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Curious about what the CBC data show? Audubon's Science team has built a tool where you can explore population trends over the last few decades!
CBC Trends Viewer Frequently Asked Questions
Q: The amount of effort that goes into a Christmas Bird Count varies quite a bit across years and count circles. How do you deal with this issue when you calculate abundance trends? How do you know that an increase in abundance isn’t simply due to an increase in count effort over the years?
A: Before we calculate CBC trends for a species, we divide the raw number of birds counted by a function that describes the statistical relationship between effort spent and birds counted. Then we look at how those resulting, effort-corrected counts change over the time, to generate the population trend.
Q: How come I am not able to find CBC trends for a specific count circle?
A: The CBC is a volunteer effort, where volunteers have a lot of flexibility regarding where, when, and how to conduct counts from year to year and from place to place. This contrasts with some other wildlife studies, where counting protocols are highly standardized and prescribed. The flexible format of the CBC encourages participation, but also has consequences for generating trends. It means that we have to pool results across several circles before we can tease statistical signals from methodological noise. We are currently working on new methods that will allow us to estimate trends at finer spatial scales, but it will be a while before we are satisfied with the performance of those methods and can report results.
Q: What is a Bird Conservation Region?
A: A Bird Conservation Region, or BCR, is an area where ecological conditions and human factors affecting bird populations are presumed to be somewhat uniform. As such, they represent convenient units for aggregating CBC circles and computing population trends. They were developed by the North American Bird Conservation Initiative (http://nabci-us.org/resources/bird-conservation-regions/).
Q: What is an a Landscape Conservation Cooperative?
A: A Landscape Conservation Cooperative, or LCC, comprises multiple resource management organizations. Their goals are to manage natural resources with the understanding that management challenges are similar across similar ecological regions and tend to cross jurisdictional and political boundaries (https://lccnetwork.org/). The boundaries of an LCC encompass those of multiple similar BCRs.
Q: It seems to me that crows are a lot more abundant than they used to be. But the CBC trend shows only a 2.61% increase.
A: CBC trends report annual percent change in relative abundance. These percent changes don’t add up over time, they actually multiply up over time, like compound interest in a bank account. To convert an annual percent change to percent change over a longer time interval, try the following. Divide the annual percent by 100, and then add 1. Then raise that value to the number of years in the new time interval, say 52 years, from 1966-2017. Then take that value and subtract 1. Finally, multiply that value by 100 to return it to a percentage. In the case of American Crows you get 2.61 / 100 = 0.0261, 0.0261 + 1 = 1.0261, 1.026152 = 3.8182, 3.8182 – 1 = 2.8182, and finally 2.8182 x 100 = 281.21%. So, American Crows have increased 282.21% over the last 52 years!
Q: How does the CBC trend analysis handle changes in taxonomy over the years? Some species have been lumped, others have been split, and still others have been lumped and split since 1966.
A: This is indeed a challenge. The situation is easiest when species are lumped. In that case, for each count circle and year, we sum the counts of all the lumped species. The situation is trickier when there is a split. If the split species spend the winter in different places, we sum the counts of all the relevant species per circle and year, but then generate trends separately for the different geographies that are unique to the split species. If the split species co-occur during the winter, then we have no choice but to calculate a trend for the collection of split species.
Q: I am working on a project, and it would be very helpful to get the processed abundance indices and abundance trends that are visualized by this CBC Trends Viewer. Is there a way to get this information? And what are the conditions of use?
A: All of the processed abundance indices and abundance trends that are visualized by this CBC Trends Viewer are available as one large (25 MB) downloadable CSV file. The file contains indices and trends for 543 species, aggregated to appropriate states, provinces, BCRs, LCCs, and countries. The file can be accessed by visiting the download site, filling out the request form, and agreeing to the conditions of use described at the site.
Q: How can I get raw data from Christmas Bird Counts? Do I have to go back to original issues of American Birds?
A: We understand that some questions can only be answered by careful analysis of raw CBC counts from individual count circles. For people who would like to do these types of analyses, summarized data can be attained at the CBC Observations Portal (http://netapp.audubon.org/cbcobservation/) and raw data can be requested here: https://form.jotform.com/81195823581159.
Q: Is there an official citation for the methods used to produce these trends?
A: Yes. The trends were generated using the methods outlined in the following publication.
Soykan, C.U., Sauer, J., Schuetz, J.G., LeBaron, G.S., Dale, K. and Langham, G.M. 2016. Population trends for North American winter birds based on hierarchical models. Ecosphere, 7(5), p.e01351.
Q: Is there an official citation for this specific version of CBC trends?
A: Yes. The citation for the current version of trends is as follows.
Meehan, T.D., LeBaron, G.S., Dale, K., Krump, A., Michel, N.L., Wilsey, C.B. 2022. Trends in relative abundance for birds wintering in the continental USA and Canada: Audubon Christmas Bird Counts, 1966-2021, version 4.0. National Audubon Society, New York, New York, USA.