Comparison of Two Double-Observer Point-Count Approaches for Estimating Breeding Bird Abundance

Abstract

Point counts are one of the most widely used and efficient approaches to survey land-bird populations. A new approach to point-count surveys involves the use of 2 observers, which allows the calculation of a detection probability for each bird species. Detection probabilities derived from 2 observers permit investigators to calculate a corrected abundance estimate that accounts for birds present but not detected. We evaluated 2 double-observer point count approaches: the dependent-observer approach and the independent-observer approach. The dependent-observer approach involves 2 observers recording data together on a single data sheet with one observer designated the primary observer and the other designated as the secondary observer. The primary observer verbally dictates the number of each species detected while the secondary observer records this information; the secondary observer also records birds that the primary observer did not detect. The independent-observer approach involves 2 observers recording data independently on separate data sheets without verbal communication between observers. This study compares the detection probabilities and associated levels of precision generated by both double-observer approaches to ascertain which technique generates data that are more accurate and more feasible to apply in the field. We conducted point counts at 137 point locations in northern West Virginia during the spring of 2000 and 2001 using both double-observer approaches. We generated detection probabilities and abundances from data collected using both double-observer approaches using program DOBSERV. The dependent-observer approach resulted in higher observer-specific and joint detection probabilities, as well as lower standard errors of detection probability across most cover types. Species-specific detection probabilities were higher across all cover types under the dependent-observer approach. Given the higher detection probabilities and associated precision combined with fewer logistical constraints, we suggest that the dependent-observer approach be used when investigators are interested in surveying birds using point counts.