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

Authors: DAVIS, KRISTIN - Colorado State University; Augustine, David; MONROE, ADRIAN - Colorado State University; ALDRIDGE, CAMERON - Colorado State University

Submitted to: The Condor: Ornithological Applications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/21/2021
Publication Date: 9/27/2021
Citation: Davis, K.P., Augustine, D.J., Monroe, A.P., Aldridge, C.L. 2021. Vegetation characteristics and precipitation jointly influence grassland bird abundance beyond the effects of grazing management. The Condor: Ornithological Applications. 123:1-15. https://doi.org/10.1093/ornithapp/duab041.
DOI: https://doi.org/10.1093/ornithapp/duab041

Interpretive Summary: Ranches make up a substantial amount of the land base in North America. These lands also support much wildlife, including many bird species of conservation concern. Some research suggests livestock grazing can be managed to benefit certain grassland bird species, but we need to better understand how and where this can be achieved. In this paper, we analyze five years of bird monitoring data from the Central Plains Experimental Range in northeastern Colorado in order to examine how environmental factors (topography, soil types, and vegetation) as well as cattle grazing management strategies affect the abundance of five grassland bird species. Variation in bird abundance could be explained in large part by environmental factors and annual variation in rainfall for all bird species, with each bird species exhibiting different relationships with the environmental factors. However, grazing management practices could also explain variation in bird abundance, particularly for two species of conservation concern, often just as well as environmental factors. These findings indicate that cattle grazing management can be an important factor influencing abundance of birds of conservation concern, while regional precipitation conditions and local vegetation structure-composition reveal additional mechanisms behind bird distribution in these rangelands. Because our study occurred during a series of years with average or above-average rainfall, additional studies during droughts are still needed.

Technical Abstract: Agricultural conversion and rangeland management practices that promote structural and compositional homogeneity of vegetation have degraded and reduced the extent of North American grasslands over the last 200 years. Populations of grassland birds have declined sharply and concurrently with this loss. Grassland birds differentially respond to variation in vegetation structure and density generated by spatiotemporally varying disturbances (e.g., herbivore grazing, weather), but we lack an understanding of the specific habitat components these species respond to in semiarid grasslands. Thus, we examined the predictive capacity of bird abundance models that represented habitat at the scale of bird point counts (~3 ha grassland patches) in terms of vegetation attributes, topography, and interannual precipitation variability. We then compared these results with models based on grazing management treatments applied at the scale of whole pastures (~130 ha) and landscape-scale edaphic variability as represented by ecological site types, which represented information more generally available to rangeland managers (vs. detailed, site-specific vegetation information). Models based on patch-scale vegetation structure and composition effectively predicted abundance of all species in a manner that was consistent with but more nuanced than a priori expectations for each species based only on vegetation structure. Precipitation also strongly affected the abundance of all focal species. For 3 of our 5 focal species, we previously showed that abundance could be modeled as a function of pasture-scale grazing treatment combined with broad edaphic groupings and interannual variation (i.e. ecological site and year effects). Here, we show that patch-scale models based on vegetation attributes and interannual precipitation variability could explain greater variability in abundance for those 3 species than our grazing models. While grazing management can be applied adaptively to influence habitat for these species, our more detailed vegetation-abiotic models identify species-specific habitat components that could be targeted for management.