A comparison of the analytical methods used in population studies of storm petrels and the implications for colony estimates

Abstract

The standard census method for burrow-nesting petrels involves using an estimated response rate to correct the number of responses to tape playback obtained across a study site. Response rates can vary significantly between sites and between surveys and are estimated by determining the level of response within a calibration plot for which the number of occupied sites must also be estimated. The field methods used for such surveys are well established. However, a range of analytical methods have been employed to determine the number of occupied sites within the calibration plot and hence the response rate. The choice of method can have a huge effect on the estimated population size. Data from surveys of Leach’s Oceanodroma leucorhoa and European Storm-petrel Hydrobates pelagicus on North Rona in 2001, 2009 and 2015 have been used to compare the three methods commonly employed to estimate the number of occupied burrows within the calibration plot. This number is usually subject to uncertainty and is key to estimating the population size. Our analysis suggests that the widely used ‘reciprocal transformation’ method can give rise to implausibly high population estimates which are up to 70% higher than those provided by other methods. We conclude that whilst potentially useful in some situations, this method should not be used in isolation. No single method appears best in all situations, but for North Rona the alternative ‘curve fitting’ and du Feu methods consistently give more plausible population estimates.