INFLUENCE OF GRAZING MANAGEMENT ON NATIVE GRASSLAND PLANT AND SONGBIRD COMMUNITIES

Grasslands are disappearing at an alarming rate, making them one of the most imperilled ecosystems on the planet. This loss has fueled a silent crisis for grassland songbirds, whose populations have plummeted more than any other avian group in North America. At the heart of conserving and recovering grassland songbirds is the critical need for habitat heterogeneity – landscapes with diverse vegetation structures. Yet, the forces that once sculpted this heterogeneity – vast herds of grazing bison and periodic fire – have largely disappeared from the landscape, leaving the grassland homogenized and less capable of supporting biodiversity. In the absence of historical disturbance regimes, thoughtfully managed grazing has emerged as one of the most viable and practical tools for restoring the complexity of these prairie ecosystems. However, questions remain about how different grazing systems, particularly those involving bison and cattle, can be strategically used to restore and sustain habitats that grassland songbirds so urgently need. This research sought to address these questions by exploring the effects of bison and cattle grazing on grassland breeding birds in the mixed-grass prairie in southwestern Saskatchewan. Specifically, I compared vegetation structure and species-specific songbird densities under the following grazing regimes: 1) Bison and cattle grazing under differing management regimes across similar summer seasonal grazing periods and comparable environmental conditions. Bison pastures were characterized by minimal management intervention and limited control over grazing parameters such as pasture rotation and stocking rates. Cattle pastures were managed more intensively with greater adaptability and increased flexibility in pasture rotations, rest periods, and stocking rates 2) Bison grazing under seasonal pasture rotation with a large summer range pasture and a smaller winter range pasture. The summer pasture was grazed from mid-spring to late fall or early winter primarily by cow-calf pairs at a moderate stocking rate. The winter range pasture was grazed by cows and bulls at moderate to high stocking rates from late fall or early winter to mid-spring and were supported by supplemental feeding 3) Cattle grazing under two distinct management systems, one characterized by low stocking rates, late spring to early fall seasonal grazing, long continuous grazing durations, and consistent use of cow-calf pairs, and the other employing moderate to moderately high stocking rates, mid-spring to mid-fall seasonal grazing, variable grazing durations, a mix of cow-calf pairs and yearlings, and periodic rest-periods. I found that bison and cattle grazing influenced habitat structure and significantly impacted grassland songbird densities. The type of grazer (i.e., bison or cattle) was the most significant factor, with seven out of ten grassland songbird species exhibiting notable differences in density between these two grazing systems. Seasonality in bison grazing contributed to subtle variations in vegetation structure and songbird densities. Winter-grazed pastures were characterized by greater cover of bare ground and forbs. Among the species assessed, brown-headed cowbirds exhibited a pronounced difference in density between seasonal treatments, whereas the differences observed for Baird’s sparrows and chestnut-collared longspurs were comparatively modest. Similarly, variations in cattle management strategies, specifically stocking rates, grazing duration, and herd dynamics, resulted in slight differences in vegetation structure and grassland songbird densities. Of the 10 species studied, the density of only two species (Savannah sparrow and Baird’s sparrow) exhibited notable variations in density between the low-intensity grazed pasture managed by the Nature Conservancy of Canada and the moderate-intensity grazed pastures managed by the Arena Community Pasture Group. The findings of this study underscore the importance of incorporating both bison and cattle grazing into management plans to create variable vegetation structures necessary to support the diverse habitat requirements of grassland songbirds. A one-size-fits-all approach to grazing is unlikely to meet the needs of species with differing ecological requirements. Instead, land managers should consider using different grazing strategies, such as those deemed effective within this study, to optimize habitat suitability for species of conservation concern while maintaining the economic viability of livestock production.

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