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

Summary and Research Needs

Factors responsible for the decline of grassland bird populations are not entirely understood (Knopf 1994), but are believed to be a combination of loss and degradation of grassland habitat (Askins 1993, Herkert 1994c), reproductive failure because of high rates of nest predation and parasitism (Johnson and Temple 1986, 1990), and shifts in agricultural practices, such as earlier and more frequent cutting of hayfields (Warner and Etter 1989). Although none of the species reviewed in this report are currently in danger of global extinction, local or even regional extinctions are likely, especially if the destruction and fragmentation of grassland habitat continues (Herkert 1994c).

As the quantity of grassland habitat declined in the Midwest, an increasing proportion was present in small, isolated patches (Samson 1980, Herkert 1991). Although the impact of habitat fragmentation has not been as well studied in grassland birds as in forest birds (Askins 1993), there is evidence that many grassland birds require a minimum area of contiguous habitat. In a study of 14 tallgrass prairies (0.5-510 ha) in Missouri, Samson (1980) found that grasshopper sparrows occurred in <30% of prairies <10 ha and that upland sandpipers and Henslow's sparrows were absent from prairies <10 ha. Herkert (1991) surveyed birds in 24 grasslands (0.5-650 ha) in Illinois. Twelve sites were tallgrass prairie remnants, 4 were restored prairie, and 8 were cool-season grass stands. The density of grassland-dependent birds was significantly lower in smaller than larger tracts. Small grasslands (<30 ha) were dominated by habitat generalists, including edge species (e.g., red-winged blackbird [Agelaius phoeniceus], field sparrow [Spizella pusilla], and song sparrow [Melospiza melodia]). Herkert (1991) did not encounter bobolinks, Savannah sparrows, grasshopper sparrows, or Henslow's sparrows on grasslands <10 ha or upland sandpipers on grasslands <30 ha. Vickery et al. (1994) surveyed 90 grasslands in coastal Maine. The incidence of upland sandpipers, vesper sparrows, savannah sparrows, grasshopper sparrows, bobolinks, and eastern meadowlarks increased with grassland area.

Species responses to reduction in habitat area are variable and estimates of minimum area requirements vary by species, region, and habitat type (Samson 1980, Herkert 1991, Vickery et al. 1994). It is difficult, therefore, to predict or determine the grassland size at which a particular species is likely to occur. Management or enhancement activities that increase the amount of contiguous grassland habitat available will benefit area sensitive species, those most affected by habitat fragmentation (Herkert 1991). Herkert et al. (1993) classified the upland sandpiper, bobolink, savannah sparrow, and Henslow's sparrow as highly sensitive to habitat fragmentation; the eastern meadowlark, western meadowlark, and grasshopper sparrow as moderately sensitive; and the vesper sparrow and dickcissel as not affected by habitat fragmentation (Table 2). The sensitivity of lark sparrows and ring-necked pheasants to habitat fragmentation has not been studied. The minimum area requirement and sensitivity to habitat fragmentation for these 11 species in Ohio must be determined.

Table 2. Summary of upland grassland bird nesting habitat requirements.a

Herbaceous vegetation
Species Area Sensitivity b Height (cm) Cover (%) Litter (%) Bare ground(%) Shrub cover(%)
Upland Sandpiper High 26(12-45) 84(75-100) 15(5-25) 10(4-25) <1
Bobolink High 49(35-65) 88(78-98) 16 4(2-5) 2
Eastern meadowlark Moderate 48(38-66) 86(74-97) 29 4(2-5) 4
Western meadowlark Moderate 44 91(81-100) 7(0-14) 3(0-5) <1
Dickcissel Low 63 74 17 9 <1
Savannah sparrow High 54(42-66) 63(22-93) 16 17(2-29) <1
Grasshopper sparrow Moderate 43(30-57) 75(51-98) 16 14(2-23) 4
Henslow's sparrow High 57(20-88) 87(74-99) 24 2 2
Vesper sparrow Low 45(20-62) 75(47-97) 17 17(2-39) 4
Lark sparrow Unknown 13 59 - 37 4
Ring-necked pheasant Unknown 48(18-70) 56(43-68) 28(23-34) 24(19-29) -

aAverage and range of values from literature cited in text.
bHigh = occurence highest on grassland tracts greater than or equal to 50 ha; moderate = occurence highest on tracts greater than 10 ha; low = occurrence similar on tracts of all sizes (Herkert et al. 1993).

Reproductive success is a better measure of habitat quality than use or abundance (Van Horne 1983). A major reason many grassland bird species are declining may be an inability to raise young (Johnson and Temple 1990). In fragmented landscapes, high rates of nest predation and nest parasitism by brown-headed cowbirds significantly reduce the ability of many avian species to successfully reproduce. Although few studies related grassland area to reproductive success, there is evidence that grassland birds have lower reproductive rates in habitat islands than in large habitat blocks. Wray et al. (1982) reported a high rate of nest predation (43% of 185 nests) for 4 sparrow species on a 42-ha reclaimed surface mine in West Virginia. The high predation rate was attributed to the close proximity of forest and pasture. Johnson and Temple (1986, 1990) compared reproductive success of 5 species of grassland birds between large (130-486 ha) and small (16-32 ha) grasslands in Minnesota. Birds nesting in small tracts experienced higher nest predation rates than those in large grasslands, and both nest predation rates and parasitism were higher in grasslands with woody edges. Grassland birds that nested <45 m from a forest edge had lower reproductive success than those that nested farther away. Additional research relating grassland area to use, nesting density, and reproductive success of grassland-nesting birds is needed.

Burger et al. (1994) examined predation rates on artificial ground nests (n = 540) in 15 tallgrass prairie remnants (4-571 ha) in Missouri. Proximity to woody cover had more influence on artificial-nest success than did tract size. Artificial nests <60 m from woody cover experienced a 28.7% predation rate compared to 7.9% for nests farther away. In small grasslands, adjacent woody habitats may allow edge and woodland predators to penetrate interior grassland areas. Management implications include the acquisition of large grassland tracts and minimization of edge effects through reduction of woody vegetation along edges and within grasslands (Wiens 1963; Wray et al. 1982; Johnson and Temple 1986, 1990; Burger et al. 1994). The effects of woody vegetation control by fire, herbicides, or mechanical means on habitat use, nesting density, and reproductive success of grassland-nesting birds must be examined (Askins 1993).

Management strategies to benefit grassland birds center on protecting or establishing large, contiguous grassland tracts, maintaining structurally diverse habitat, eliminating catastrophic midseason mowing, reducing edge, and eliminating or controlling woody encroachment (Herkert 1991). Herkert (1991) and Vickery et al. (1994) believed that large grasslands were necessary to support a diverse grassland avifauna as small grasslands were dominated by habitat generalists and, thus, were of little conservation value to most grassland birds. Herkert et al. (1993) recommended that grassland restorations aimed at benefiting bird species most sensitive to habitat fragmentation be >/=50 ha, preferably >100 ha. Small grasslands (<30 ha) benefit grassland birds with a moderate or low sensitivity to habitat fragmentation. Where grassland restorations >/=30 ha are not possible, Herkert et al. (1993) recommended establishing several small grasslands, 6-8 ha minimum size, within 0.4-km of each other, and using adjacent grassland habitats (e.g., pastures, hayfields, waterways) as corridors between tracts.

In the absence of management (mowing, grazing, or burning) grassland vegetative productivity declines and extensive invasion of woody plants occurs (Bragg 1982, Hulbert 1986). These factors lead to the rapid transformation of grassland into Savannah or forest. Despite the widespread use of mowing, grazing, and burning as management techniques, their effects on breeding bird communities inhabiting midwestern grasslands are poorly understood (Herkert 1991; Table 3).

Table 3. Effect of habitat management practices on abundance of upland grassland-nesting birds.a

Species Mowingb Light Moderate Heavy Burningd
Upland Sandpiper + 0 - - +
Bobolink + 0 - - u
Eastern Meadowlark + 0 - - 0
Western Meadowlark u + + - u
Dickcissel u - - - u
Savannah sparrow + 0 - - u
Grasshopper sparrow + + u - +
Henslow's sparrow - 0 - - -
Vesper sparrow u 0 + 0 u
Lark sparrow u + + + u
Ring-necked pheasant - u u u u

a+ = positive; - = negative; 0 = neutral; u = unknown.
bSpring (prior to 1 May) and fall (after 1 Aug).
cLight = greater than or equal to 40% vegetation cover greater than or equal to 25 cm tall; moderate = 20-40% vegetation cover greater than or equal to 25 cm tall; heavy = less than 20% vegetation cover greater than or equal to 25 cm tall (Herkert et al. 1993).
dSpring (Mar-Apr) and fall (Oct-Nov).

Each species has a unique set of habitat requirements consisting of area and vegetative composition and structure (Table 2). Some grassland-nesting birds prefer short, sparse vegetation (upland sandpiper, vesper sparrow, and lark sparrow), others prefer tall, dense vegetation (bobolink, dickcissel, and Henslow's sparrow), and some prefer intermediate levels of vegetation height and density (eastern meadowlark, western meadowlark, savannah sparrow, grasshopper sparrow, and ring-necked pheasant). Under proper management, a 50-ha grassland tract in Ohio might support breeding populations of eastern meadowlarks, Savannah sparrows, grasshopper sparrows, and ring-necked pheasants, but that same tract may be unsuitable for upland sandpipers and Henslow's sparrows. Small grassland tracts are dominated by species that show a minor response to management (i.e., habitat generalists) and should be managed as a single unit. Management of large grasslands is more complex because they typically support the most management-sensitive species (Herkert 1991). Management of large grasslands is further complicated by the fact that management-sensitive species occupy opposite ends of the management spectrum (Table 2). Renken and Dinsmore (1987) and Herkert (1994a,b) recommended a rotational system of management in which sections (20-30%) of large grasslands are treated (mowed, grazed, or burned) on a regular rotating schedule. Such a rotating schedule would provide a diversity of habitats within a grassland each year and ensure the availability of suitable cover for birds at both ends of the management spectrum. The size of the optimal management unit requires further research, but Zimmerman (1988) recommended >/=30 ha.

Additional research is needed to determine the effects of spring (prior to 1 May) versus fall (after 1 Aug) mowing on the habitat use, nesting density, and reproductive success of grassland-nesting birds. Mowing of grasslands must be avoided during the nesting period, 1 May through 1 August. Theoretically, light grazing should benefit species with intermediate vegetation height and density preferences, moderate to heavy grazing should benefit species with low vegetation height and density preferences, and no grazing should benefit species that prefer tall, dense vegetation (Table 2). Herkert et al. (1993) suggested burning grasslands managed for breeding bird habitat in early spring (Mar-Apr) or late fall (Oct-Nov). They recommended burning 25-30% of grasslands >/=50 ha annually.

Although much has been written regarding nesting ecology and nesting habitat requirements of grassland-nesting birds, many aspects remain poorly understood. A summary of research needs addressing these is presented in the Appendix.

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