A program of the National Audubon Society, The Christmas Bird Count (CBC) is a citizen-science based early-winter bird survey involving volunteers across the US, Canada and many countries in the Western Hemisphere since 1900. Over a 24 hour period, participants follow specified routes through a designated 15-mile diameter circle, counting every bird they see or hear. Birds are counted throughout the day, giving an indication of the total number of birds in the circle that day. Compilers are responsible for collating and submitting the data for their circle. All individual CBC’s are conducted in the period from December 14 to January 5 each season, and each count is conducted in one calendar day.
The long term perspective made possible by the Christmas Bird Count is useful for conservationists. It informs strategies to protect birds and their habitat, and helps identify environmental issues with implications for people as well. For example, local trends in bird populations can indicate habitat fragmentation or signal an immediate environmental threat, such as groundwater contamination or poisoning from improper use of pesticides. Long-term, broad-scale analyses can inform continental conservation planning efforts and guide further research by revealing regional or continental patterns of population change. CBC is an easy way for people to become involved in counting birds; 30,000 participants counting birds in 2000 circles per year. Current and historical data are available on-line by count circle, state and species displayed graphically and numerically (raw count data). Most of the field methods and standardizations currently employed date to 1950, providing the ability to assess patterns of populations change for the past 60 years, at least for some species and some regions of the country.
While the CBC’s historical longevity is an advantage, the inferences that one can make are limited due to its biases: coarse spatial scale, unequal sampling intensity, and lack of ways to correct for observer skill differences, time-of-day, mode of transportation, and circle coverage. CBC count circles are set up based on volunteer interest with no stratification or statistical design employed. If any inferences are to be made from CBC data, they should first be standardized by the number of hours of observer participation on that count. However, new analytical tools are being employed that have greatly improved the ability to draw reasonable inferences about population change with the CBC data and statistically account for some biases. Bayesian methods are now used to fit hierarchical models that estimate change at the strata level (states, BCRs, etc.) and this addresses some concerns. These methods also provide the additional advantage of improving the comparability to BBS analyses because they are analogous. A disadvantage, however, is that these analyses are complex, so not easily implemented.
Examples of use
The data collected by observers over the past century allow researchers, conservation biologists, and other interested individuals to study the long-term health and status of bird populations across North America. When combined with other surveys such as the Breeding Bird Survey (Link and Sauer 1999), the CBC provides a picture of how the continent's bird populations have changed in time and space over the past hundred years. This link provides examples of several species specifically addressed through CBC: http://birds.audubon.org/how-christmas-bird-count-helps-birds
For more information:Link, W.A and J. R. Sauer. 1999. Controlling for Varying Effort in Count Surveys: An Analysis of Christmas Bird Count Data. Journal of Agricultural, Biological, and Environmental Statistics 4(2):116-125
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