(This first section describes the summer abundance maps. For information about the winter abundance maps, see The Audubon Christmas Bird Count (CBC) halfway down this page.)
Summer Abundance Maps
Citation:
Sauer, J. R., B. G. Peterjohn, S. Schwartz, and J. E. Hines. 1996.
The North American Breeding Bird Survey Home Page, version 95.1
Patuxent Wildlife Research Center, Laurel, MD
These maps indicate the number of birds seen on BBS (Breeding Bird Survey) routes, grouped into convenient categories of relative abundance. The maps predict the average number of birds of the species that could be seen in about 2.5 hours of birdwatching along roadsides (by very good birders).
The maps have some error associated with both the underlying data and the mapping procedure. On rare occasions, the procedure inserts edges that omit some marginal routes. Also, note that the lowest relative abundance category occasionally indicates the presence of a species outside its recognized range of the species, representing an accidental observation. Finally, the mapping procedure causes the lowest relative abundance category to extend slightly beyond the survey routes on which the species occurs. We have left these edges and accidental observations in the map to emphasize that unusual observations (and misidentifications) occur in any survey that is based on data collected by many observers.
Also, bird populations are changing, and species can be more (or less) abundant than indicated by the 29-year mean count on a nearby survey route.
So, be warned! You will not always be able to see the species at all locations where the maps indicate they occur at low abundances.
The Details
Many investigators have used bird survey data to develop contour maps of bird abundance based on mean counts on survey routes. Root (1988) provided a grid of smoothed relative abundances for species observed on Christmas Bird Counts. Sauer and Droege (1989) mapped relative abundances of Eastern Bluebirds (Sialia sialis) just after severe winters in the mid 1970's and after their populations returned to pre-winter levels. We have also used relative abundance maps to document the ranges of several species (e.g., Droege and Sauer 1990) for applications and discussions regarding mapping of survey data.
We developed a map of starting locations of BBS routes. Latitude and longitude (degree-minute) of the starting locations were taken from topographic maps of the route path. Of course, the route is 24.5 miles in length, hence any point used to characterize the route is arbitrary.
We estimated average counts from the interval 1966 -1994 on each route for each species, and copied them into database files. We developed contour maps of bird relative abundances, using the route relative abundances as input to smoothing procedures (Isaaks and Srivastava 1989, Cressie 1992).
We used inverse distancing (Isaaks and Srivastava 1989) to prepare a smooth of the data. This procedure estimates the abundance at a location as a distance-weighted average of counts from nearby survey routes. We used inverse distancing to estimate abundances for a grid of points overlaid on the survey area, then used Arc/Info to make a contour map from the estimated abundances (Environmental Systems Research Institute 1991).
Arc/Info provided an arc coverage of contours that connect points having the same value. Depending on the maximum relative abundance of the species, we used levels of 1, 3, 10, 30, and 100 for contours. The maps end at a minimum level of 0.1, which was chosen as a possible edge-of-range index after some comparisons of contours with known edges of ranges (S. Droege and D. Bystrak, Personal Communication), and the larger cutpoints were chosen as a series of powers of 3, rounded up for ease of presentation.
The maps presented here are quite similar to the maps in the 1966û1992 version of the Home Page, but several differences exist between the procedures used to prepare the earlier maps and these maps. To make the 1966û1992 maps, we used Kriging, a procedure in which a variogram is estimated for the species and is used to define the distance-covariance relationship for the smooth (Cressie 1992). In theory, the Kriging should provide a more accurate surface than a procedure such as inverse distancing, which never uses information from the data to adjust the weighting. However, in our experience the variograms were not particularly informative, suggesting that at the scale of the BBS there is little advantage in using Kriging.
We acknowledge, however, that the maps provided here are designed to provide a large-scale summary of the data, and if a species is of particular interest, a more intensive analysis should be conducted using Kriging or some other smoothing procedure. Kriging is a model-based estimation procedure, and if the model is appropriate for the data we can put confidence intervals on the resulting surface. By developing a semivariogram model that more accurately portrays the spatial covariance among routes, the resulting Kriged surface will better reflect the patterns of change among the routes. Often features such as directionality of the semivariogram and trend in the data will require the use of more complex models. See Isaaks and Srivastava (1989) for a useful discussion of the technical details of fitting semivariograms to data, and Cressie (1991) for a more technical discussion of all aspects of spatial modeling.
Literature Cited
Cressie, N. 1992. Statistics for spatial data.
Wiley, New York. 900pp.
Droege, S., and J. R. Sauer. 1990. Northern bobwhite, Gray
partridge, and ring-necked pheasant population trends
(1966-1988) from the North American Breeding Bird Survey.
Pages 2-20 in K. E. Church, R. E. Warner, and S. J. Brady,
eds. Perdix V: Gray partridge and ring-necked pheasant
workshop, Kans. Dept. Wildl. and Parks, Emporia.
Environmental Systems Research Institute. 1991. Surface Modeling
with TIN. Environmental Systems Research
Institute, Inc., Redlands, CA.
Isaaks, E. H., and R. M. Srivastava. 1989. An introduction to
applied geostatistics. Oxford University Press, New York.
561pp.
Root, T. 1988. Atlas of wintering North American birds.
University of Chicago Press, Chicago, IL.
Sauer, J. R., and S. Droege. 1990. Recent population trends
of the eastern bluebird. Wilson Bull 102:239-252.
Sauer, J. R., S. Orsillo, and B. G. Peterjohn. Using GIS to
model bird distributions from the North American Breeding
Bird Survey. Proc IV U. S. Fish and Wildlife Service GIS
Workshop, In Press a.
Sauer, J. R., S. Orsillo, and B. G. Peterjohn. Geographic
patterns in relative abundances and population trends
of breeding and wintering Loggerhead Shrikes in North
America. Proc. Int. Shrike Symp. In Press b.
Sauer, J. R., G. W. Pendleton, and S. Orsillo. Mapping of bird
distributions from point count surveys. Proc. Point Count
Workshop, In Press c.
Disclaimer
Although these data have been processed successfully on a computer system at the National Biological Service, no warranty expressed or implied is made regarding the accuracy or utility of the data on any other system or for general or scientific purposes, nor shall the act of distribution constitute any such warranty. This disclaimer applies both to individual use of the data and aggregate use with other data. It is strongly recommended that these data are directly acquired from a National Biological Service server, and not indirectly through other sources which may have changed the data in some away. It is also strongly recommended that careful attention be paid to the contents of the metadata file associated with these data. NBS data are released on the condition that neither the National Biological Service nor the U. S. Government may be held liable for any damages resulting from its authorized or unauthorized use.
The Audubon Christmas Bird Count (CBC)
These winter abundance maps are designed to display temporal and geographic patterns in populations of wintering North American birds. They use summarized CBC results from the period 1959 - 1988. PLEASE NOTE: the abundance levels shown in these winter maps are NOT comparable to the abundance levels in the summer Breeding Bird Survey maps.
Visit the Audubon web site to see results of all past Christmas Bird Counts
What Is The National Audubon Society Christmas Bird Count?
The National Audubon Society Christmas Bird Count (CBC) is an early-winter survey of birds. Although counts occur in Central and South America, most CBCs occur in North America. The sample area for a count is a circle that is 15 miles in diameter, and varying numbers of volunteers count all birds they see in the circle during a single day, which is within 2 weeks of 25 December. (Butcher 1990)
Although this analysis only considers the interval 1959 - 1988, the CBC was begun in 1900. The number of circles and participants has changed dramatically since the early years. Butcher (1990) notes that 1,508 circles were surveyed in 1986-1987. Unfortunately, the number of birds counted is a function of effort, and analysis of change over time must incorporate some effort adjustment (Butcher and McCulloch 1990). In this analysis, we standardized the counts to birds/100 party hours, but we acknowledge that more research is needed into methods of adjusting counts.
Each year, American Birds publishes a summary of the previous years count (e.g., American Birds 47(4):1-1026) that notes interesting features of the counts and summarizes birds, effort, and volunteers for each circle. Also, numerous articles have appeared in American Birds that summarize data for individual species and groups of species. Root (1988) has summarized patterns of regional relative abundance for wintering North American birds.
Taxonomic problems with the database
Several species are grouped in the database, and several transcription errors have occurred in collating the data.
Because the CBC is an early winter survey, many migrant species appear in the database. Because species seen on few circles could provide misleading views of population trends, we have eliminated species with small sample sizes (fewer than 40).
Is the CBC really a survey?
The CBC "is the oldest and largest wildlife survey in the world" (Butcher 1990). The National Audubon Society sponsors the survey and publishes results. It is designed as a series of circular count areas, and birders count birds within these "circles" each year on a prespecified day around 25 December. With lots of circles (over 1,500) and a long history (the CBC was started in 1900), it is hard to dispute Greg Butcher's "oldest and largest" label for the survey (Butcher 1990).
The CBC is the biggest event in the birding year, with over 40,000 participants (Butcher 1990). All these eyes observe (and count) a lot of birds, but the observers differ greatly in ability. It is still an open question regarding whether it is possible to adequately accommodate differences in effort and abilities in analyses of these data. In creating these abundance maps, we make the big assumption that effort can be adjusted by dividing a count by the number of party-hours. Please keep this problem in mind when reviewing these results!
Another possible problem with the CBC is that the circles are not randomly located across North America. For the CBC to be a survey that provides unbiased results, we have to prevent areas with more samples have having too much influence on the overall results. In this analysis, we attempt to do this by using physiographic strata within states as the basic unit for analysis. In these relatively small areas, samples tend to be more randomly distributed.
Why are these data on an NBS Home Page?
We view the CBC as a valuable supplement to the North American Breeding Bird Survey, in that many wintering species are counted in the CBC but not in the BBS. The CBC also provides an alternative view of population distribution and change for many species that are well-surveyed by the BBS.
These abundance maps present the first trend analysis of the data conducted by the National Biological Service, using data provided by the Cornell Laboratory of Ornithology and the National Audubon Society. The analysis is preliminary, and we present the results with the goal of determining both the status of the database and the feasibility of conducting trend analyses on the CBC data. Please contact us with your comments on both the quality of the data and the relevance of the results.
Thayer Birds of North America CD-ROM contains relative winter abundance maps. These maps indicate the number of birds seen on CBC circles, grouped into convenient categories of relative abundance (in Birds/100 party hours). They predict the average number of birds of the species that could be seen in about 100 party-hours of birdwatching in a CBC circle.
There are additional maps shown on the NBS Home Page. These other maps include:
Numerical estimates of trends and average counts on CBC circles by region, for the period 1959 - 1988.
Annual indices of abundance, presented as graphs of annual indexes for specific regions where the species occurs
A comparison of standardized indices of CBC data and Breeding Bird Survey data, for those species for which trends could be estimated in both surveys.
Details files, with technical information regarding how the maps, trends, and indices were generated.
Comments and Disclaimers
NBS welcomes comments on the home page. We intend to improve it as new methods for summary of survey data are developed, and in response to user feedback. We caution that, as always in data analyses, the possibility exists for errors in the analyses and summaries.
NBS Data Liability Disclaimer
Although these data have been processed successfully on a computer system at the National Biological Service, no warranty expressed or implied is made regarding the accuracy or utility of the data on any other system or for general or scientific purposes, nor shall the act of distribution constitute any such warranty. This disclaimer applies both to individual use of the data and aggregate use with other data. It is strongly recommended that these data are directly acquired from a National Biological Service server, and not indirectly through other sources which may have changed the data in some way. It is also strongly recommended that careful attention be paid to the contents of the metadata file associated with these data. The National Biological Service shall not be held liable for improper or incorrect use of the data described and/or contained herein.
So, these data are provided "as is" and without any express or implied warranties, including, without limitation, the implied warranties of merchantability and fitness for a particular purpose. Also, use of trade names or commercial products in this home page is solely for the purpose of providing specific information, and does not imply recommendation or endorsement by the U.S. Government.
Literature Cited
Butcher, G. S. 1990. Audubon Christmas Bird Counts. Pp. 5-13 in J. R. Sauer and S. Droege, eds. Survey designs and statistical methods for the estimation of avian population trends. U.S. Fish Wildl. Serv. Biol. Rep. 90(1).
Butcher, G. S., and C. E. McCulloch 1990. The influence of observer effort on the number of individual birds recorded on Christmas Bird Counts. Pp. 120-129 in J. R. Sauer and S. Droege, eds. Survey designs and statistical methods for the estimation of avian population trends. U.S. Fish Wildl. Serv. Biol. Rep. 90(1).
Root, T. 1988. Atlas of wintering North American birds. University of Chicago Press, Chicago, Il.