Field evaluation of distance-estimation error during wetland-dependent bird surveys

Abstract

Context The most common methods to estimate detection probability during avian point-count surveys involve recording a distance between the survey point and individual birds detected during the survey period. Accurately measuring or estimating distance is an important assumption of these methods; however, this assumption is rarely tested in the context of aural avian point-count surveys. Aims We expand on recent bird-simulation studies to document the error associated with estimating distance to calling birds in a wetland ecosystem. Methods We used two approaches to estimate the error associated with five surveyor’s distance estimates between the survey point and calling birds, and to determine the factors that affect a surveyor’s ability to estimate distance. Key results We observed biased and imprecise distance estimates when estimating distance to simulated birds in a point-count scenario (error = –9 m, s.d.error = 47 m) and when estimating distances to real birds during field trials (error = 39 m, s.d.error = 79 m). The amount of bias and precision in distance estimates differed among surveyors; surveyors with more training and experience were less biased and more precise when estimating distance to both real and simulated birds. Three environmental factors were important in explaining the error associated with distance estimates, including the measured distance from the bird to the surveyor, the volume of the call and the species of bird. Surveyors tended to make large overestimations to birds close to the survey point, which is an especially serious error in distance sampling. Conclusions Our results suggest that distance-estimation error is prevalent, but surveyor training may be the easiest way to reduce distance-estimation error. Implications The present study has demonstrated how relatively simple field trials can be used to estimate the error associated with distance estimates used to estimate detection probability during avian point-count surveys. Evaluating distance-estimation errors will allow investigators to better evaluate the accuracy of avian density and trend estimates. Moreover, investigators who evaluate distance-estimation errors could employ recently developed models to incorporate distance-estimation error into analyses. We encourage further development of such models, including the inclusion of such models into distance-analysis software.

Experimentally derived detection distances from audio recordings and human observers enable integrated analysis of point count data

“…Although we demonstrate that simultaneous comparisons of HPC and ARU data potentially enable the calculation of EDR and densities of birds from ARU recordings, this approach still relies on accurate distance estimation during HPCs, an assumption that is frequently violated during avian surveys (Alldredge et al 2007, Nadeau andConway 2012). Errors in distance estimation can bias EDR and bird density calculations and will persist when using our correction approach for ARU data.…”

Evaluating autonomous acoustic surveying techniques for rails in tidal marshes

“…Accurately estimating distances of birds to observers and ARUs is difficult to achieve during most avian surveys because birds are often detected aurally instead of visually (Scott et al , Alldredge et al , Nadeau and Conway ). Biased or imprecise distance estimations often result from a combination of localization error (i.e., the ability of an observer to locate a bird) and distance estimation error (i.e., the ability of an observer to accurately estimate distance from the bird to themselves; Alldredge et al ).…”

“…Biased or imprecise distance estimations often result from a combination of localization error (i.e., the ability of an observer to locate a bird) and distance estimation error (i.e., the ability of an observer to accurately estimate distance from the bird to themselves; Alldredge et al ). Previous studies have indicated that a suite of factors leads to errors in distance estimation, including habitat structure (Richards and Wiley ), species of the bird (Richards , Schieck , Hobson et al , Alldredge et al , Nadeau and Conway ), distance from the bird to the observer (Nadeau and Conway ), volume of the vocalization (Nadeau and Conway ), orientation of the bird (Alldredge et al ), and various environmental factors such as wind and noise (Simons et al ). These factors can ultimately lead to overestimating distances of birds close to the observer and underestimating distances of birds farther away (Nadeau and Conway ).…”

Habitat models to predict wetland bird occupancy influenced by scale, anthropogenic disturbance, and imperfect detection

“…Surveyors received training and were reasonably accurate at estimating distance to rails within 225 m ( Fig. 1b in Nadeau and Conway 2012). If no distance was recorded for a given detection (9.4% of detections), we assumed that detection was within 225 m. Our analyses included data from a total of 36,378 surveys conducted at 2473 survey points (mean = 14.71 surveys conducted per survey point).…”