Effects of imperfect detectability on inferences from monitoring
August 2009 - July 2014
- Associate Director, USGS-BRD
The value of bird monitoring has come under increasing scrutiny recently due to concerns about imperfect detectability. In particular, because the probability of detecting a bird in the area surveyed often is less than one, counts are only indices of abundance, rather than actual estimates of abundance. A variety of methods that attempt to adjust counts for imperfect detectability have been advanced; using these methods requires additional effort, however. A critical question is the extent to which that additional effort is rewarded by improved results. Clearly, estimates of abundance will be affected by adjustments for detectability; what is not known are the consequences on estimates of population trajectories. The goal of this study is to evaluate the influence of imperfect detectability on inferences about population changes. Specific objectives: - Determine the variation in detectability values that occur in avian surveys and understand factors that influence detectability. This objective will be addressed through literature reviews, discussions with bird surveyors, consultations with experts (e.g., Dr. Theodore Simons, North Carolina Cooperative Fish and Wildlife Research Unit), and possibly field trials. - Understand the influence that imperfect detectability can have on results of monitoring. This will be accomplished through investigations into the analytic methods applied to monitoring data, and simulation studies. - Determine the extent to which methods that adjust counts for imperfect detectability affect results from monitoring programs and inferences drawn from those results. This objective will be addressed by analyzing simulated and possibly actual field data with those methods. - Determine how detectability is affected by abundance and variety of birds and other features not well understood. This will require carefully designed field studies. The extent to which this objective will be necessary, and the exact features to be investigated, will depend in part on topics not addressed by Dr. Simons and his students.