Nest-site selection and productivity of Vesper Sparrows breeding in grazed habitats
Livestock grazing in the shortgrass steppe of the Intermountain region of British Columbia may have a negative impact on ground-nesting birds, but evidence of such an impact is lacking. We examined nest-site selection and productivity of ground-nesting Vesper Sparrows (Pooecetes gramineus) across sites with different grazing histories. From 2006 to 2008, we monitored Vesper Sparrow nests and measured vegetation characteristics known to be affected by grazing within nest patches. We used an information-theoretic approach to test the relative importance of grazing-affected vegetation variables as predictors of nest-site selection, nest survival, and nestling condition. Vesper Sparrows selected nest sites with greater cover of late-seral grass species that decrease in occurrence in response to grazing (i.e., “decreasers”) than was available in random patches in the same territories. Daily nest survival was also lower for nests surrounded by shorter vegetation (odds ratio = 1.12). However, “decreaser” cover was not associated with either of the two indices of productivity measured (daily nest survival probability and nestling condition). In addition, vegetation height, although an important driver of success, was not linked with nest-site selection, and no vegetation-cover variable was positively associated with productivity, despite nest concealment being central to our predictions. This suggests that predation risk for nests in areas with shorter vegetation was being elevated through some factor unrelated to concealment. Our results show that grazing reduced both the availability of suitable habitat for and nesting success of Vesper Sparrows, indicating that grazing could pose a threat to population persistence at a broader scale and could potentially contribute to observed declines. Additional research is needed to determine if grazing guidelines in the Intermountain region of British Columbia should be amended, better enforced, or both to prevent regional declines in populations of ground-nesting grassland birds.
Document Type: Research Article
Affiliations: Centre for Wildlife Ecology, Department of Biological Sciences, Simon Fraser University, 8888 University Drive, Burnaby, British Columbia V5A 1S6, Canada
Publication date: 2011-06-01