Presence–nonpresence surveys of golden-cheeked warblers: detection, occupancy and survey effort

Surveys to detect the presence or absence of endangered species may not consistently cover an area, account for imperfect detection or consider that detection and species presence at sample units may change within a survey season. We evaluated a detection–nondetection survey method for the federally endangered golden-cheeked warbler (GCWA) Dendroica chrysoparia. Three study areas were selected across the breeding range of GCWA in central Texas. Within each area, 28–36 detection stations were placed 200m apart. Each detection station was surveyed nine times during the breeding season in 2 consecutive years. Surveyors remained up to 8min at each detection station recording GCWA detected by sight or sound. To assess the potential influence of environmental covariates (e.g. slope, aspect, canopy cover, study area) on detection and occupancy and possible changes in occupancy and detection probabilities within breeding seasons, 30 models were analyzed. Using information–theoretic model selection procedures, we found that detection probabilities and occupancy varied among study areas and within breeding seasons. Detection probabilities ranged from 0.20 to 0.80 and occupancy ranged from 0.56 to 0.95. Because study areas with high detection probabilities had high occupancy, a conservative survey effort (erred towards too much surveying) was estimated using the lowest detection probability. We determined that nine surveys of 35 stations were needed to have estimates of occupancy with coefficients of variation 20%. Our survey evaluation evidently captured the key environmental variable that influenced bird detection (GCWA density) and accommodated the changes in GCWA distribution throughout the breeding season.