• Rocky Mountain Research Station
• 240 West Prospect
• Fort Collins, CO 80526
• (970) 498-1100

Factors Affecting Grassland Bird Richness

The study was conducted on 29 transects that were located along secondary roads -- each transect being 25 miles in length. These transects are part of a national bird monitoring network called the North American Breeding Bird Survey and these surveys have been conducted annually since the mid-1960s. Each year a competent observer drives along the transect, stopping every 1/2 mile to count all birds seen or heard within a 3-minute period of time. The actual paths of each transect were digitized and brought into a geographic information system along with satellite imagery (Landsat 5 Thematic Mapper) to characterize land use and habitat patterns around the bird survey transects (see images). We classified land use and habitat into 15 categories. We also linked in grasshopper data (grasshoppers are an important prey item of grassland birds) from the Animal and Plant Health Inspection Service's grasshopper surveys that are also conducted along roadsides. We used the land use, habitat, and grasshopper data to estimate statistical models that predicted grassland bird richness across the 29 bird transects.

Our best model suggested that grassland bird richness was positively related to the amount of grassland habitat near bird transects, negatively related to how fragmented grassland habitats were, positively associated with edge habitats, and positively associated with the variety of grasshoppers. These results suggest that the distribution of grassland bird species in influenced by a complex mixture of factors that includes habitat area effects, landscape pattern, and the availability of a variety of prey species that allow grassland bird species to coexist.

Studies at broad spatial scales are needed for a more complete understanding of the effects of human-induced landscape change, including the loss of open space and the natural habitats they contain, on biodiversity in general and grassland birds in particular. The results provide information to those who manage grassland habitats on the kinds of factors that are likely affect grassland bird richness. Because much of the remaining grassland habitats east of the continental divide are privately owned, our results inform the Agency's State and Private Forestry Programs directed at providing information to land owners on how to best manage natural habitat for species conservation.

More information on this study can be found in: "Factors associated with grassland bird species richness: the relative roles of grassland area, landscape structure, and prey." Landscape Ecology 21:569-583.