Academic journal article The American Midland Naturalist

Influence of Topography on Density of Grassland Passerines in Pastures

Academic journal article The American Midland Naturalist

Influence of Topography on Density of Grassland Passerines in Pastures

Article excerpt

ABSTRACT.-Pastures provide substantial habitat for grassland birds of management concern in the Driftless Area of southwestern Wisconsin. The rolling topography in this region is characterized by lowland valleys surrounded by relatively steep and often wooded slopes which are set apart from more expansive treeless uplands. We hypothesized that there would be lower densities of area sensitive grassland passerines in lowland grasslands compared to upland grasslands because of their preference for larger more open grasslands. To test this hypothesis and assess how well pasture area and vegetation structure predicted grassland passerine density compared to upland/lowland status, we conducted point counts of birds in 60 pastures in May-June 1997 and 1998. Upland pastures generally supported greater densities of grassland passerines than lowland pastures. Densities of Savannah sparrow (Passerculus sandwichensis) and bobolink (Dolichonyx oryzivorus) were significantly higher in upland pastures than in lowland pastures. Grasshopper sparrow (Ammodramus savannarum) density was significantly higher on uplands in one of the study years. The density of eastern meadowlark (Sturnella magna), western meadowlark (S. neglecta) and sedge wren (Cistothorus platensis) did not differ significantly between uplands and lowlands. Grassland passerine density was also predicted by pasture size and vegetation structure. Densities of bobolink and grasshopper sparrow were higher in larger pastures. Bobolink and Savannah sparrow occurred on pastures with greater vegetation height-density and less bare ground; bobolink also preferred shallower litter depths. Lowland pastures supported grassland bird species of management concern and should not be neglected. However, we recommend that pasture management for grassland passerines in areas of variable topography favor relatively large upland pastures that will contain higher densities of species of management concern.

INTRODUCTION

Grassland birds breeding in pastures in the southwestern Wisconsin Driftless Area include species of management concern (Sample and Mossman, 1997; Wisconsin Natural Heritage Program, 1999) such as the Savannah sparrow (Passerculus sandwichensis), grasshopper sparrow (Ammodramus savannarum), eastern meadowlark (Sturnella magna), western meadowlark (S. neglecta), bobolink (Dolichonyx oryzivorus) and sedge wren (Cistothorus platensis). Despite a consistent declining trend in Midwest pasture acreage that continues today, pasture is still the dominant grassland type in the region (Herkert et al., 1996).

Pastures appear to provide habitat for some grassland birds that is valuable relative to other grasslands. For example, grazed grasslands in Wisconsin have greater grassland bird species richness and abundance than hayfields and some idle grasslands (Sample, 1989). Skinner et al. (1984) found more species and individuals on grazed grasslands than on hayed lands in Missouri and Smith and Smith (1992) found a high frequency of occurrence of grasshopper sparrows in New York pastures. Conservation Reserve Program (CRP) lands provide some habitat for grassland birds of management concern, but establishment, maintenance and management of these lands are subject to political changes (Warner et aL, 2000). In addition, some CRP management techniques prevent successful nesting (Herkert, 1994a, b). Pastures suffer some of the same problems but provide additional habitat for obligate grassland species.

Understanding which pastures are most frequently used by grassland bird species of management concern is necessary for guiding management. Relative avian use of upland vs. lowland pastures with streams, regardless of management, has not been evaluated in the Driftless Area. Grasslands in the Driftless Area of Wisconsin are unusual because they have an undulating topography compared to the relatively flat landscape common to the surrounding glaciated area. However, undulating topography is found elsewhere in the midwestern United States, such as the Loess Hills of Iowa, the Flint Hills of Kansas and the Sand Hills of Nebraska. …

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