Vegetation characteristics and precipitation jointly influence grassland bird abundance beyond the effects of grazing management

Grassland birds have experienced some of the steepest population declines of any guild of birds in North America. The shortgrass steppe contains some of North America's most intact grasslands, which makes the region particularly important for these species. It is well known that grassland birds differentially respond to variation in vegetation structure generated by spatiotemporally varying disturbance like grazing management. However, understanding how species respond to characteristics beyond vegetation structure or grazing could better inform management for these species in the shortgrass steppe. We analyzed point count data for 5 grassland bird species breeding on the Central Plains Experimental Range in northeastern Colorado from 2013 to 2017 to examine the predictive capacity of models representing fine-scale (∼5 ha) vegetation attributes (vegetation structure and cover type) and topography, combined with interannual precipitation variability (i.e. vegetation-abiotic models). We then compared these models to models based on grazing management treatments (applied to whole pastures, ∼130 ha) and edaphic conditions (ecological sites), which represented information more generally available to rangeland managers. Precipitation, vegetation structure, and vegetation cover type influenced all species in a manner consistent with, but more nuanced than, vegetation structure alone. These models also explained more variation in abundance for species that responded to grazing management. Thus, while grazing management can be applied adaptively to improve habitat for these species, our more detailed vegetation-abiotic models identified species-specific habitat components that could be targeted for management. For example, not grazing pastures with extensive, homogenous stands of mid-height grasses (e.g., Hesperostipa comata) for an entire growing season during wet years could be one strategy to enhance Grasshopper Sparrow (Ammodramus savannarum) abundance and stockpile residual forage for future utilization by livestock. Our models provide a better understanding of and reveal nuances in the suite of environmental conditions to which grassland birds respond in shortgrass steppe rangelands. LAY SUMMARY We lack clear understanding of which environmental characteristics might support declining bird species breeding in the shortgrass steppe of the United States. These bird species often breed on lands managed for cattle grazing and are known to respond to vegetation structure generated by grazing and/or soil conditions. We sought to evaluate how precipitation and different types of vegetation cover (e.g., shrubs, shortgrasses) could explain additional variation in bird abundance in this system. We found vegetation cover, vegetation structure, and precipitation could explain more variation in abundance than cattle grazing management and/or ecological sites (i.e. unique soil types with associated plant communities) for most bird species. For bird species that did not respond to grazing management, ecological site and year effects explained more variation in abundance than vegetation characteristics and precipitation. Precipitation conditions, types of vegetation cover, and ecological sites often are not considered when developing grazing management plans to support grassland birds; incorporating these factors into management plans may help support populations of these declining species.