Research design considerations to ensure detection of all species in an avian community

Abstract

Recent advances in the estimation of species richness from count data have allowed avian ecologists to incorporate incomplete detectability of species when comparing richness across space or time. Raw counts from single or repeated visits to sample point(s) are nonetheless still used for assessing community composition, and the failure to account for detectability when making these evaluations may lead to incorrect inferences about the community. We estimated detection probabilities (P) for a suite of bird species and used these detection probabilities to determine the minimum number of visits at a single point and the minimum number of points in a grid required to confidently (≥90%) detect the full community of birds for rare, moderately rare, and common species. We used occupancy modelling to estimate the detection probabilities for species from two study sites in Nebraska and Saskatchewan. Some common or highly detectable species were confidently detected in a single visit to a point, whereas others with low detection probabilities (P < 0·20) required more than ten visits to be confidently detected at a point. The grid size required to detect a species in an area varied from a single point for a common highly detectable species to over 30 points for a rare species with low detectability. Detection probabilities of the least detectable species in a study area can be used to determine the number of visits to a single point or the number of points in a grid to be confident that the full community is detected. Biologists can conclude that a species is most likely absent from the community if it remains undetected using the appropriate sampling effort.