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Nesting Ecology and Nesting Habitat Requirements of Ohio's Grassland-nesting Birds: A Literature Review

Results and Discussion

Henslow's Sparrow

Michigan and Ohio support the highest breeding populations of Henslow's sparrows within the species' range (Hands et al.1989). Henslow's sparrows were not regularly reported from Ohio until the 1920s. They started declining in northern and central Ohio during the 1940s and disappeared from most of this range by the 1960s, largely because of intensive agricultural land-use. While Henslow's sparrows declined in glaciated Ohio, numbers increased in southern and unglaciated counties where the birds found abundant nesting habitat on reclaimed surface mines. Today, they nest regularly in most counties southeast from Clermont, Highland, Ross, Pickaway, Licking, Knox, Richland, Stark, and Columbiana (Peterjohn 1989, Peterjohn and Rice 1991).

Most Henslow's sparrows return to their breeding grounds in Ohio between 20 April and 15 May. Nest construction is initiated in May and most first clutches are produced by 10-25 May. First broods fledge between 20 June and 10 July. Second broods and renests are most frequent during July, and produce fledglings between 25 July and 15 August. The bulk of Ohio's Henslow's sparrow population migrates south during October (Peterjohn 1989, Peterjohn and Rice 1991).

In Ohio, Henslow's sparrows nest most often in idle fields overgrown with weeds, grasses, and small trees and shrubs (Peterjohn 1989). They also nest in reclaimed surface mines, grassy hayfields, and hillside grasslands dominated by broomsedge (Andropogon virginicus). In northern and central Ohio, they formerly nested in wet prairies, bogs, and sedge-bulrush (Carex-Scirpus) margins of swamps (Peterjohn 1989, Peterjohn and Rice 1991). Most studies reported Henslow's sparrows nesting in native and restored tallgrass prairie (Robins 1971b, Zimmerman 1988, Sample 1989, Verser 1990, Smith 1992, Volkert 1992, Reinking and Hendricks 1993, Seibert 1993, Herkert 1994b), unmowed hayfields (Henninger 1910; Graber and Graber 1963; Speirs and Orenstein 1965; Bent 1968; Robins 1971a,b; Peterson 1983; Sample 1989), and undisturbed (idle) grasslands (Hyde 1939, Wiley and Croft 1964, Wiens 1969, Bowles et al. 1981, Peterson 1983, Sample 1989, Herkert 1994b). Henslow's sparrows also have been reported nesting in old fields (Graber and Graber 1963, Speirs and Orenstein 1965), pastures (Sutton 1959, Graber and Graber 1963, Peterson 1983), meadows (Hyde 1939, Graber and Graber 1963, Robins 1971b), marshes (Robins 1971b), and small grains (Sample 1989).

Characteristics of Henslow's sparrow nesting habitat replete in the literature included a deep litter layer and abundant standing dead vegetation (Wiens 1969, Robins 1971a, Samson 1980, Zimmerman 1988, Verser 1990, Clawson 1991, Herkert 1991, Reinking and Hendricks 1993), tall, dense herbaceous vegetation (Sutton 1959, Wiley and Croft 1964, Wiens 1969, Robins 1971a, Samson 1980, Peterson 1983, Hands et al. 1989, Sample 1989, Clawson 1991, Smith 1992), and little woody cover (Wiens 1969, Baskett et al. 1980, Peterson 1983, Zimmerman 1988, Hands et al. 1989, Sample 1989, Verser 1990, Clawson 1991). In Kansas, Henslow's sparrow territories contained a greater amount of standing dead vegetation, taller grasses, and a lower percent cover of woody vegetation than was found outside territories (Zimmerman 1988). Cover of grasses and standing dead vegetation were greater within than outside Henslow's sparrow territories in Missouri (Clawson 1991). Occupied areas had a significantly taller mean herbaceous canopy, denser vegetation, and a higher proportion of standing dead plant material than unoccupied areas in Illinois (Herkert 1994b).

Wiens (1969) found higher values of litter depth and cover, standing dead forb cover, and vegetation height and density in Henslow's sparrow territories than in those of other grassland-nesting birds. He reported that Henslow's sparrow nesting territories were characterized by 82% grass cover, 17% forb cover, and 1% bare ground; litter depth at nest sites averaged 4 cm. Mean vegetative characteristics of habitats occupied by nesting Henslow's sparrows in Wisconsin were <2% woody cover, 74% herbaceous vegetation cover, 24% litter cover, <2% bare ground, 88-cm maximum vegetation height, and 40-cm vegetation height-density (Sample 1989). Robins (1971a) described Henslow's sparrow nesting territories as having a continuous cover of grasses and sedges about 80 cm tall. In Kansas, mean height of live herbaceous vegetation in Henslow's sparrow territories ranged from 25-51 cm (Zimmerman 1988), similar to the 20-40cm range observed in Missouri (Clawson 1991). Herkert (1994b) reported that areas occupied by Henslow's sparrows in Illinois had an average grass height of 28 cm and a mean herbaceous canopy height of 47 cm.

Sample (1989) noted that Henslow's sparrow abundance was positively associated with maximum vegetation height, vegetation height-density, and percent cover of standing residual vegetation and negatively correlated with percent bare ground. In New York, Henslow's sparrow abundance was positively related to density of herbaceous vegetation (Peterson 1983). Baskett et al. (1980) reported that Henslow's sparrow abundance increased markedly as grass height exceeded 15 cm.

Peterson (1983) believed that any woody vegetation decreased the suitability of an area for nesting Henslow's sparrows. Occupied fields on his New York study area were either pure herbaceous communities or had a few, widely scattered, short (<1 m tall), woody stems. In Missouri, Henslow's sparrow territories were devoid of woody stems >2.5-cm diameter breast height (Clawson 1991). Wiens (1969) reported that Henslow's sparrow territories on his study area contained no trees, posts, or fence lines but considered fields with scattered woody vegetation suitable nesting habitat. Sample (1989) noted that Henslow's sparrows nested in habitats with <2% woody cover <1 m tall in Wisconsin. In Missouri, Baskett et al. (1980) described optimal Henslow's sparrow habitat as having <3 shade-producing tree or shrub clumps/50 m2.

Henslow's sparrows foraged on the ground, mostly in grassy areas where the vegetation was 10-15 cm tall (Wiens 1969). Food of adults between April and October was 82% animal and 18% plant matter. Animal matter came primarily from Orthoptera, Coleoptera, Heteroptera, Lepidoptera, and Hymenoptera. Seeds of ragweed (Ambrosia artemisiifolia), smartweed (Polygonum spp.), and sedges were the main plant foods eaten (Hyde 1939, Robins 1971b). About 80% of the nestling diet in 2 Michigan studies consisted of grasshoppers and butterfly larvae (Hyde 1939, Robins 1971b). Major foods fed to nestlings included crickets, grasshoppers, mantids, caterpillars, and brome grass florets (Robins 1971b).

Loss of habitat to agricultural development and urbanization or grassland abandonment and subsequent reversion to shrubland and forest has been implicated as the most likely factor causing declines in Henslow's sparrow populations throughout their range (Bowles et al.1981, Smith 1992, Herkert 1994b). Habitat suitability must be maintained by setting back succession periodically and producing tall (>30 cm), dense herbaceous vegetation with no, or few, scattered trees and shrubs.

In Illinois, Herkert (1994a) reported that regular mowing significantly reduced Henslow's sparrow nesting densities, despite the fact that mowing occurred outside the nesting season (Jan-Apr). Henslow's sparrows did not avoid mowed grasslands, but nesting densities were 90% lower on mowed (within 4 months of 1 May) than unmowed areas (Herkert 1994b). Lower densities of Henslow's sparrows on mowed than unmowed areas also were observed in Wisconsin (Sample 1989) and they were not encountered on recently mowed grasslands in Missouri (Baskett et al. 1980). Smith (1992) reported that, in New York, Henslow's sparrows nested in grasslands that were mowed annually after mid-August and considered mowing a viable option for maintaining suitable habitat for this species. Thus, early fall mowing permitted sufficient vegetative regrowth for Henslow's sparrows prior to the subsequent nesting season, but spring mowing did not.

Because they require tall, dense herbaceous cover, Henslow's sparrows were typically associated with ungrazed or lightly-grazed areas (Baskett et al. 1980, Peterson 1983, Zimmerman 1988, Hands et al. 1989, Verser 1990). Herkert et al. (1993) recommended that grasslands not be grazed to provide optimal Henslow's sparrow nesting habitat. Moderate grazing (resulting in 20-40% vegetative cover greater than or equal to 25 cm tall) of grasslands in Kansas rendered them unsuitable for Henslow's sparrows for up to 1 year after removal of cattle (Zimmerman 1988). However, Smith (1992), summarizing unpublished data, reported that Henslow's sparrows nested successfully in pastures grazed by cattle from mid-May to mid-October in Missouri and New York. Grass height in the pastures averaged 20-30 cm in Missouri and 61 cm in New York.

Research in Kansas (Zimmerman 1988, 1992), Oklahoma (Verser 1990, Reinking and Hendricks 1993), Missouri (Baskett et al. 1980, Clawson 1991), Illinois (Herkert 1994a,b), and Wisconsin (Sample 1989, Volkert 1992) indicated that Henslow's sparrows avoided grasslands burned the previous spring (Apr) because of the paucity of litter and standing dead vegetation. Several researchers reported that Henslow's sparrows nested in grasslands the second summer following spring burning and concluded that this species required residual vegetation from >/=1 growing season (Zimmerman 1988, Clawson 1991, Reinking and Hendricks 1993). Herkert (1994b) found that burning prevented the nesting of Henslow's sparrows the summer immediately following spring and fall fires and significantly reduced nesting densities in ensuing years as well. He reported that densities of Henslow's sparrows in grasslands the second growing season (13-16 months) post-fire were half the densities in grasslands the third or more growing season post-fire. Henslow's sparrows used recently burned grasslands by mid-July and throughout late summer and fall as foraging and loafing areas (Herkert 1994a). Herkert (1994a) also found Henslow's sparrow nests in small patches (1 ha) of unburned grassland surrounded by 120 ha of burned grassland. Zimmerman (1988) pointed out that although Henslow's sparrows did not nest in recently burned grasslands, burning was a valuable tool for this species as it stimulated aboveground productivity of grasses and produced dense standing dead vegetation in subsequent years.

Henslow's sparrows are typically restricted to large grassland tracts in the Midwest (Herkert 1994b) and Northeast (Peterson 1983, Smith 1992). Henslow's sparrows were categorized as having zero tolerance to habitat fragmentation (Herkert 1991) and grassland area was considered the major factor influencing habitat selection (Herkert 1994b). Several studies indicated that grassland area positively influenced the probability of encountering Henslow's sparrows (Peterson 1983, Herkert 1991, Smith 1992). In New York, Peterson (1983) noted that Henslow's sparrows occupied only fields on or near hilltops with an unbroken view of the horizon; fields in valley bottoms or those surrounded by trees were never occupied. Although Henslow's sparrows defended small territories in Wisconsin (0.3-1.1 ha; Wiens 1969) and Michigan (0.1-1.0 ha; Robins 1971a), they occupied only grassland tracts >40 ha. Herkert (1994c) reported that the average size of a grassland tract occupied by Henslow's sparrows in Illinois was 421 ha and reached 50% incidence at 55 ha; Henslow's sparrows were not encountered on tracts <10 ha. Samson (1980) estimated that 10-100 ha represented the minimum amount of contiguous grassland habitat required to maintain a viable breeding population of Henslow's sparrows. Six fields occupied by Henslow's sparrows in southwestern Ohio averaged 12 ha, the smallest being 8 ha (D. Nolin and J. Ritzenthaler, unpubl. data). Three CRP grasslands in Ohio used by Henslow's sparrows averaged 13 ha and ranged from 1 to 26 ha (Oh. Div. Wildl., unpubl. data).

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