Influences of Management Regimes on Breeding Bird Densities and Habitat in Mixed-Grass Prairie: An Example from North Dakota

Abstract

It is well known that North American grassland bird populations appear to be declining (Igl and Johnson 1997, Sauer et al. 2004). Most of these birds breed and winter in North America, so declines are likely associated with continental processes (Knopf 1994). Scientists have also observed parallel declines among species that have overlapping breeding ranges but disparate wintering distributions (Igl and Johnson 1997). These patterns suggest declines may be linked to problems on the breeding grounds. Across the Great Plains, the breeding grounds for many of these species, one primary land-use is grazing by domestic cattle. Before European settlement, most of this region was grazed by freeranging herbivores, particularly bison (Hartnett et al. 1997). With settlement, bison were mostly extirpated from the region, and cattle grazing systems were established. This species replacement may have transformed the landscape because bison and cattle grazing regimes differ somewhat (Harnett et al. 1997). Bison and cattle possess slightly different foraging preferences and patterns (Hartnett et al. 1997), and cattle herds are spatially constrained whereas historic bison herds roamed freely (Steuter and Hidinger 1999). Moreover, grassland fire is usually suppressed and rarely prescribed in modern cattle-managed systems, yet during the time of the bison, naturally occurring fires burned at varying frequencies in the Great Plains (Steuter and Hidinger 1999). Today, some land managers are trying to reestablish this ecological condition by reintroducing smaller bison herds to various locations in the region, along with prescribed fire (Hartnett et al. 1997, Griebel et al. 1998). Given this recent trend, it is an opportune time to compare how a widespread, modern land-management regime and a re-created historic regime affect breeding grassland birds. Only Zimmerman (1997) and Griebel et al. (1998) have compared effects of bison and cattle grazing regimes on birds, in tallgrass prairie and sandhills prairie, respectively. Moreover, only Griebel et al. (1998) studied the effects of a bison regime that included fire. Thus, our study is the first to compare effects of these regimes in mixed-grass prairie. Of these 3 studies, ours is also unique because we estimate bird densities using distance sampling. We evaluated bird population densities and their habitats under cattle and bison management regimes in mixedgrass prairie. Our objectives were to 1) determine the influence of these different range-management regimes on habitat structure, composition, and heterogeneity; and 2) determine whether bird breeding densities differ between regimes.