Detection rates of spotted owls based on acoustic-lure and live-lure surveys

Most survey protocols for northern spotted owls (Strix occidentalis) assume that 3 surveys from roads at night are sufficient to document occupation of territories by spotted owls. The 3-survey protocol was based on the information available at the time and did not use radiotelemetry to determine the adequate number of surveys needed. We used vocal-lure and live-lure surveys to assess detection rates of territorial northern spotted owls in Oregon. The sample included 8 pairs, in each of which 1 or both members were radio-marked. All males were detected within 3 visits with a 10-minute acoustic-lure survey, but 37% of females never responded to either a 10- or 20-minute acoustic lure survey. When we used the acoustic-lure and live-lure techniques in combination, all individuals were confirmed within 5 visits and 94% were located within 3 visits. We conclude that the acoustic-lure and live-lure techniques used in combination will locate most pairs of spotted owls in 3 repeat surveys. By itself, the acoustic-lure technique will tend to underestimate the proportion of females in the territorial population.