The influence of sample size on the determination of population trends in the vulnerable Lesser Kestrel Falco naumanni overwintering in South Africa

Abstract

Reliable measures of population sizes of endangered and vulnerable species are difficult to achieve because of high variability in population sizes and logistic constraints on sample sizes, yet such measures are crucial for the determination of the success of conservation and management strategies aimed at curbing declines in population size. In this study, we use bootstrap confidence intervals to assess the effects of sample size on the reliable determination of roost size in the vulnerable Lesser Kestrel Falco naumanni overwintering at a large communal roost in South Africa. Using results from a unique five-season daily survey, we show that the currently recommended number of replicate sample counts (international action plan for the Lesser Kestrel) of three censuses per season is too low to reliably assess roost size. Bootstrap confidence intervals asymptote at 40–60 censuses per season, with 50 being the modal value. We recommend 50 censuses per season as the optimal sample size. We were also able to show that the population size of Lesser Kestrels at this roost significantly increased over each of the first four seasons of the study period and decreased slightly in the final season.