Many species of birds are well adapted to the vast grasslands that characterized the central United States before European settlement. Changes in this landscape over time have resulted in steep declines in populations of many of these species, and the extinction of some. Begun in 1966, the BBS collects information annually from more than 4,000 routes across the United States and Canada.
Volunteers collect data along roadside survey routes during peak nesting season.
Each route is 24.5 miles long, with 50 stops at 0.5 mile intervals. A 3-minute point count is conducted at each stop, during which the observer records all birds heard or seen within 0.25 mile. Routes are randomly located to sample representative habitats. In analyzing population trends, data are weighted to adjust for variances caused by the sampling methodology, including both observer effects and missing years of data. Physiographic regions are used to develop regional trends.