Where Have All The Birds Gone?

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! All users of CBC data must agree to our Terms of Use and review the appropriate README file and documentation. 

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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.  

Q: What exactly is an abundance index? 

A: An abundance index is not a measure of true abundance. Rather it is a relative measure of the number of birds counted per unit counting effort. We assume that it is proportional to true abundance in a given sampling area. If it is, indeed, proportional to true abundance, then trends in true abundance will be reflected in abundance indices. Abundance indices are tricky to interpret. Our abundance index can be interpreted as the typical count of birds in a typical CBC within a specified region, given a typical constant effort. 

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: It seems to me that American Ravens (Corvus corax) are a lot more abundant than they used to be. But the CBC trend for the US and Canada shows only a 3.9% 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 57 years, from 1966-2023. Then take that value and subtract 1. Finally, multiply that value by 100 to return it to a percentage. In the case of Common Ravens, which have increased by 3.9% per year, you get 3.9 / 100 = 0.039, 0.039 + 1 = 1.039, 1.039 raised to the power 57 = 8.85, 8.85 – 1 = 7.85, and finally 7.85 x 100 = 785%.  So, Common Ravens have increased nearly 8-fold over the last 57 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 abundance indices and trends that are visualized by this CBC Trends Viewer are available as two large downloadable CSV files. These files contain annual relative abundance indices or trends aggregated to a variety of spatial units. The files can be accessed by visiting the download site, filling out the request form, and agreeing to the terms of use described at the site. Data files come with README text files that describe information in the data files.

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.

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. 

Smith, A.C., Binley, A.D., Daly, L., Edwards, B.P., Ethier, D., Frei, B., Iles, D., Meehan, T.D., Michel, N.L. and Smith, P.A., 2024. Spatially explicit Bayesian hierarchical models improve estimates of avian population status and trends. Ornithological Applications, 126(1), p.duad056. 

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., Farr, C., Tarbox, N., Bateman, B., Michel, N.L., Haywood, B.K., Wilsey, C.B. 2025. Trends in relative abundance for birds in the continental United States and Canada during the boreal winter: Audubon Christmas Bird Counts, 1966-2023, version 2023.0. National Audubon Society, New York, New York, USA.