Effects of changes in agriculture and pasture based dairy farming on grassland bird species
Changes in land use, agricultural practices, and the subsequent reduction of mosaic grassland, which vary spatially and temporally in structure, have resulted in dramatic and range wide population declines of grassland birds. These grassland species have exhibited more substantial and continuous population declines than any other behavioral or ecological guild. To understand the impact of agricultural practices, we investigated if grassland bird communities differed on dairy pastures and grassland fragments and if vegetation structure and composition contributed to bird community differences in southwest Michigan. Rather than relying on bird counts, we created utilization distributions (UD) to analyze these bird communities. A UD was created for each species included in our analysis, these likelihood values were used in all subsequent analyses. Vegetation were also sampled across all study sites, horizontal and vertical cover was recorded and used to determine species association with specific structural components and which species were found at each site.
Correspondence analysis indicated that pasture and grassland bird communities differed. Based on this analysis, specific species were shown to be more strongly associated with dairy pastures or grassland fragments, with very little overlap of bird species between sites. This finding suggests that each site meets specific habitat characteristics, not found at the other sites, necessary to support certain species. Canonical correlation analysis confirmed that vegetation structure and composition contributed to variation in species distributions, suggesting that the species-specific associations found in the correspondence analysis were at least partially due to the vegetation structure of the dairy pastures and grassland fragment. Species-specific models indicated that some grassland birds were associated with unique vegetation characteristic. However a few species were not associated with any of the analyzed vegetation characteristics. Interestingly while there was some overlap in vegetation association no two species had the same relationships with vegetation. We concluded that species-specific habitat requirements are generally fulfilled through mosaic grasslands and that both grasslands and agriculture fields should be managed to maintain mosaics vegetation structure that varies spatially and temporally in order to maintain a diverse community of grassland bird species.