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

Results and Discussion

Grasshopper Sparrow

Grasshopper sparrows nest in every Ohio county, but their relative abundance varies considerably across the state; highest densities occur in the western and central counties (Peterjohn and Rice 1991). Numbers of breeding grasshopper sparrows in Ohio peaked during the 1930s and have declined significantly since (Peterjohn 1989). While recent increases of grasshopper sparrows on reclaimed surface mines in eastern Ohio have partially compensated for their declines in the central and western counties (Peterjohn and Rice 1991), their current statewide population continues to decline (Table 1).

Grasshopper sparrows normally return to breeding grounds in Ohio during late April and early May. First clutches are generally laid during the last half of May and early June. Most young fledge between 20 June and 5 July. Renesting attempts and second broods produce clutches through mid-July and recently fledged young through mid-August. Most grasshopper sparrows leave Ohio for the winter during August (Peterjohn 1989, Peterjohn and Rice 1991).

The grasshopper sparrow is noted for its patchy distribution and marked between year fluctuations in abundance, even when apparently suitable habitat is available (Smith 1963). Researchers reported grasshopper sparrows nesting in hayfields (Walkinshaw 1940, Johnston and Odum 1956, Smith 1963, Bent 1968), pastures (Dambach and Good 1940, Good and Dambach 1943, Lowther 1984), old fields (Johnston and Odum 1956, Smith 1963, Bent 1968), meadows (Walkinshaw 1940, Graber and Graber 1963), native and restored tallgrass prairie (Kendeigh 1941, Bent 1968, Elliott 1978, Blankespoor 1980, Sample 1989, Volkert 1992), reclaimed surface mines (Whitmore and Hall 1978; Whitmore 1980, 1981; Anderson et al. 1994), fields bordering airport runways (Alsop 1979), roadside rights-of-way (Warner 1992, Camp and Best 1994), railroad rights-of-way (Braband 1984), grassed waterways in rowcrop fields (Bryan and Best 1991,1994), small grains (Dambach and Good 1940, Good and Dambach 1943, Smith 1963, Bent 1968), cultivated rowcrops (Johnson and Schwartz 1993a) and no-tillage corn and soybean fields (Wooley et al. 1985, Basore et al. 1986). In Ohio, grasshopper sparrow nesting densities were higher in meadows (31 nests/40 ha) than in corn and small grain fields (6 nests/40 ha) (Good and Dambach 1943). Grasshopper sparrows avoided rowcrops during the nesting season in Wisconsin (Sample 1989). In the northcentral United States, grasshopper sparrow nesting densities were >40 times higher in CRP fields than in adjacent cropland (Johnson and Schwartz 1993a).

The grasshopper sparrow was described as a species of open, treeless habitats (Sutton 1959, Smith 1963, Speirs and Orenstein 1965, Wiens 1969). Grasshopper sparrow nesting territories in Georgia (Johnston and Odum 1956) and North Carolina (McNair 1984) were characterized by almost no woody vegetation. Grasshopper sparrow nesting densities in old fields in West Virginia decreased as percent shrub cover increased (Smith 1963). Johnston and Odum (1956) and Bent (1968) reported that grasshopper sparrows were not found in areas with > or equal to 35% tree or shrub cover. However, in Montana, Walcheck (1970) classified the grasshopper sparrow as a forest edge species because it was most abundant in multiflora rose (Rosa multiflora) thickets bordering cottonwood (Populus spp.) forests. Sample (1989) described grasshopper sparrows in Wisconsin as tolerant of woody vegetation. Although the Florida grasshopper sparrow (A. s. floridanus) selected nesting habitats with higher percent shrub cover than the eastern grasshopper sparrow (A. s. pratensis) (19.2 vs.0.3%, respectively), percent grass cover and the dominance of bunchgrass species, rather than sodforming types, were similar in the nesting territories of the 2 races (Wiens 1969, Whitmore 1981, Delany et al. 1985).

Throughout their range, researchers reported highest grasshopper sparrow nesting densities in low, sparse vegetation (Smith 1963; Cody 1968; Wiens 1969, 1973a) and avoidance of tall, dense cover (Smith 1963, Shugart and James 1973, Whitmore 1979a, Sample 1989). Johnson and Schwartz (1993b) reported a negative correlation between grasshopper sparrow abundance and percent legume cover, consistent with the species' preference for grass-dominated habitats (Johnston and Odum 1956, Shugart and James 1973). Herkert (1994c) found grasshopper sparrow abundance negatively correlated with mean grass height and positively related to percent grass cover.

Whitmore (1979a) stated that grasshopper sparrows required areas free of dense living vegetation and litter with a minimum of 24% bare ground. Several researchers noted that dense vegetation and accumulated litter precluded effective foraging of grasshopper sparrows (Smith 1963; Wiens 1969, 1973a,b; Janes 1983). On reclaimed surface mines in West Virginia, 3 habitat-structure variables changed significantly over a 3-year period (Whitmore 1979a): mean vegetation height decreased, percent bare ground increased, and percent grass cover decreased. Corresponding to this opening up of the vegetation was an increase in grasshopper sparrow density (pairs/100 ha) from 21.7 in 1976 to 79.5 in 1978 (Whitmore 1979a).

Sample (1989) found most grasshopper sparrow nests in portions of pastures and hayfields with cover 30-60 cm tall. In Florida, grasshopper sparrows nested most frequently in shrub and grass cover 50-70 cm tall (Delany et al.1985). Frawley and Best (1991) reported that grasshopper sparrow nesting density in alfalfa fields decreased significantly when alfalfa reached a height of 30 cm.

Thirty grasshopper sparrow nesting territories in Wisconsin were characterized by 75% grass cover, 23% forb cover, 2% bare ground, and an average herbaceous canopy height of 41 cm (Wiens 1969). Corresponding values from 100 grasshopper sparrow nesting territories in West Virginia were 26% grass cover, 25% forb cover, 22% bare ground, and a mean herbaceous canopy height of 44 cm (Whitmore 1981). Mean vegetative characteristics of habitats used by nesting grasshopper sparrows in Wisconsin were 3% woody cover, 76% herbaceous vegetation cover, 16% litter cover, 8% bare ground, 57-cm maximum vegetation height, and 14-cm vegetation height-density (Sample 1989). Habitats occupied by nesting grasshopper sparrows in Arizona were characterized by 5% woody cover, 72% grass cover, 4% forb cover, 23% bare ground, and a mean grass height of 30 cm (Bock and Webb 1984).

Wiens (1969) and Whitmore (1981) compared vegetation structure within grasshopper sparrow nesting territories to that outside territories. Wiens (1969) reported that forb density, forb height, vegetation density, and litter depth were significantly greater outside territories. Similarly, Whitmore (1981) found that territories were more sparsely vegetated than non-territories, having lower values of percent grass, forb, and shrub cover and vegetation height and higher values of percent bare ground.

Grasshopper sparrows remain in the nest for about 9 days (Walkinshaw 1940), but both adults and young usually remain in the general vicinity of the nest until fall (Smith 1963). Grasshopper sparrows generally foraged in grass cover < or equal to 5 cm tall, but frequently fed in grass cover 6-15 cm tall (Wiens 1969, 1973a). Insects, primarily grasshoppers and their nymphs, were the main foods of grasshopper sparrows during the nesting season; grass and forb seeds constituted < or equal to 20% of the diet (Wiens and Rotenberry 1979, Kaspari 1991). In North Carolina, soybeans and other monoculture crops were important foraging habitats, insects and grass seeds being the major food items (McNair 1984).

Fields cut during May for grass silage or hay in West Virginia supported few nesting grasshopper sparrows (Smith 1963). Grasshopper sparrows did not abandon fields after hay cutting if their nest had not been destroyed, but parasitism and predation increased because of the reduced cover (Smith 1963). Alsop (1979) considered hay cutting between early May and mid-July a contributing factor to the decline of grasshopper sparrows in Tennessee. Grasslands mowed outside the nesting season (after 15 Jul) were used by nesting grasshopper sparrows in Georgia (Johnston and Odum 1956) and Arkansas (Shugart and James 1973).

Herkert (1991, 1994a) compared grasshopper sparrow densities between mowed (cut 1-4 months prior to 1 May) and unmowed (not mowed for 12 months prior to 1 May) areas in Illinois and found that densities were twice as high on the mowed areas. Nesting densities of grasshopper sparrows were 5 times greater on roadsides not mowed until after 1 August compared to those mowed repeatedly during the summer in Illinois (Warner 1992). In Iowa rowcrops, grasshopper sparrows nested only on grassed waterways that had been mowed the previous year (Bryan and Best 1994).

Alsop (1979) believed the effects of grazing on grasshopper sparrows was poorly understood and needed further research. Although grasshopper sparrows were common in ungrazed sites in western grasslands (Wiens 1973a, Bock and Webb 1984), they nested only in grazed portions of tallgrass prairie in South Dakota (Blankespoor 1980) and Oklahoma (Risser et al. 1981). Grazing at a rate of 1 AUM/8 ha was considered compatible with grasshopper sparrow management in Florida (Delany et al. 1985). In Georgia, grazing at a rate of 7 cows/ha did not affect nesting grasshopper sparrows (Johnston and Odum 1956). Herkert et al. (1993) believed that light grazing, resulting in greater than or equal to 40% vegetative cover greater than or equal to 25 cm tall, would benefit the grasshopper sparrow because of its intermediate vegetation height and density preferences. In Missouri, Skinner (1975) and Baskett et al. (1980) concluded that moderate grazing, resulting in 20-40% vegetative cover greater than or equal to 25 cm tall, was required to ensure peak numbers of nesting grasshopper sparrows. In the northern Great Plains, Kantrud (1981) and Kantrud and Kologiski (1982) found that grasshopper sparrow densities were significantly greater in lightly-grazed than heavily-grazed plots.

Prescribed burning during winter (Nov-Jan) at 2-3-year intervals was considered the best management practice for grasshopper sparrow nesting habitat in Florida (Delany et al.1985) and West Virginia (Whitmore 1979a, 1981). Higher densities of grasshopper sparrows were reported on grasslands the first and second growing seasons post-fire than on those the third or more season post-fire in Florida (Nicholson 1936, Delany and Cox 1986), Arkansas (Shugart and James 1973), and Illinois (Herkert 1991,1994a). In Kansas (Zimmerman 1992), Missouri (Baskett et al. 1980), Wisconsin (Volkert 1992), and South Dakota (Force et al. 1984), low densities of grasshopper sparrows on grasslands 2-3 years post-fire were attributed to the vegetation becoming too tall and dense. In Montana, grasshopper sparrow abundance was significantly lower in burned compared to unburned shrub steppe communities (Bock and Bock 1987).

Grasshopper sparrows occupy large grassland tracts and avoid narrow strips of grassy vegetation (Peterjohn and Rice 1991). Herkert (1991) did not encounter grasshopper sparrows on grasslands <10 ha and classified them as moderately sensitive to habitat fragmentation. Grasshopper sparrow incidence rose sharply as grassland area increased and reached 50% at about 30 ha in Illinois (Herkert 1994c) and 100 ha in Maine (Vickery et al.1994). In Ohio, grasshopper sparrow abundance was positively related to size of CRP fields (Swanson et al. 1995). Samson (1980), based on work in Missouri, estimated that 1-10 ha represented the minimum amount of contiguous grassland habitat required to maintain a viable breeding population of grasshopper sparrows. In Florida, Delany et al. (1995) suggested that 240-1,348 ha of contiguous prairie habitat were required to maintain a viable population of 50 breeding pairs. In a study of 8 grassland fragments in Minnesota, Johnson and Temple (1986,1990) found that grasshopper sparrows were more likely to nest in large (130-486 ha) fragments and that nest predation rates were lower than in small (16-32 ha) fragments. In southwestern Ohio, 8 fields used by grasshopper sparrows averaged 16 ha (D. Nolin and J. Ritzenthaler, unpubl. data). Nine CRP fields occupied by grasshopper sparrows in Ohio averaged 11 ha and ranged from 1 to 21 ha (Oh. Div. Wildl., unpubl. data).

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