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Henslow’s Sparrow

Background

 

1.1   Description

Henslow’s Sparrow is a small (13 cm, 10–15 g) grassland sparrow. The head is pale olive-green and has two black stripes on the top, separated by a pale median stripe. The feathers on the back are black edged with white, creating a scaled appearance. The rump, wings, and tail are chestnut coloured, with black in the middle of the feathers, and the breast, sides, and flank are buff with black streaks. Adult males and females look alike, but young Henslow’s Sparrows can be distinguished from adults by the lack of streaking on the buff underparts. This is a very secretive species; it is rarely seen and difficult to flush. It is most easily detected when males sing during the breeding season. The song is an insect-like “tsi-lick.”

1.2    Populations and Distribution

1.2.1   Global Breeding Distribution

Henslow’s Sparrow breeds in the northeastern United States, from eastern South Dakota, Minnesota, New York, and central New England south to Kansas, Missouri, Kentucky, North Carolina, and eastern Texas. In Canada, it has been known to breed in southern Ontario and southwestern Québec (Figure 1). Throughout this range, it has a very scattered and localized distribution. Less than 9% of the global range occurs in Canada (NatureServe 2006).

Figure 1. North American distribution of Henslow’s Sparrow

Figure 1. North American distribution of Henslow’s Sparrow.

1.2.2   Canadian Breeding Distribution

Henslow’s Sparrow was recorded during the breeding season in southwestern Québec (e.g., Hull, Eccles-Hill, Montreal) between 1943–1950 and 1965–1968 , but it has not been recorded breeding there since 1968 (Godfrey 1972; Knapton 1982). It is now considered only a vagrant there (Gauthier and Aubry 1996). For this reason, Henslow’s Sparrow is not listed as a species at risk in Québec.

In Ontario, the historical range of Henslow’s Sparrow is considered to be southern Ontario, north to Barrie and Ottawa and east to at least Morrisburg. However, the breeding range has contracted substantially since the 1950s. In the early 1980s, Knapton (1982, 1986) found the main concentration of breeding pairs to be in the southern part of Hastings, Lennox-Addington, Frontenac, and Prince Edward counties. In the early 1990s, a thorough search for the birds revealed only a single singing male (Austen 1994). Recent surveys in eastern Ontario have yielded similar results. At least seven singing males were heard on occasion in the Regional Municipality of Halton in 2000 (M. Austen pers. comm.). Breeding evidence was documented in a total of nine locations in Ontario during the 2001–2005 Ontario Breeding Bird Atlas surveys. (Figure 2).

Figure 2. Breeding distribution of Henslow’s Sparrow in Canada from 1981 to 1985 and from 2001 to 2005

Figure 2. Breeding distribution of Henslow’s Sparrow in Canada from 1981 to 1985 and from 2001 to 2005. [Reproduced with permission from Ontario Breeding Bird Atlas.]

1.2.3   Winter Distribution

Henslow’s Sparrow is a short-distance migrant, wintering primarily in the southeastern United States. The winter range is not well known, but is believed to include eastern Texas, southern Louisiana, southern Mississippi, southern Alabama, Florida, southern Georgia, eastern South Carolina, and southeastern North Carolina.

The species is difficult to detect during migration, and so migration patterns, pathways, and behaviour are poorly understood.

1.2.4   Population Size and Trends

The continental population of Henslow’s Sparrow has experienced a significant decline during the period 1966–2004, averaging an 8.7% annual decline. Breeding Bird Survey data suggest that Henslow’s Sparrow populations have declined in Michigan, Ohio, and Wisconsin. Data from other states in the north-central United States are insufficient to allow meaningful trends to be calculated, although Breeding Bird Atlas data suggest that population trends in these states are variable. For example, in Illinois, the establishment of large areas of grassland through the Conservation Reserve Program has resulted in a 10-fold increase in Henslow’s Sparrow numbers in recent years (Herkert 2005). In western Pennsylvania, reclaimed surface mines have created an estimated core area of 35 373 ha of grassland habitat where at least 4884 Henslow’s Sparrows were present in 2001 (Mattice et al. 2005). Despite this increase in suitable habitat, Henslow’s Sparrow range in Pennsylvania appears to have remained relatively stable between 1984 and 2004 (Pennsylvania Breeding Bird Atlas 2006).

The State of New York noted a significant decline in Henslow’s Sparrow throughout its range in surveys conducted between 1980–1985 and 2000–2004 (New York State Department of Environment and Conservation 2005). Currently, the largest area of suitable habitat appears to be concentrated in Jefferson County, in the vicinity of Fort Drum Military Reserve, where the population has also been declining (C. Norment pers. comm.). In this county, 151 fields were surveyed for Henslow’s Sparrow in 1997, and 18 (12%) were occupied by a total of 47 male birds; by 2005, only four male Henslow’s Sparrows were recorded at a total of 3 of 156 fields (2%) (C. Norment, pers. comm.).

In Michigan in 2005, 20 singing males were recorded in the southern part of the Lower Peninsula, and one singing male was recorded in the northern part of the Lower Peninsula; no singing males were recorded in the Upper Peninsula of Michigan (J. Gibson pers. comm.). The decline of Henslow’s Sparrow in Michigan since the 1970s may correspond with the more intensive use of grasslands occurring there in the mid-1970s (R. Adams pers. comm.).

The first Ontario Breeding Bird Atlas (1981–1985) reported Henslow’s Sparrow in only 38 squares, and confirmed breeding was reported in only three of these (Cadman et al. 1987). In the early 1980s in Ontario, it was estimated that there were fewer than 50 pairs remaining in the southern part of Hastings, Lennox-Addington, Frontenac, and Prince Edward counties (Knapton 1987). In the early 1990s, a thorough search for the birds in these areas revealed only a single singing male. The results of surveys in 1992 and 1993 suggested that there were probably fewer than 10 pairs nesting in Ontario at that time (Austen 1994). Breeding evidence was documented in nine locations in the 2001–2005 Ontario Breeding Bird Atlas and suggests that at least one breeding territory may exist in Ontario each year. The conservation status for Henslow’s Sparrow across its range is outlined in Table 1.

 

Table 1. Conservation Status
StatusJurisdiction
N1 (critically imperilled nationally)Canada(N1B)
N3 (vulnerable to extirpation nationally or extinction)United States(N3B N4N)
S1 (critically imperilled in the province/state)Ontario (S1B), Arkansas (S1B, S2N), Maryland (S1S2B), Minnesota (S1B), Massachusetts, Nebraska, New Jersey (S1B), Tennessee (S1B), Vermont (S1B), Virginia (S1B), West Virginia (S1B)
S2 (imperilled in the province/state)Alabama(S2N), District of Columbia (S2S3N), Illinois, Michigan (S2S3), North Carolina (S2B, S1N) , Oklahoma, Texas (S2S3N, SXB), Wisconsin (S2S3B)
S3 (vulnerable to extirpation in the province/state)Georgia, Indiana (S3B), Iowa (S3B), Kansas (S3B), Kentucky (S3B), Louisiana (S3N), Missouri, New York (S3B)
S4 (apparently secure)Ohio, Pennsylvania (S4B)
SNA (conservation rank not applicable)Mississippi, South Carolina
SUB (unrankable due to lack of information or conflicting information)South Dakota
SNR (not yet assessed)Quebec, Florida(SNRN)
SX (presumed extirpated)Rhode Island
SHB (historical breeding)Connecticut(SHB, SHN), Delaware (SHB, S1N), New Hampshire

Source: NatureServe (2006)

 

1.3      Needs of Henslow’s Sparrow

1.3.1     Habitat and Biological Needs

Biological Needs

Birds arrive on the breeding grounds in Ontario in late April and early May. Males begin singing as soon as they arrive on the breeding grounds, with the frequency and vigour of this song increasing until mid-May (Herkert et al. 2002). Singing begins approximately one half hour before sunrise and stops approximately one half hour after sunset, with singing intensity greatest at dawn and dusk.

Males defend their territory; territories may be clustered to form a loose colony (Wiens 1969; Cully and Michaels 2000). In Michigan, the average territory size was 0.3 ha (Robins 1971); in Wisconsin, the average territory size is larger (0.7 ha ± 0.26 SD, n = 4; Wiens 1969); and in Pennsylvania, the territories on reclaimed surface mines are often smaller (0.18 ha ± 0.05 SD, n = 22; Piehler 1987). Henlow’s Sparrows are generally monogamous. Females build the nest in about 5–6 days (Hyde 1939). The cup-shaped nest is constructed of dead vegetation (typically grass) and placed at the base of grass clumps, resting on litter usually 2.5 cm to several centimetres above the ground (Robins 1967 1971). The nest is generally not fastened to the standing vegetation. A new nest is constructed for each nesting attempt (Robins 1971). Typically, 4–5 eggs (range 2–5 eggs) are laid, one per day. Incubation over a 10- to 12-day period is performed by the female, as is brooding. Young are tended by both parents and fledge at 9–10 days. In Michigan, two clutches may be raised in a single year; it is not known whether Henslow’s Sparrows are double-brooded in Ontario. Birds leave the breeding grounds in Ontario during September or early October. Very low numbers of banded birds are recaptured in successive years at active colonies (Herkert et al. 2002), suggesting either low site fidelity by individual sparrows or high mortality. However, colonies will remain active year after year if suitable habitat is available.

Breeding Habitat

Henslow’s Sparrows occupy open fields. They are believed to have originally been adapted to the tallgrass prairie community (Knapton 1982), wet fields, and marshes. Many of these grassland and prairie habitats in both the United States and Canada have been converted to agricultural lands, developed, or degraded through intense grazing pressure (Smith 1992); others have grown in with woody species in the absence of fire. Less than 1% of Canada’s tallgrass prairie remains; tallgrass prairie remnants are in southern Manitoba and Ontario (Morgan et al. 1995). Today, Henslow’s Sparrows in Ontario inhabit mainly pastureland and uncut and abandoned hayfields.

The key elements of the breeding habitat, based upon studies from the United States and Ontario, are summarized below. Henslow’s Sparrow has highly specific habitat requirements on the breeding (and wintering) grounds. However, as population density in an area increases, a wider range of habitat elements may be selected, and the importance of the following features may decline (J.R. Herkert pers. comm.).

Tall, dense grass cover – In Ontario, colonies have been located in abandoned fields, ungrazed or lightly grazed pasture, fallow hayfields with high clover and alfalfa content, grassy swales in open rolling farmland, wet meadows, or, infrequently, mowed fields (Cuddy 1984). The key feature of these habitats has been a high percentage of cover and a moderate to high density of grasses and sedges. The dense vegetation is typically over 30 cm tall. Herkert (1998) reviewed the habitat associations of Henslow’s Sparrow and found that their abundance was positively related to maximum herbaceous vegetation height and maximum vegetation density and negatively correlated with the amount of bare ground.

Thick thatch layer – A thick mat of dead plant material from previous years’ vegetation is generally found in the ground layer. In Kansas (Zimmerman 1988), Wisconsin (Wiens 1969), and Illinois (Herkert 1994a), occupied areas had a higher density of standing dead vegetation than unoccupied areas. Areas with high litter depth readings (Wiens 1969; Winter 1999) and greater litter coverage (Wiens 1969; Kahl et al. 1985) appear to be favoured and may be associated with greater nest success (Winter 1999). However, Henslow’s Sparrows were negatively correlated with this feature in Missouri (Skinner et al. 1984).

Lack of emergent vegetation – Henslow’s Sparrows appear to avoid sites with hills or treelines nearby and sites with posts, fence lines, wires, or trees (Wiens 1969). They will also avoid grassland with emergent shrubs or trees. Long, unbroken views to the horizon may be essential (Peterson 1983). In New York, Henslow’s Sparrow territories had fewer than 10 woody stems (average height 0.5 m) per 250 m2 and shrub cover <1% (Krebs 2002). In Kansas, Henslow’s Sparrow habitat had significantly lower tree (>4 m tall) densities (mean 0.54 trees/ha) than random sites (6.67 trees/ha; Cully and Michaels 2000). 

Large areas of grassland habitat – Henslow’s Sparrow was described as an area-sensitive species in Illinois; grassland size had a significant positive influence on the probability of occurrence for Henslow’s Sparrow, and a grassland fragment of 55 ha was required for the probability of occurrence to equal 50% of its maximum value (Herkert 1994b). The average size of an occupied grassland patch was 421 ha (Herkert 1994a). J.R. Herkert (pers. comm.) suggests that as population density increases and birds become more common, breeding birds are increasingly found in smaller fields; large tracts of grassland may be required for birds to establish and maintain active colonies when densities are low.

Restored grasslands should be greater than 50 ha in size, preferably greater than 100 ha. Contiguous grassland greater than 30 ha should be provided. Smaller grasslands are generally dominated by generalist species and less likely to support viable populations of area-sensitive species such as Henslow’s Sparrow (Herkert 1998). However, small fragments surrounded by other grassland habitat and near large grassy areas may also provide suitable habitat, but support lower densities (Winter and Faaborg 1999). A rotational system of management, where management (e.g., mowing, burning, grazing) is applied to small sections of the grassland on a regular rotating schedule, may be most appropriate and would best be facilitated in large grasslands. Management units should be approximately 30 ha in size (Herkert 1998).

Low-lying wet areas – In Ontario, a number of historical locations contained, or were adjacent to, low-lying areas that were seasonally flooded during the spring. Canada blue-joint (Calamagrostis canadensis) or reed canary grass (Phalaris arundinacea) were common in these habitats (Cuddy 1984). In Michigan, Henslow’s Sparrow occupied habitat with an intermediate moisture range; very wet or very dry areas were avoided (Robins 1971). J.R. Herkert (pers. comm.) states that of 11 grassland fields studied for 11 years in Illinois, the field containing the most stable population between years was also the wettest; he speculates that this native prairie field contained habitat with the most stable vegetation structure from year to year (even after fire) and that this stability was attributable to the wetness of the location. At Fort Drum Military Reserve in New York, Henslow’s Sparrow breeding pairs appeared to select microhabitats with standing water (C. Norment pers. comm.).

Migration Habitat

Because Henslow’s Sparrows are believed to migrate singly or in small groups at night over a short period (1–2 weeks), migrating individuals are rarely observed. They have been found in grassland habitats, adjacent to grassland habitats in hedgerows, and at the edges of shrubby areas.

Winter Habitat

Little is known about habitat selection on the wintering grounds. Typical habitat appears to be open longleaf pine (Pinus palustris) savannas that have a dense ground cover; fire intervals are important for maintaining appropriate forest structure (Chandler and Woodrey 1995; McNair 1998; Plentovich et al. 1999; Fuller et al. 2005; Johnson et al. 2005; Thatcher et al. 2005).

1.3.2     Limiting Factors

Henslow’s Sparrows require large areas of grassland for breeding, rearing, and feeding, little disturbance, and a sufficient supply of invertebrates for food. Undisturbed grassland habitat other than the necessary management through fire, grazing, or mowing is essential. The following activities can result in destruction of habitat:

·      drainage or infilling of low-lying areas;

·      urbanization;

·      aforestation;

·      cultivation without extended periods of fallow;

·      regular mowing that prevents the formation of tall, dense herbaceous cover;

·      heavy grazing that prevents the formation of tall, dense herbaceous cover;

·      succession of grasslands to shrubland or forest; and

·      fire that prevents the formation of tall, dense herbaceous cover with a dense thatch layer.

Management of grassland habitat using fire, grazing, or mowing is periodically required to ensure the long-term availability of both breeding and wintering habitat. The timing of these events appears critical. Historically, prairie grasslands were swept repeatedly by wildfire, which prevented the invasion by woody vegetation. The time taken by Henslow’s Sparrow to recolonize grassland following a fire appears variable and location specific and may be related to the timing, intensity, and heterogeneity of the burn. In Kansas, Henslow’s Sparrows returned in low densities to a field managed on a three-year burn rotation one year after the fire and at much higher densities after 2–3 years (Austen et al. 1997). In Minnesota, Henslow’s Sparrow recolonized a burned field 4–5 years after a fire, around the same time that woody forbs reestablished, and were potentially eliminated when rotation periods less than four years were used (Austen et al. 1997).

1.4     Threats

Threats to the survival of Henslow’s Sparrow are presented in order of significance.

1.4.1   Loss/Degradation of Breeding Habitat

The decline of Henslow’s Sparrows in the United States and Canada appears to track the loss of grassland or old-field habitats on the breeding grounds (Knapton 1986; Hands et al. 1989; McPeek 1991; Peterjohn and Rice 1991; Smith 1992). Industrial and residential development and changes in agricultural practices are the key factors involved in habitat loss and decline. Changes to agricultural practices that degrade habitat include row crop production, fodder and grain production, the continual use of fields with no fallow periods, earlier and more frequent cutting of hay crops, overgrazing, and aforestation. Natural events, such as succession of grassy fields to shrub and forest or flooding of low-lying areas, also destroy habitat. Henslow’s Sparrow requires large patches of suitable habitat, and so fragmentation of habitat through changing land use practices also threatens habitat. Recent Henslow’s Sparrow population increases in some areas of the United States (a 10-fold increase in Illinois) appear to be associated with the creation of undisturbed grassland habitat by the Conservation Reserve Program (Herkert 1997; Herkert et al. 2002), suggesting that habitat creation could reverse the negative population trend for this species over time.

1.4.2   Loss of Wintering Habitat

The typical wintering habitat, longleaf pine savannas, are threatened by many of the same processes that threaten breeding habitat. Primary threats include changes due to a decreasing frequency of fire, habitat degradation, or habitat loss through drainage, urbanization, and conversion to agriculture or pine plantations (Herkert et al. 2002). For example, in Mississippi, pine savannas managed on a three- to four-year fire cycle appear to provide suitable wintering habitat for Henslow’s Sparrow (Chandler and Woodrey 1995); few sparrows are recorded when fire intervals are longer.

1.4.3   Catastrophic Disturbance

The small population size and clumped breeding distribution due to both the limited availability of suitable habitat and the semi-colonial breeding behaviour of Henslow’s Sparrows suggest that localized catastrophic disasters such as poorly managed or uncontrolled fire, incompatible agricultural practices, and extreme weather events would pose a threat to the species.

Localized catastrophic events (e.g., intense storms and hurricanes) on the wintering grounds may also pose a threat to the species. Currently, insufficient information is known about the winter distribution of Henslow’s Sparrow to enable an assessment of its vulnerability.

1.4.4   Low Adult and Juvenile Survival

Few birds banded at breeding sites have returned to those same sites the following year (Robins 1967; Hands et al. 1989; Skipper 1998), suggesting that adult or juvenile mortality may be high before or during migration or on the wintering grounds. However, Henslow’s Sparrow may also not be faithful to individual breeding locations due to the unpredictable nature of its habitat (Hands et al. 1989), and so the lack of banded birds returning does not prove a high level of mortality. Increased levels of monitoring are required to confirm this.

1.4.5   Threats to Reduce Breeding Productivity

Very little information is available on either nest success rates or predation rates for Henslow’s Sparrow. In Michigan, Robins (1971) found that six of 11 nests (55%) had at least one young and that all young were successfully raised in only one of those 11 nests (9.1%). From 46 eggs, 17 young were produced (37%; Robins 1971). Because nests are placed so close to the ground, mammals such as skunks, weasels, raccoons, and snakes are expected to be important nest predators (Robins 1971; Smith 1992; Winter 1999; Winter et al. 2000). Predation may be higher in small grassland fragments, particularly grassland habitats close to woody cover. Predation rates on artificial ground nests in tallgrass prairie fragments were examined in Missouri. Nests close to woody vegetation (<60 m) experienced a 28.7% predation rate, compared with a 7.9% predation rate for nests farther away (Burger et al. 1994). For Henslow’s Sparrow, nest success was lower in areas less than 50 m from a shrubby edge, presumably because of predation (Winter et al. 2000).

Nests in Michigan and Ontario are infrequently parasitized by Brown-headed Cowbirds (Molothrus ater) (Robins 1971; Peck and James 1987). In Ontario, one of 12 nests examined had been parasitized by cowbirds (Peck and James 1987), representing a parasitism rate of 8.3%. In Oklahoma and Missouri, parasitized nests that successfully fledged young fledged both Henslow’s Sparrow and cowbird young (Winter 1999; Reinking et al. 2000). 

Competition for habitat, particularly with other sparrows, may limit breeding success. Aggressive interactions between Henslow’s Sparrow and Bobolinks (Dolichonyx oryzivorus), Savannah Sparrows (Passerculus sandwichensis), Grasshopper Sparrows (Ammodramus savannarum), and Red-winged Blackbirds (Agelaius phoeniceus) have been observed (Wiens 1969; Robins 1971). Savannah and Grasshopper sparrows in particular have a high degree of habitat overlap with Henslow’s Sparrow (Hands et al. 1989; Smith 1992; Smith and Smith 1992), although Henslow’s Sparrow appears to have a larger area requirement (Smith and Smith 1992) and requires taller, denser grassland habitat.

The most significant threat to breeding productivity may be habitat disturbance early in the breeding season from agricultural activities such as grazing and mowing. If these activities do not prevent territory establishment, they may delay the onset of breeding until vegetation height and density are sufficient to provide breeding habitat; as a minimum, grass tussocks are required (Winter 1999). Mowing during the breeding season will result in a high rate of nestling and fledgling mortality and is incompatible with Henslow’s Sparrow persistence. However, mowing later in the summer may be acceptable. For example, in New York, hayfields that were mown in September, leaving unmown strips or unmown habitat in a checkerboard pattern, provided suitable habitat for Henslow’s Sparrow the following spring (S. Lazazzero pers. comm.).

1.5    Actions Already Completed or Under Way

In 1995, a draft habitat management plan for Henslow’s Sparrow was prepared (Enright 1995). This plan provides broad guidelines on habitat area size and shape, grass mixtures to plant, and prescribed burn, grazing, haying, and woody vegetation management. The management plan proposed to restore approximately 1000 ha of grassland habitat in South Cayuga, Ontario, predominantly on land owned by the Ontario Ministry of Natural Resources. The management plan was not implemented.

In 1998, an adaptive habitat management project was initiated at Ostrander Point in Prince Edward County. Approximately one third of the area identified for treatment was mowed and cleared of brush. In 1999, bird surveys were conducted to determine if the mowing and clearing of brush had had a positive impact on Henslow’s Sparrow; several singing males were heard in 1999 and again in 2000, indicating that with careful habitat management, this species may again breed there (Environment Canada 2006).

While no habitat stewardship projects in Ontario have focused exclusively on Henslow’s Sparrow, a few habitat securement projects were completed between 2000 and 2006, such as on Walpole Island First Nation lands, and habitat restoration projects that could benefit the species were completed in Alderville First Nation and Pelee Island, among other locations.

1.6    Knowledge Gaps

In addition to information gaps related to the identification of critical habitat, there is currently inadequate information available on:

·        the size, status, and distribution of the Henslow’s Sparrow population in Canada;

·        productivity and factors affecting productivity;

·        management techniques to maintain, create, or enhance habitat for Henslow’s Sparrow in Ontario;

·        sources of the birds that immigrate into Ontario from the United States;

·        migration and wintering habitat needs and the location of each for the Canadian population; and

·        significance of migration and wintering habitat threats to the Canadian population.

Henslow’s Sparrow will respond to taped calls of conspecifics. Call playback was used for censusing Henslow’s Sparrow in the early 1990s (M. Austen pers. comm) and could be used for attracting them to a new breeding site (through conspecific attraction).