Estimating population size using capture-recapture encounter histories created from point-coordinate locations of animals

Abstract

Summary 1. Estimating population size is a fundamental objective of many animal monitoring programmes. Capture–recapture methods are often used to estimate population size from repeated sampling of uniquely marked animals, but capturing and marking animals can be cost prohibitive and affect animal behaviours, which can bias population estimates. 2. We developed a method to construct spatially explicit capture–recapture encounter histories from locations of unmarked animals for estimating population size with conventional capture–recapture models. Prior estimates of the maximum distance individuals move in the population is used to set a summary statistic and process subsequent capture–recapture survey data. Animal locations are recorded as point coordinates during survey occasions, and the parameter of interest is abundance of individual activity centres. 3. We applied this method to data from a point-coordinate capture–recapture survey of burrowing owls Athene cunicularia in the Imperial Valley of California, USA. We also used simulations to examine the utility of this technique for additional species with variable detection probabilities, levels of home range overlap and distributions of activity centres within a survey area. 4. The estimates from empirical and simulation studies were precise and unbiased when detection probabilities were high and territorial overlap was low. 5. This method of estimating population size from point locations fills a gap in non-invasive census and long-term monitoring methods available for conspicuous species and provides accurate estimates of burrowing owl territory abundance. The method requires high detection probabilities, low levels of home range overlap and that individuals use activity centres. We believe that these requirements can be met, with suitable survey protocols, for numerous songbird and reptile species.