Accounting for non-detection in colonial bird breeding distributions derived from opportunistic site visits
Opportunistic site surveys are useful for obtaining species distribution data over a large spatial scale, but often entail logistical constraints that reduce the detectability of resident species. Antarctic birds breed at discrete single- or multi-species colonies, and although several of these species have been well-studied at particular sites, a detailed spatial analysis of the breeding occurrence of many species has been impossible due to the logistical challenges of doing comprehensive coastal surveys. Recent advances in occupancy modeling allow for explicit estimation of species non-detection, which greatly improves the utility of opportunistic surveys that may cover large areas but miss species that are cryptic or rare. We surveyed over 205 breeding bird sites from vessels of opportunity and combined those data with published census records to map the breeding distributions of all 16 nesting bird species from 61º to 68º S latitude along the Antarctic Peninsula. By using Bayesian site-occupancy models to estimate non-detection and accounting for the spatial pattern of observed breeding locations, we project estimated breeding range maps for the entire avian community.
Consistent with expectations, detection probabilities ranged from very high in densely colonial open-ground-nesting species, such as penguins, to very low in crevice-nesting species, such as storm-petrels. Our model incorporates observations of both adult presence and confirmed breeding, and allows us to assign testable probabilities of occurrence to all potential nesting areas. We will present a probabilistic biogeographic atlas of Antarctic avian communities unbiased by heterogeneity in survey effort. Changes in site occupancy are frequently used as a measure of conservation threat status; opportunistic surveys coupled with occupancy modeling provide a baseline against which to assess population status for future conservation assessments.