Northern Prairie Wildlife Research Center
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.
|Species||Area Sensitivity b||Height (cm)||Cover (%)||Litter (%)||Bare ground(%)||Shrub cover(%)|
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).
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).
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).
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.