Estimating colony and breeding population size for nocturnal burrow-nesting seabirds

Estimating colony areas, locations, population sizes, and trends are all important aspects of managing animal populations. The ability to assess population trends and delineate important wildlife areas remains a top priority for managers and conservation biologists. Yet, out- dated laborious estimation methods remain in high use. By simulating known populations on known island sizes and using established transect and quadrat survey methods, we asked whether using inverse distance weighting (IDW) interpolations in ArcGIS improved estimates of colony area and population size for nocturnal burrow-nesting seabirds over conventional global interpo- lation methods. We performed 100 simulations for each of 3 population sizes (500, 1000, and 50 000 breeding pairs) on 3 island sizes (10, 50, and 500 ha), excluding the largest population size on the smallest island size, for a total of 800 simulated islands. We estimated colony area and population size for each simulated island using both IDW interpolations and an established global interpola- tion method, and the accuracy of each estimate was then calculated. Using an information theory approach, we found that IDW interpolation estimates were overall more accurate when estimating population size, but we found no difference in colony area accuracy between interpolation meth- ods. We recommend using IDW interpolations to estimate colony area and population size along with consistency in survey structure both among study sites and years. We also recommend main- taining a consistent transect length whenever possible to ensure that observer bias does not influ- ence areas surveyed.