Efficacy of Northern Goshawk Broadcast Surveys in Washington State

Abstract

Statewide surveys conducted for northern goshawks (Accipiter gentilis) by the Washington Department of Fish and Wildhfe (WDFW) in the early 1990s had relatively low detection rates throughout the state and different rates of detection between easte n (1 bird/55 kn and western Washington (1 bird/174 km To investigate the possibility that survey methods or regional habitat characteristics affected detection rates, we tested broadeast calls at 40 nest stands known to be occupied by nesting northern goshawks. During 439 station visits and 210 trials, we recorded 109 detections, 68% of which were vocalizations. Northern goshawks at 37 of the 40 (93%) occupied territories re detected at least once, including 4 occupied sites that failed. Detectability was eater at successful nests Juvenile northern goshawks were more responsive than adults, which increased detection rates at successful nests by 37% during the postfledging period and accounted for 87% of responses. Logistic regression modeling identified distance of the surveyon to the nest as the only factor correlated with detection rates (P < 0.001). Probability of detecting northern goshawks at occupied stands increased as nests were approached from 400 m (0.20), 250 m (0.25), and 100 in (0.42). Binomial expansion of detection probabilities at a single nest visit found broadcasting to attain ≥90% detections required 5 visits at 100 m, 8 at 250 m, and 10 at 400 m. Because analysis of detection rates by area did not show an effect of vegetation screening, slope, or topography, differences in the relative abundance of northern goshawks from earlier surveys reflect true variation in abundance, differences in nest success, or some combination of both. Results lend more justification for use of broadcast calling for northern goshawk in Pacific Northwest torests and provide a hypothetical model for estimating survey costs as a function of detection probability, trial frequency, and station spacing.