During the 1960s, Chandler Robbins and his associates at the Migratory Bird Population Station (now the Patuxent Environmental Science Center) in Laurel, Maryland developed the concept of a continental monitoring program for all breeding birds. The roadside survey methodology was field tested during 1965, and the North American Breeding Bird Survey (BBS) was formally launched in 1966 when approximately 600 surveys were conducted in the U.S. and Canada east of the Mississippi River. The survey spread to the Great Plains states and prairie provinces in 1967. By 1968, approximately 2000 routes were established across southern Canada and the contiguous 48 states, with more than 1000 routes surveyed annually.
The BBS continued to grow as more birders became aware of the program. During the 1980s, the BBS expanded into the Yukon and Northwest Territories of Canada, and Alaska. Additional routes have been added in a number of states, so by 1994, there are approximately 3700 active BBS routes across the continental U.S. and Canada, of which nearly 2900 are surveyed annually.
Breeding Bird Surveys are conducted during the peak of the nesting season, primarily during June, although surveys in desert regions and some southern states are conducted in May. Each route is 24.5 miles long, with a total of fifty stops located at 0.5 mile intervals along the route.
A three-minute point count is conducted at each stop, during which the observer records all birds heard or seen within 0.25 mile of the stop.
The BBS was designed to provide a continent-wide perspective of population changes. To achieve this perspective, the routes are randomly located in order to sample habitats that are representative of the entire region. Other requirements such as consistent methodology and observer expertise, visiting the same stops each year, and conducting surveys under suitable weather conditions are needed to produce comparable data over time. A large sample of routes is needed to average local variations and reduce the effects of sampling error.
Route density varies considerably across the continent, reflecting regional densities of skilled birders. The greatest densities are in the New England and Mid-Atlantic states, while densities are lower elsewhere.
Once the data are recorded in the field, it is sent to the BBS office at Patuxent where it is computerized. Data are recorded at each stop, and then totaled over the entire 50 stop route.
The BBS data are very challenging to analyze. The survey produces an index of relative abundance rather than a complete count of breeding bird populations. The data analyses assume that fluctuations in these indices of abundance are representative of the population as a whole.
Despite its complicated analyses, the BBS has proven to be a very valuable source of information on bird population trends. The following examples provide an indication of the types of analyses that can be performed on these data.
BBS data can be used to produce continental relative abundance maps. When viewed at continental or regional scales, these maps provide a reasonably good indication of the relative abundances of species that are well sampled by the BBS. They should be viewed with some caution, however. Where species approach the edges of their ranges, they tend to be rare, locally distributed, and likely to be poorly represented along BBS routes. The procedures used to produce these maps also tend to distort the edges of the ranges. Hence, BBS abundance maps provide only an approximation of range edges. Here is an example of a BBS relative abundance map, for the Bobolink.
Analyzing population changes on survey routes are probably the most effective use of BBS data, but these data do not provide an explanation for the causes of population trends. To evaluate population changes over time, BBS indices from individual routes are combined to obtain regional and continental estimates of trends. Although some species have consistent trends throughout the history of the BBS, most do not. For example, populations of permanent resident and short-distance migrant (birds wintering primarily in the U.S. and Canada) species are adversely affected by periodic episodes of unusually harsh winter weather. Examples include the Carolina Wren whose populations dramatically declined during the harsh winters of 1976-1977 and 1977-1978 but rapidly recovered during the 1980s.
Few species have consistent trends across their entire ranges, so geographic patterns in trends are of considerable interest to anyone concerned with the status of the continent's birds. Route- specific trends can be smoothed to produce trend maps that allow for the identification of regions of increase and decline. The following map for Bobolink is typical of BBS trend maps, and shows widespread declines in much of its range. Similar declines have been experienced by most grassland birds.
Although trends at the species level will always be a basic part of BBS data analyses, combining species into groups with similar life-history traits, known as guilds, provides additional insight into patterns of population trends. Consistent trends within a guild are indicative of the overall health of the habitat factors defining the group, and establish a community approach for analyses of BBS data.
Neotropical migrant birds have generated considerable interest in recent years, although reports of their declines have been exaggerated by some.
With interest in the BBS at an all-time high among observers conducting surveys and researchers using the data, its prospects appear bright. Expansion in portions of central and western North America will result in improved geographic coverage and more accurate trend estimates for many species. We are also studying the feasibility of making it a truly North American project by expanding into northern Mexico. As part of a three-year study, twenty-six surveys were conducted by Mexican biologists in 1993 and 27 in 1994. The continued success of the BBS is a direct result of the efforts of the state and provincial BBS coordinators, and of the thousands of volunteers who conduct surveys each year. These dedicated people deserve our thanks.