Effects of point-count duration on estimated detection probabilities and occupancy of breeding birds

Increasingly, point-count data are used to estimate occupancy, the probability that a species is present at a given location; occupancy accounts for imperfect detection, the probability that a species is detected given that it is present. To our knowledge, effects of sampling duration on inferences from models of bird occupancy have not been evaluated. Our objective was to determine whether changing count duration from 5 to 8 min affected inferences about the occupancy of birds sampled in the Chesapeake Bay Lowlands (eastern United States) and the central and western Great Basin (western United States) in 2012 and 2013. We examined the proportion of species (two doves, one cuckoo, two swifts, five hummingbirds, 11 woodpeckers, and 122 passerines) for which estimates of detection probability were ≥ 0.3. For species with single-season detection probabilities ≥ 0.3, we compared occupancy estimates derived from 5- and 8-min counts. We also compared estimates for three species sampled annually for 5 yr in the central Great Basin. Detection probabilities based on both the 5- and 8-min counts were > 0.3 for 40% ± 3% of the species in an ecosystem. Extending the count duration from 5 to 8 min increased the detection probability to ≥ 0.3 for 5% ± 0.5% of the species. We found no difference in occupancy estimates that were based on 5- versus 8-min counts for species sampled over two or five consecutive years. However, for 97% of species sampled over 2 yr, precision of occupancy estimates that were based on 8-min counts averaged 12% ± 2% higher than those based on 5-min counts. We suggest that it may be worthwhile to conduct a pilot season to determine the number of locations and surveys needed to achieve detection probabilities that are sufficiently high to estimate occupancy for species of interest. Progresivamente se utilizan datos de conteo por puntos para estimar la ocupación, la probabilidad de que una especie esté presente en un lugar determinado; la determinación de la ocupación se incluye la detección imperfecta, la probabilidad de que una especie que está presente sea detectado. A nuestro conocimiento, no han sido evaluados los efectos de la duración del muestreo sobre las inferencias de los modelos de ocupación de aves. Nuestro objetivo fue determinar si el cambio de la duración del conteo, de 5 min a 8 min, afectó las inferencias sobre la ocupación de las aves tomadas en las tierras bajas de la bahía de Chesapeake (este de Estados Unidos) y en el centro y