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 this home page, 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.
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.
This Home Page presents the first trend analysis of the data conducted by the Biological Resource Division of the U.S. Geological Survey, 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.
The options include:
(1) A relative abundance map. This map indicates 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.
(2) Numerical estimates of trends and average counts on CBC circles by region, for the period 1959 - 1988.
(3) Annual indices of abundance, presented as graphs of annual indexes for specific regions where the species occurs
(4) 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.
(5) Details files, with technical information regarding how the maps, trends, and indices were generated.
We welcome 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.
Although these data have been processed successfully on a computer system at the USGS-BRD, 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 USGS-BRD 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 USGS-BRD 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).