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Article Excerpt
Grassland birds have exhibited more significant and consistent population declines than any other avian guild highlighting the urgency for conservation action (Peterjohn and Sauer 1999, Brennan and Kuvlesky 2005). These declines in the United States have largely resulted from extensive degradation of midwestern native grasslands (Johnson and Schwartz 1993, Samson and Knopf 1994, Warner 1994, Noss et al. 1995) and conversion of native habitats to row-crop agriculture (Herkert 1994). Grassland birds in agricultural landscapes are largely reliant on non-crop, strip habitats for food, nest sites, song perches, and escape cover (Rodenhouse et al. 1993, Best 2000). However, the diversified farming techniques and small farm fields that historically facilitated strip habitat on farmlands (Warner 1994) have been largely replaced with modern techniques that promote crop intensification to maximize yield. These land-use alterations have contributed to declines in grassland bird populations through reduced presence of weedy, strip-habitats (Herkert 1995).
The Conservation Reserve Program (CRP) helped stabilize many avian populations by providing suitable wildlife habitat on private lands, although some species have continued to decline (Ryan et al. 1998). The Dickcissel (scientific names in Table 1) was of particular concern having endured population sinks in some CRP habitats (McCoy et al. 1999). Conservation buffers are an effort to increase habitat options and enhance wildlife benefits through federal Farm Bill programs such as CRP. Conservation buffers are non-crop strips of vegetation that enhance soil and water quality, and provide suitable wildlife habitat (Best 2000). Whereas most conservation buffers are restricted to riparian zones, CP33-Habitat Buffers for Upland Birds (hereafter, field borders) are non-crop, linear strips of herbaceous vegetation that target grassland birds in upland habitats (Burger et al. 2006). Field borders benefit the avian community year-round by providing nesting habitat, foraging habitat, roosting sites, movement corridors, and escape cover (Marcus et al. 2000, Conover 2005, Smith et al. 2005, Conover et al. 2007). The contribution of herbaceous strips to adjacent plant communities may also enhance avian benefits on agriculture-wooded field ecotones, where avian abundances are typically elevated (Best et al. 1990). The vegetative diversity in strip habitats can provide other environmental benefits, including increased avian diversity (Rotenberry 1985, Bryan and Best 1994), greater abundance of agronomically-beneficial insects (Marshall and Moonen 2002), and suppression of agronomic weeds (Davison 1941, Marshall and Moonen 2002).
The high rate of field border acceptance by landowners in 35 states (80% of an available 101,174 ha enrolled in first 2 years) heightens our need to understand the interactions among field borders, farmland wildlife, and crop production. Width is a particularly important factor for bird use of linear habitats, as it may mitigate their vulnerability to edge effects (Gates and Gysel 1978, Ratti and Reese 1988, Paton 1994). Wider borders provide habitat farther from the wooded edge, which can benefit birds that use edges (Paton 1994) and those that avoid them (Johnson and Temple 1986, Herkert 1994, Vickery et al. 1994). However, previous studies of grassland bird responses to woody edges commonly focused on 50 m thresholds (Paton 1994, O'Leary and Nyberg 2000, Jensen and Finck 2004) and provide tenuous predictions for these field borders, as all except one are <50 m wide. The augmented field border area that results from increased width can also result in enhanced vegetative heterogeneity (Rodenhouse and Best 1983) and subsequently, increased avifaunal density and diversity (Warner 1992).
Our primary study objectives investigated the hypotheses that: (1) field margins with borders would have greater density and richness than without, and (2) wide-bordered margins would have greater avian density and richness than narrow-bordered margins.
METHODS
Study Area.--We conducted fieldwork on six farms in the lower Mississippi Alluvial Valley (MAV; Bird Conservation Region 26; Sunflower County, MS, USA) over two consecutive breeding seasons (May-Jun; 2003-2004). This region was historically bottomland-hardwood forest, and field borders do not constitute native habitat restoration, but semi-natural vegetation in a landscape largely devoid of non-crop vegetation. All farms were within 12 km and representative of the MAV landscape, which has nominal topographic relief dominated by large (171.14 [+ or -] 34.20 ha) row-crop fields of soybeans (58%), cotton (16%), milo (10%), and other crops (16%, e.g., corn, wheat, etc.), and fragmented by wooded fencerows and linear riparian zones (e.g., drainage ditches, rivers, and streams). Agricultural fields had varied tillage (till and minimal-till) regimes, although this should not influence avian community patterns on field margins (Castrale 1985).
Soil associations were primarily Dundee silt loam or Forestdale silt loam, which are stratified alluvium soils of fine to coarse texture with poor to moderate drainage and varying acidity levels (Powell et al. 1952). Average precipitation over 3 years was highest during June (20.1 cm) and lowest in May (8.7 cm), and July (10.1 cm). Total precipitation was somewhat lower during 2003 (39.2 cm) than 2004 (58.5 cm; USDA-NRCS Beasley Lake, MS weather station).
Field Borders.--Field borders were established between a wooded field margin (typically an old fencerow) that enclosed a drainage ditch, and an agricultural row-crop field. Borders were planted in spring 2002, although adequate vegetative cover did not emerge until late June. Field borders were 400 m...
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