Bias introduced by detection errors is a well‐documented issue for abundance and occupancy estimates of wildlife. Detection errors bias estimates of detection and abundance or occupancy in positive and negative directions, which can produce misleading results. There have been considerable design‐ and model‐based methods to address false‐negative errors, or missed detections. However, false‐positive errors, or detections of individuals that are absent but counted as present because of misidentifications or double counts, are often assumed to not occur in ecological studies. The dependent double‐observer survey method is a design‐based approach speculated to reduce false positives because observations have the ability to be confirmed by two observers. However, whether this method reduces false positives compared to single‐observer methods has not been empirically tested. We used prairie songbirds as a model system to test if a dependent double‐observer method reduced false positives compared to a single‐observer method. We used vocalizations of ten species to create auditory simulations and used naive and expert observers to survey these simulations using single‐observer and dependent double‐observer methods. False‐positive rates were significantly lower using the dependent double‐observer survey method in both observer groups. Expert observers reported a 3.2% false‐positive rate in dependent double‐observer surveys and a 9.5% false‐positive rate in single‐observer surveys, while naive observers reported a 39.1% false‐positive rate in dependent double‐observer surveys and a 49.1% false‐positive rate in single‐observer surveys. Misidentification errors arose in all survey scenarios and almost all species combinations. However, expert observers using the dependent double‐observer method performed significantly better than other survey scenarios. Given the use of double‐observer methods and the accumulating evidence that false positives occur in many survey methods across different taxa, this study is an important step forward in acknowledging and addressing false positives.
Temperate grassland ecosystems are one of the most threatened ecosystems worldwide, and their loss endangers the grassland songbirds that rely upon them. This guild of birds has shown long‐term declines in North America. At the same time, American bison (Bison bison) are becoming more common through reintroductions, and they may make significant modifications to grassland songbird habitat. To support conservation for this guild, we sought to understand the importance of bison grazing and ecosystem productivity to the species richness, occupancy, and abundance of this avian community. We conducted dependent double‐observer bird counts, measured bison grazing intensity with patty counts, and used remote‐sensed normalized difference vegetation index (NDVI) data to measure ecosystem productivity. Our work took place in the National Bison Range near Moiese, Montana and in Yellowstone National Park in Wyoming. We found that species richness was positively correlated with patty counts and had a weak negative correlation with NDVI. Occupancy probability for six of seven grassland songbird species was positively correlated with patty counts, and for six of seven species was negatively correlated with NDVI. Abundance of vesper sparrow (Pooecetes graminueus) and western meadowlark (Sturnella neglecta) were positively correlated with patty counts, although for western meadowlark, this trend became less positive with increasing patty counts. Our work suggests that managers may want to encourage a broad range of bison grazing intensities to ensure that vegetative conditions related to bison grazing are present for all species.
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