Detectability and Response Rates of Ferruginous Pygmy-Owls


Flesch, A. D., and R. J. Steidl

Survey techniques that are both reliable and efficient are necessary to accurately estimate population parameters, especially for rare species. Cactus ferruginous pygmy-owls (Glaucidium brasilianum cactorum; hereafter pygmy-owls) have declined in southwestern North America and are surveyed often to comply with federal law. We studied owl responses to broadcasted calls to quantify how detectability and response rates (owls/station/transect) vary with environmental, spatial, temporal, and weather-related factors. We surveyed owls along 392 transects (1,113 km) throughout Sonora, Mexico, including a subset of 14 transects (47.2 km) that we surveyed repeatedly to assess factors that affected response rates. We challenged 17 adults and 23 juveniles that were radiomarked, adults attending 50 occupied nests, and adults attending 6 groups of radiomarked juveniles to respond to broadcasted calls to assess factors that affected detectability. Across Sonora, response time averaged 2.6 6 0.1 minutes (x ̄ 6 SE, n 1⁄4 520), with 99 6 0.4% of owls detected in 8 minutes; response distance averaged 251 6 7 m, with 91 6 1% of owls detected at 400 m. Response time decreased by an average of 4 6 2% and response distance decreased by 12 6 3 m with each half-month period from early courtship through brooding (P 0.035). Response time averaged 39 6 24% faster during morning than midday at occupied nests. Detectability was 1.0 6 0.0 when surveyors were 100 m from occupied nests and decreased to 0.78 6 0.10 when surveyors were 500 m from occupied nests. Detectability was higher during incubation, brooding, and natal dispersal (0.89 6 0.05–1.0 6 0.0) than during fledgling-dependency (0.50 6 0.20–0.67 6 0.19). Response rates of males did not vary from early courtship to brooding (P 1⁄4 0.84), yet those of females decreased systematically to zero across the same period (P , 0.001). Because detectability of pygmy-owls remains consistently high during nesting, response rates generated from carefully designed surveys can provide reliable estimates of occupancy and abundance.

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Refereed Articles