Breeding Bird Density CV Tool


Patuxent Wildlife Research Center	Managers' Monitoring Manual

Managers' Monitoring Manual > CV Tools > Breeding Bird Density CV Tool

Coefficients of Variation Tool for Breeding Density Estimates of North American Birds from Select Territory Mapping Plots

You may use this tool to estimate the number of samples needed to address questions about changes in bird populations.

Select species Search by common name (Numbers in brackets indicate the total number of records in that category.)
Select state/province (U.S./Canada only)	(optional)
Select minimum length of time series	(from 4 to 24)

The CV (coefficient of variation) is calculated by doing a linear regression of the time series data (where time is the independent variable and count is the dependent variable), doing a standard deviation of the residuals and dividing it by the mean of the counts. See an example of how the CV is calculated.

The purpose of this web site is to provide estimates of the year-to-year variability of territory mapping estimates. Those estimates of variation can be used in power and sample size calculations to further estimate if sample sizes are adequate to meet the statistical and biological goals of studies that use territory mapping in research or monitoring.

Details of this database

This database was kindly provided to us by Deanna Dawson and includes Breeding Bird Census reports and other published bird census data that were compiled in a computer database for an analysis of North American breeding bird community patterns (Noon et al. 1985, see full ref below). Census reports included in this database have the following characteristics:

	1.	They were collected using territory-mapping or spot-mapping techniques.
	2.	Each plot is larger than 9 hectares.
	3.	Is there any way to change our management actions that will best allow us to learn from our mistakes and experiences?
	4.	All plots are located in North America.

Included are data from a total of 105 study plots, yielding 3663 separate count series for 335 bird species. While additional appropriate data have been collected and published since these data were compiled in the early 1980's, we have not added those records. More recent data for Canada are included in the Canadian Breeding Bird (Mapping) Census Database (this links to a .pdf file). Anyone interested in expanding this database with more recent data would be doing a great service and are welcome to contact Sam Droege (sam_droege@usgs.gov) to learn how to assist. All such efforts will be gratefully acknowledged on this web site.

We made some changes to the original dataset to reflect taxonomic changes that have occurred in recent years. For example, the Rufous-sided Towhee was split into Spotted Towhee and Eastern Towhee. Based on published breeding range maps (Sibley Guide to Birds, copyright 2000), we substituted the new species name if the plot location was clearly in one species' range and not the other.

Our purpose in creating this web site is both to make these data easily available to researchers and to provide coefficients of variation of the density estimates. A coefficient of variation (CV) is calculated by dividing the standard deviation by the mean. To account for the effects of trend over time, which elevates variation in counts, we used the standard deviation of the residuals from a regression of the counts run against time. Associated with this method are inherent assumptions, which you may find disagreeable due to sticky statistical issues like model misspecification, autocorrelation, and effect of survey length on variance. For this reason, we provide the raw counts so you can analyze the data using your preferred method. Coefficients of variation provide estimates of variability of counts that can be used to estimate the number of samples needed to address questions about changes in bird populations.

In the future we plan to add additional databases to the web using data from other counting techniques. Similar data are available for amphibians at our Amphibian Count Database. As always, we look forward to feedback on all aspects of this site. Please drop us a line if you know of studies we missed, methods relating to count data from which we might benefit, or if you would consider contributing data to the project.

Literature cited:

Noon, Barry R., Deanna K. Dawson, and John P. Kelly. 1985. A search for stability gradients in North American breeding bird communities. The Auk 102: 64-81.