Line transect estimation of bird population density using a Fourier series

-A general approach to the estimation of bird population density from line transect data is discussed. This method is based on a nonparametric statistical analysis technique: the Fourier series method. The Fourier series estimator is both robust and efficient; i.e., it is not dependent on specific distributional assumptions about the detection probability of birds at various perpendicular distances from the transect line to provide relatively precise density estimates. The method is especially easy to compute for ungrouped, perpendicular distances, but can also be applied to grouped data commonly taken when sampling birds. A comprehensive computer program, TRANSECT, implements the Fourier series method, under a variety of options, by conducting hypothesis testing and point and interval estimation of population density. Examples of the Fourier series method based on nongame breeding bird transect data are provided. Because results will only be as reliable as the data collected, brief guidelines on field procedures and sample size are given. Finally, comments on other methods of analysis of line transect data are presented. Line transect sampling to estimate the abundance of biological populations has been in use for over 40 years. However, only within about the last 10 years have there been substantial efforts to apply line transect sampling to the problem of estimating abundance of nongame birds. Similarly, it has only been within recent years that the statistical properties of this method have been intensively studied. Line transect sampling is now (almost) an established method for estimating densities of some species of nongame birds, especially breeding birds. Reliable results appear possible if good field practices are used to collect the data and robust, efficient analysis methods are used to analyze these data. The objective of this paper is to bring to the attention of ornithologists a general, robust, reliable data analysis method for use with line transect sampling data. Line transect sampling embodies the explicit recognition of the fact that the probability of detecting birds decreases with increasing distance from the transect line. Because of this, distance data to birds detected are recorded. Estimation of bird abundance involves using these distance data to “correct” the sample size for the detectability of birds. This can be viewed as a refinement on strip transect sampling which requires the assumption that all birds are detected within a fixed perpendicular distance (i.e., within the strip) of the transect line. Strip transect sampling predates line transects; it was used as early as 1906 (as reported in Forbes and Gross 1921). The use of distance ’ U.S. Fish and Wildlife Service, 2625 Redwing Road, Ft. Collins,