in 1988, and 22 in 1989.

Considered rather ephemeral in nature, Henslow’s Sparrow populations often fluctuate considerably from year to year (Herkert et al. 2002). Their abundance at the state park in the late 1980s was followed by four consecutive years (1992–995) when they were entirely absent from the site despite intensive field searches (Minnesota Ornithologists’ Union 2016; Svingen 1998). These lean years were followed by an abundance of reports throughout the state beginning in 1996 when a total of 21 birds were reported from 5 separate locations (Svingen 1998). In 2000 they were reported from 10 counties across the southern two-thirds of the state, and in 2005 they were reported from 32 counties, including 28 singing males at renamed Great River Bluffs State Park! The only regions lacking reports were north-central and northeastern Minnesota (Minnesota Ornithologists’ Union 2016). Faber speculated that the unprecedented number of birds in Minnesota could be due to increased grassland management efforts and/or to the influx of birds from areas farther south that were experiencing a significant drought (Faber 2006).

Field biologists surveying grasslands throughout Minnesota as part of the Minnesota Biological Survey documented 80 Henslow’s Sparrow breeding season locations, all south of a line from southern Otter Tail County in the west to southern Crow Wing County in the east (Minnesota Department of Natural Resources 2016). Henslow’s Sparrows are exceedingly difficult to find, and the number of confirmed nesting records in the state remains low. Since 1970, Hertzel and Janssen (1998) documented a total of only 2 counties with nesting records: Hennepin and Winona. Indeed these remained the only 2 counties where nesting had been documented for nearly 40 years, until the MNBBA began in 2009.

During the MNBBA, participants reported 185 Henslow’s Sparrow records in just 2.1% (99/4,740) of the surveyed atlas blocks and in 1.6% (37/2,337) of the priority blocks. Breeding evidence was confirmed in only 3 atlas blocks (Figures 2 and 3; Table 1). The birds were observed in 46 counties, which covered the southern half of the state and extended north through the Red River valley and the Tallgrass Aspen Parklands Province (3 blocks straddled 2 counties each). Breeding was confirmed in 3 counties: Carver, Murray, and in a block in the Murphy-Hanrehan Park Reserve that straddled Scott and Dakota Counties. Detailed notes that accompanied the latter record confirmed that the record was in the Scott County portion of the block. The Interactive Map, however, displays the location of the block and therefore highlights both counties. Submitted by Bruce Fall, the notes for this 2012 record provide further details on the presence of the birds: “There are 7-8 Henslow’s Sparrow territories within the block in unburned restored prairie . . plus another 16 territories in the adjacent block to the west.”  Similar to the situation Hanson (1994) found at Great River Bluffs State Park, the density of breeding pairs suggests the species is “loosely colonial” (Herkert et al. 2002). The breeding records from Murphy-Hanrehan and from Carver County were observations of adults feeding young; the record from Murray County was of a juvenile sited in a block where an adult male was singing earlier in the season.

Despite the large number of counties where the Henslow’s Sparrow was reported, it remains a sparsely distributed and uncommon species. Nevertheless, Roberts (1932) prediction that the birds might be more common in southern Minnesota than the few records in the early 1900s suggested has finally proven true!

In their comprehensive review of the species, Herkert et al. (2002) provided a detailed description of the historical changes in the Henslow’s Sparrow’s distribution. In the 1800s, the species was represented by two disjunct populations: one in the central grasslands of Nebraska and Kansas, and one in the coastal marshes of New England. As the eastern forests were cleared, local populations increased and expanded north in the Great Plains and east throughout New England and the northern Ohio River valley. However, by the mid-1900s populations were declining, especially in New England. An extensive survey of more than 1,000 grassland sites in 1997 found no birds in New England. Populations in New York were limited to just 2 counties. Apart from a few remnant populations, the species has largely disappeared from the Atlantic Coast and is now confined primarily to the eastern tallgrass prairie biome of the north-central United States.

*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 Henslow’s Sparrow in Minnesota based on the Breeding Bird Atlas (2009 – 2013).

Print Map
Figure 3.

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

Breeding statusBlocks (%)Priority Blocks (%)
Confirmed3 (0.1%)1 (0.0%)
Probable29 (0.6%)16 (0.7%)
Possible67 (1.4%)20 (0.9%)
Observed0 (0.0%)0 (0.0%)
Total99 (2.1%)37 (1.6%)
Table 1.

Summary statistics for the Henslow’s Sparrow 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).

Breeding Habitat

An obligate grassland species, Henslow’s Sparrow historically was found utilizing coastal marshes, wet meadows, and weedy pastures and fields along the Atlantic coast. In the Upper Midwest, it was found in tallgrass prairie, hayfields, and wet meadows (Figure 4). For many years, moisture was considered an important component of suitable breeding habitat given the species’ prevalence in coastal marshes and wet meadows. More recent studies in the Midwest indicated vegetation structure is a far stronger predictor of occupied habitat (Burhans 2002). Important components include a dense litter layer and standing residual vegetation (Herkert et al. 2002). The litter layer is used for nesting, foraging, and protection, and residual forbs are used for song perches (Herkert [1998] 2002; Herkert et al. 2002; Hanson 1994; Burhans 2002; Sample and Mossman 1997). Woody cover is generally minimal to absent on most sites. Such structural features are not only present on native grasslands and idle hayfields but are also available on undisturbed Conservation Reserve Program lands (Herkert [1998] 2002; Cooper 2012). Grasslands on reclaimed surface mines also provide suitable habitat (Cooper 2012).

In Minnesota, Hanson (1994) conducted an in-depth assessment of the sparrow’s habitat requirements. A dense litter layer was essential and averaged 7.1 cm in depth. It was important that the litter not be compressed to the ground but provide an open area underneath to allow easier mobility for the birds. This structure was a primary feature that differentiated sites that were used by the sparrows from grasslands that were not used. The mean height of standing dead vegetation was 59 cm. Unlike other studies, Hanson’s documented no significant difference in woody cover between used and unused sites.

Some studies suggest that loose colonies are more frequently found on larger sites. In both New York and Illinois, area was the best predictor of the species’ occurrence (Herkert [1998] 2002). Other studies, however, suggest that larger sites (>100 ha) are simply preferred. When population levels are high, the birds can be found using small tracts (3–20 ha) as well as larger sites, but the larger sites are occupied first; when population levels are low, only the larger sites will be occupied (Burhans 2002). The 23 sites across Minnesota known to provide habitat to 1 or more singing males from 1968 to 1988 ranged in size from just a few hectares to more than 100 ha (Hanson 1994). The amount of habitat utilized by the species at Great River Bluffs State Park, the only site in the late 1980s that was known to support a breeding population for consecutive years, was only 23.1 ha.

Cooper (2012) warned that most in-depth habitat studies have been conducted while the species has been experiencing a dramatic, long-term population decline. When population levels are low, species are often restricted to the highest-quality habitat, providing a skewed view of suitable habitat (Smith 1992). Recent field studies in Illinois support this premise. Henslow’s Sparrow numbers have experienced a measurable increase in the state since 2000 and are now utilizing smaller disturbed sites, habitat that is generally considered less desirable (Sauer et al. 2017; Cooper 2012).

Figure 4.

Breeding habitat of the Henslow’s Sparrow in eastern Minnesota (© Lee A. Pfannmuller).

Population Abundance

Data gathered by the federal Breeding Bird Survey (BBS) have been used to model population abundance and generate a North American population estimate of 390,000 breeding adults (Rosenberg et al. 2016). In 2013, the statewide estimate for Minnesota was 6,000 birds, or 1.6% of the continental population (Partners in Flight Science Committee 2013). The species was too rare during the MNBBA to generate an updated statewide estimate.

Sparsely distributed throughout its limited breeding range, the Henslow’s Sparrow is rarely encountered along BBS routes (Figure 1). Across the United States the average number of birds observed per route per year ranges from a low of 0 to a high of only 0.18 birds per route in Indiana. In Minnesota, on the northern periphery of its range, an average of only 0.03 birds is detected per route (Sauer et al. 2017).

When the birds find suitable habitat, local breeding densities can be fairly high. Across their range, densities averaged 23 pairs per 40 ha, ranging from a low of 12 pairs to a high of 61 pairs per 40 ha (Potter et al. 2007). In Indiana, where breeding densities are relatively high, 9 to 11 males per 40 ha have been reported on large prairie tracts. In Minnesota, during the 1987 breeding season, 10 Henslow’s Sparrow breeding territories occupied 23 ha of habitat at Great River Bluffs State Park for a breeding density of 17 pairs per 40 ha. A recently discovered population near Whitewater Wildlife Management Area in Winona County supported 7 singing males in a restored prairie tract that was less than 4 ha in size, for a comparable breeding density of 28 pairs per 40 ha (D. Zumeta, pers. comm.).

Because the Henslow’s Sparrow is so sparsely distributed, the BBS does not provide statistically reliable population trends. Nonetheless, it is the only long-term population data available and shows a steady decline averaging 1.53% per year since 1966 throughout its range. BBS data in recent years suggest the population may be slowly increasing (Sauer et al. 2017). BBS data from Minnesota also suggest a slowly increasing population but with wide confidence intervals. Population gains are largely restricted to the Central Hardwoods and Central Mixed Grass Prairie regions, while large decreases are being observed farther north (Figure 5). When the second atlas was conducted in Ohio, Henslow’s Sparrows were observed in 37% fewer priority blocks (Rodewald et al. 2016); the same decline was reported in Pennsylvania during the the state’s second atlas (Wilson et al. 2012). In New York, the birds were present in 348 blocks during the first atlas and in only 70 during the second atlas (New York State Department of Environmental Conservation 2006). In all three states the first atlas was conducted in the 1980s, and the second in the first decade of the 21st century.

Overall, the species’ decline is largely attributed to the loss and degradation of grassland habitat (Cooper 2012). Federal farm policies can significantly impact species dependent on the availability of undisturbed tracts of grass. Indeed, research biologists at a 2007 workshop speculated that the Henslow’s Sparrow was at a relatively high population level in 1966, when the BBS began, due to the establishment of the Soil Bank Program in 1956. Designed to reduce farm commodity surpluses, at its peak the program retired 28.7 million acres in 10-year contracts in 1960. However, most contracts expired by 1969, which coincides with the start of the sparrow’s decline. Not until the Conservation Reserve Program (CRP) was initiated in 1985 did populations begin to rebound (Cooper 2007).

Recent decreases in the northeastern United States are attributed to intensive agricultural practices that have converted pastures and hayfields to row crops, and to the loss of habitat to development. Inactive management of idle grasslands that results in woody encroachment also degrades otherwise suitable habitat (Cooper 2012). Factors responsible for the species’ increase in the southwestern portion of its range are less clear (Cooper 2007).

Figure 5.

Population trend map for the Henslow’s Sparrow in North America for 1966–2015 based on the federal Breeding Bird Survey (Sauer et al. 2017).

Conservation

The plight of the Henslow’s Sparrow has garnered the attention of many state and federal resource agencies and conservation organizations. It is a federally endangered species in Canada. A petition to federally list it in the United States was submitted in 1998, but the U.S. Fish and Wildlife Service concluded that listing was not warranted (U.S. Fish and Wildlife Service 1998). It has been assigned a Continental Concern Score of 14/20 by Partners in Flight and added to their Yellow Watch List. The latter is a group of 12 North American birds that “require constant care” to prevent further declines (Rosenberg et al. 2016). Officially classified as a state endangered or threatened species in 13 states (Cooper 2012), it is officially classified as a state Endangered Species in Minnesota and has been designated a Species in Greatest Conservation Need (Minnesota Department of Natural Resources 2015). A national conservation plan was prepared in 2012 (Cooper 2012).

For the past several years, biologists have considered changes to the CRP as the single most immediate threat to the Henslow’s Sparrow. When the program was reauthorized in 2014, several changes were enacted that could negatively impact Henslow’s Sparrows and many other grassland-dependent species. Included among the changes was a lower enrollment cap (32 million acres decreased to 24 million acres by FY2018) and allowances for emergency harvesting and grazing without a reduction in payments. These changes also coincided with a period of high commodity prices, which have reduced overall enrollment in the CRP (Stubbs 2014).

Longer term, warming temperatures could pose serious challenges for the species. The 2010 “State of the Birds” report classified the Henslow’s Sparrow’s vulnerability to climate change as medium (North American Bird Conservation Initiative 2010), while a recent assessment by the National Audubon Society predicted a 100% loss of the bird’s current winter range, forcing it to shift northward (Langham et al. 2015National Audubon Society 2016). The species was classified as “climate threatened.”

Habitat loss and climate change are not the only threats the Henslow’s Sparrow has to contend with. Annual mortality at communication towers was identified as a major concern, comprising an estimated 1.6% of annual mortality (Longcore et al. 2013).

Numerous studies have focused on identifying management actions that improve habitat conditions. Overall, the keys to maintaining suitable habitat focus on protecting large grassland tracts (a minimum of 30 ha and preferably 100 ha or larger) and actively managing each site to control succession and woody encroachment (Herkert [1998] 2002). Grazing, burning, and mowing all improve habitat by reducing woody growth but also negatively impact the litter layer, a critical habitat component. As a result, one of the most important aspects of habitat management is to protect large sites that enable portions of the tract to be actively managed in a given season while suitable habitat conditions prevail elsewhere on the tract (Herkert [1998] 2002; Hanson 1994). Because much of the species’ habitat is on privately owned lands, it is particularly important that resource agencies promote private land conservation programs such as the CRP, CRP Enhancement, and Partners for Fish and Wildlife programs.

In Minnesota, the increasing number of reports of Henslow’s Sparrows in the southern and northwestern regions of the state is encouraging. Recent interagency efforts focused on protecting and restoring critical grassland and wetland habitats will also help to secure the future of Audubon’s little “mouse with wings” (Minnesota Prairie Plan Working Group 2011).

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