Temperate grassland ecosystems are imperiled globally, and habitat loss in North America has resulted in steepdeclines of endemic songbirds. Commercial livestock grazing is the primary land use in rangelands that supportremaining bird populations. Some conservationists suggest using livestock as “ecosystem engineers” to increasehabitat heterogeneity in rangelands because birds require a spectrum of sparse to dense vegetation cover.However, grazing effects remain poorly understood because local studies have not incorporated broad-scaleenvironmental constraints on herbaceous growth. We surveyed grassland birds across a region spanning26 500 km2 in northeast Montana, United States to assess how distribution and abundance were affected byweather, soils, and grazing. We modeled bird abundance to characterize regional response to herbaceouscover, experimentally manipulated grazing to isolate its effect, and then scaled back up to evaluate how the regionalenvironment constrains bird response to grazing. Regional models predict that a quarter of our studyregion was productive grassland where managed grazing could benefit specialist species; the remainder wasnongrassland or low-productivity soils where it had low potential to affect habitat. Grassland species distributedthemselves along a gradient of herbaceous coverwith predictable shifts in community composition.We demonstratedexperimentally that grazing influences bird communities within productive grasslands, with higherutilization promoting more Chestnut-collared Longspur (Calcarius ornatus) and fewer Baird’s Sparrow(Ammodramus bairdii). Results inform a new conceptual framework for grazing that explicitly incorporates therole of broad-scale environmental constraints.
