Passing cars throwing off breeding bird counts, study finds

Bay-breasted Warbler (Dendroica castanea). Credit, Mdf.Passing cars may be throwing off breeding bird counts according to a new study by researchers from the U.S. Geological Survey. This is likely because increases in traffic noise make it more difficult for the volunteers conducting the surveys to detect birds by hearing their calls.

The good news is that statistical models developed from the study can help us adjust the numbers from breeding bird surveys to more accurately reflect avian populations.

Emily Griffith and study co-authors looked specifically at how traffic levels affect volunteer bird counts along the North American Breeding Bird Survey routes.

Data from the survey is used to estimate poplulation trends of birds that breed in North America. Volunteers follow their respective routes stopping every ½ mile to count all the birds that they see and hear within a ¼ mile radius over the course of 3 minutes.

During recent years, the volunteers have also been counting the number of cars that pass by.

The researchers looked at 7 years of data across the same routes and focused on 37 species. According to the study findings published in the journal The Auk, 30 of the species showed decreased counts with increasing numbers of cars passing by.

While it is possible these findings are due to increased traffic having a direct negative effect on birds, the results from the modeling suggest that reduced detectability is the main cause.   

Overall the effects of the traffic on the bird counts were fairly small. "For example, for the Nashville warbler, if the vehicle count increased from 10 to 50, with other effects in the simpler model constant, we would expect the bird count to decrease by only .07," the authors write.

Nonetheless this effect could become problematic if underlying trends in vehicle traffic give the false appearance that populations are either increasing or decreasing.

Therefore the authors advise, " Using these data can help improve breeding bird survey models and lead to better inferences about population trends for North American birds. "

--by Rob Goldstein

Griffith, E., Sauer, J., & Royle, J. (2010). Traffic Effects on Bird Counts on North American Breeding Bird Survey Routes The Auk, 127 (2), 387-393 DOI: 10.1525/auk.2009.09056