A statistical methodology for tracking long-term change in reporting rates of birds from volunteer-collected presence-absence data

The ability to track change in biodiversity is essential to guide sustainable management and meet biodiversity monitoring, evaluation and reporting requirements, yet long-term data are usually scarce. Birds Australia has developed a simple survey methodology for use by their nationwide network of volunteers; it involves the collection of data on the presence-absence of species at repeatedly visited sites. Here we present a statistical methodology for use with these binary data to examine long-term change, using as an example records from a major bioregion of eastern Australia, 1999-2007. Regression splines were employed to model trend as a smooth nonlinear function of time within a generalised linear modelling framework. Confidence intervals based on bootstrap resampling provided a basis for assessing the significance of change, and a method was incorporated for identifying important change points in the trajectory from second derivatives of the curve. The methodology proved sensitive to change and the impact of extended dry periods was evident. The populations of several woodland species were found to be in significant decline. Two composite indices to track change common to a group of birds were developed and/or adapted from the existing literature. The results confirm the usefulness of repeated 2-ha presence-absence survey data to provide insight into patterns of long-term trends in bird populations. The statistical methodology described offers a means of tracking trends and identifying important time points and is particularly useful in situations where surveys of presence-absence of species are the most efficient way to gather long-term data.