Note that the data published in the 2002 State of the Nation’s Ecosystems Report as well as the 2003 and 2005 Web-Only Updates have been superseded by the 2008 Report and thus should be used with caution. For the most recent data, purchase the 2008 Report from Island Press.

The Indicator

This indicator reports the change in population of invasive and native, non-invasive grassland/shrubland birds. The invasive birds include both non-native birds and some native birds that spread aggressively because of a favorable change in conditions. The non-invasive birds are native birds that depend on high-quality native grasslands and shrublands. There was some interest in separating the groups of this indicator by native/non-native; however, given the low number of birds involved, a decision was made to maximize the number of species in each group to improve the statistical reliability of the results. Thus, both natives and non-natives were included in the invasive category.

The Data

Data Source: This indicator incorporates population trend estimates for 15 invasive non-native and 35 native grassland bird species. Estimates are based on data collected for the North American Breeding Bird Survey (BBS), and were obtained from the Patuxent Wildlife Research Center (PWRC), United States Geological Survey, U.S. Department of the Interior. Trends were estimated for BBS Physiographic Strata (regions) 6–8, 32–56 and 80–91, in seven 5-year intervals from 1966 to 2000 (http://www.mbr-pwrc.usgs.gov/bbs/physio.html).

Following is a list of the invasive species included in this indicator and the reason the species is considered invasive: American crow, habitat conversion to agriculture; American robin, habitat fragmentation due to suburban development; black-billed magpie, habitat conversion and fragmentation; bronzed cowbird, forage in association with livestock; brown-headed cowbird, forage in association with livestock; cattle egret, Old World native, habitat conversion to agriculture, and forage in association with livestock; common grackle, habitat fragmentation and conversion to agriculture; European starling, Old World native; gray partridge, Old World native, habitat conversion to agriculture; great-tailed grackle, habitat conversion to agriculture; house finch, habitat fragmentation due to suburban development; house sparrow, Old World native; mourning dove, habitat conversion and fragmentation; ring-necked pheasant, Old World native, habitat conversion to agriculture; and rock dove (domestic pigeon), Old World native, habitat conversion, and fragmentation.

Native, non-invasive species, which are restricted to those native species known to be dependent upon relatively intact and high-quality native grasslands and shrublands, included Baird’s sparrow, black-throated sparrow, LeConte’s sparrow, bobolink, loggerhead shrike, Brewer’s sparrow, long-billed curlew, burrowing owl, McCown’s longspur, Cassin’s sparrow, mountain plover, chestnut-collared longspur, northern harrier, common nighthawk, prairie falcon, dickcissel, sage grouse, eastern meadowlark, sage sparrow, ferruginous hawk, sage thrasher, golden eagle, savannah sparrow, grasshopper sparrow, sharp-tailed grouse, greater prairie chicken, Sprague’s pipit, Henslow’s sparrow, Swainson’s hawk, horned lark, upland sandpiper, lark bunting, vesper sparrow, lark sparrow, and western meadowlark.

Data Collection Methodology: The BBS is jointly coordinated by the PWRC and the Canadian Wildlife Service, Environment Canada. It is conducted along randomly located routes on secondary roads throughout the contiguous United States and southern Canada. Routes are 24.5 miles long, with 50 survey points at 0.5-mile intervals. Observers survey each route annually during June (May in some southern states and desert areas). At each survey point, the observer counts all birds seen or heard within a 0.25-mile radius during a 3-minute census. The first BBS routes in 1966 were run only east of the Mississippi River. The BBS was extended to the central United States in 1967, with full coverage of the contiguous United States by 1968. The number of BBS routes has increased over time, so recent years provide more comprehensive data than early years. Summaries of the BBS methodology are provided by Peterjohn and Sauer (1993) and Sauer et al. (2000a,b), and a review of the program is provided by O’Connor, et al (2000).

Data Manipulation: W. Mark Roberts, an independent researcher, obtained trend estimates (change in population size as a percentage per year) for each species in each physiographic stratum (region) and time interval from a server program provided by John R. Sauer at PWRC (http://www.mbr-pwrc.usgs.gov/bbs/trend/tfmb.html). The program uses an “estimating equations estimator” (described in Link and Sauer 1994) to calculate each stratum’s trend estimate from individual route data. Dr. Roberts performed subsequent manipulations: To reduce the influence of less reliable estimates, each stratum estimate was weighted toward the survey-wide estimate, proportionately to the variances of both estimates. Weighting used an empirical-Bayes formula (Equation 1 in Link and Sauer 1996). The mean of the variance-weighted stratum estimates was calculated for each species and time interval. The summary indicator is the proportion of species with positive (increasing) mean variance-weighted estimates. To compare native with invasive birds, Yates-corrected Chi-square statistical tests were performed on the frequencies of positive and negative mean variance-weighted estimates.

Data Quality/Caveats: Bird species differ in habits, habitat, abundance, and range, all factors that may bias trend estimates for certain species more than for others (see Droege 1990 and http://www.mbr-pwrc.usgs.gov/bbs/introbbs.html). The BBS methodology and data have been subjected to peer review, results of which are available at http://www.mp2-pwrc.usgs.gov/bbs/bbsreview/. The trend analysis program (Sauer and Hines 2001) and manipulations performed by Dr. Roberts are based on peer-reviewed methodology. Output of these manipulations has not, however, been independently verified.

Data Access: Trend estimates are the output of a draft program (http://www.mbr-wrc.usgs.gov/bbs/trend/tfmb.html), placed on the PWRC server but not linked to public pages. Though accessible without charge, this program should not be used without permission from John R. Sauer at PWRC.

References

Droege, S. 1990. The North American Breeding Bird Survey, pp. 1-4. 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 Report 90(1).

Link, W.A., and J.R. Sauer. 1994. Estimating equations estimates of trends. Bird Populations 2:23–32. Link, W.A., and J.R. Sauer. 1996. Extremes in ecology: Avoiding the misleading effects of sampling variation in summary analyses. Ecology 77(5):1633–1640.

O’Connor, R.J., E. Dunn, D.H. Johnson, S.L. Jones, D. Petit, K. Pollock, C.R. Smith, J.L. Trapp, and E. Welling. 2000. A programmatic review of the North American Breeding Bird Survey: Report of a peer review panel to USGS Patuxent Wildlife Research Center, Laurel, MD. http://www.mp2-pwrc.usgs.gov/bbs/bbsreview/.

Peterjohn, B.G., and J.R. Sauer. 1993. North American Breeding Bird Survey annual summary 1990-1991. Bird Populations 1:1–15.

Sauer, J.R., and J.E. Hines. 2001. Trend analysis form: Draft version. Version 2001.00, 2 April 2001 DRAFT, USGS Patuxent Wildlife Research Center, Laurel, MD. http://www.mbr-pwrc.usgs.gov/bbs/trend/tfmb.html.

Sauer, J.R., J.E. Hines, I. Thomas, J. Fallon, and G. Gough. 2000a. Details of the BBS. In The North American Breeding Bird Survey, results and analysis 1966–1999. Version 98.1, USGS Patuxent Wildlife Research Center, Laurel, MD. http://www.mbr-pwrc.usgs.gov/bbs/introbbs.html.

Sauer, J.R., J.E. Hines, I. Thomas, J. Fallon, and G. Gough. 2000b. General introduction–BBS. In The North American Breeding Bird Survey, results and analysis 1966–1999. Version 98.1, USGS Patuxent Wildlife Research Center, Laurel, MD. http://www.mbr-pwrc.usgs.gov/bbs/genintro.html.