Breeding Bird Survey: Population Trends 1966-92 | ||
by Bruce G. Peterjohn National Biological Service John R. Sauer National Biological Service Sandra Orsillo National Biological Service |
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The North American Breeding Bird Survey (BBS) was begun in 1966 to collect standardized data on bird populations along more than 3,400 survey routes across the continental United States and southern Canada. The BBS has been used to document distributions and establish continental, regional, and local population trends for more than 250 species. | ||
We summarize here survey-wide patterns in the 1966-92 population trend estimates for 245 species of birds observed on a minimum of 40 routes with a mean relative abundance of 1.0 bird per route. Survey-wide trend estimates are also summarized for six groupings of birds, providing insight into broad geographical patterns of population trends of North American birds. | ||
Methods | ||
The BBS routes are located along secondary roads and surveyed each year during the peak of the breeding season by observers competent in bird identification. Each route is 39.4 km (24.5 mi) long, with 50 stops placed at 0.8-km (0.5-mi) intervals (Robbins et al. 1986). To estimate population change, we used a procedure called route regression, described in greater detail by Geissler and Sauer (1990). | ||
We examined population change in several ways. First, we estimated overall population change for individual species over the entire survey area. Second, we looked for temporal and geographic patterns in individual bird species (e.g., Sauer and Droege 1990). | ||
Additionally, we analyzed overall patterns of population change for several species of particular management interest. Groups of birds were defined by migration status (nonmigratory, short-distance, and Neotropical migrants) or by breeding habitat (grassland, shrubland, or woodland; see also Peterjohn and Sauer 1993). For each group, we determined the percentage of species with positive (> 0) trends. If population change is not consistent within the group, about half (50%) of the species should show positive trends. Clearly, some species will show very significant declines (or increases) over the interval, and these species can be identified in the Appendix. However, the percentage of species with positive population trends is a convenient summary of information from all species within the group to demonstrate overall trend patterns. | ||
Finally, to display regional patterns of population change, we calculated the mean trend for the species in each group for each survey route. We used an Arc/Info geographic information system to summarize and display geographic patterns of population change (Isaaks and Srivastava 1989; ESRI 1992). | ||
Trends | ||
Of the 245 species considered, 130 have negative trend estimates, 57 of which exhibit significant declines. Species with negative trend estimates are found in all families, but they are especially prevalent among the mimids (mockingbirds and thrashers) and sparrows. A total of 115 species exhibits positive trends, 44 of which are significant increases. Flycatchers and warblers have the largest proportions of species with increasing populations. |
Fig. 4. Geographic patterns in the mean trends for Neotropical migrant bird species during 1966-92. | |
Neotropical migrants have received considerable attention in recent years, yet as many species have increased as have decreased during 1966-92 (Table). A region with apparently declining populations extends from the southern Great Plains across the southeastern states and along the Appalachian Mountains to southern New England (Fig. 4). Increasing mean populations prevail across the northern Great Plains and throughout much of western North America. The pattern of population decline in the eastern United States noted by Robbins et al. (1989) occurred after 1978 and is not reflected in these long-term trends. |
Fig. 5. Geographic patterns in the mean trends for short-distance migrant bird species during 1966-92. | |
Short-distance migrants and permanent residents have slightly greater percentages of decreasing species (Table). Both groups have negative mean trends in the southeastern states and from the lower Great Lakes into the Appalachian Mountains, but the patterns elsewhere are mixed (Figs. 5, 6). |
Fig. 6. Geographic patterns in the mean trends for permanent resident bird species during 1966-92. | |
These results indicate that grassland and shrubland birds are experiencing the most consistent and widespread declines of any group of species. Whenever possible, appropriate conservation measures should be undertaken to enhance the population trends of these species. While the BBS data indicate the population trends for breeding birds, these data are not designed to identify the factors responsible for these trends. To understand how bird populations are responding to the changing habitat conditions in North America, additional studies are needed that would combine the BBS results with regional data on land-use changes, weather conditions, and other variables. |
Appendix. Population trends of birds from the North American Breeding Bird Survey. To appear in this list, the species must have been seen on > 40 routes at an average count of > 1 bird/route. We present trends (%/year), probability (P), and the number of routes on which the species was seen. See Peterjohn and Sauer 1993 for group classification. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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National Biological Service Patuxent Environmental Science Center Laurel, MD 20708 |
References | |
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ESRI. 1992. Understanding GIS: the Arc/Info method. Environmental Systems Research Institute, Inc., Redlands, CA. 416 pp. Geissler, P.H., and J.R. Sauer. 1990. Topics in route-regression analysis. Pages 53-56 in J.R. Sauer and S. Droege, eds. Survey designs and statistical methods for the estimation of avian population trends. U.S. Fish and Wildlife Service Biological Rep. 90(1). Isaaks, E.H., and R.M. Srivastava. 1989. An introduction to applied geostatistics. Oxford University Press, New York. 561 pp. |
Peterjohn, B.G., and J.R. Sauer. 1993. North American Breeding Bird Survey annual summary 1990-1991. Bird Populations 1:52-67. Robbins, C.S., D. Bystrak, and P.H. Geissler. 1986. The Breeding Bird Survey: its first fifteen years, 1965-1979. U.S. Fish and Wildlife Service Resour. Publ. 157. 196 pp. Robbins, C.S., J.R. Sauer, R.S. Greenberg, and S. Droege. 1989. Population declines in North American birds that migrate to the Neotropics. Proceedings of the National Academy of Science USA 86:7658-7662. Sauer, J.R., and S. Droege. 1990. Recent population trends of the eastern bluebird. Wilson Bull. 102:239-252. |