Radar and audiovisual surveys are important tools for identifying nesting habitat and developing inland conservation strategies for the marbled murrelet (Brachyramphus marmoratus), a threatened seabird that nests in old-growth forests in the Pacific Northwest, USA. Theoretically, counts from these 2 approaches (radar and audiovisual surveys) in different habitats depend on both habitat-specific densities and detection probabilities, which could result in spurious or mask differences among habitats if murrelets are more detectable in certain habitat types. Therefore, we used simultaneous audiovisual and radar surveys to estimate detection probabilities and quantify the relationship between detection probabilities and habitat in coastal redwood forests of northern California, USA. Radar and audiovisual counts were highly correlated (r = 0.65) based on 156 simultaneous surveys, but audiovisual surveys detected only 20.2% (2.6 SE) of murrelets detected by radar, whereas radar detected 75.6% (6.3 SE) of murrelets detected by audiovisual surveys. Murrelets tended to be easier to detect with both audiovisual and radar surveys when there were relatively large areas (>35 ha) of unharvested, old-growth forest at the survey site, probably because birds tended to fly slower and circled more over old growth. Detection probabilities were strongly affected by a variety of other factors including weather, time relative to sunrise, observer, and murrelet behavior. Murrelet counts were positively but weakly correlated to the area of unharvested old growth at the survey site for audiovisual and radar surveys, but this relationship disappeared when we corrected for the effect of habitat on detection probabilities. Our results indicate that raw counts should not be used as indices of nesting densities in different habitats and underscore the need to model heterogeneity in detection probabilities among habitats using available sampling designs and statistical methods. Nevertheless, we recommend using radar instead of audiovisual surveys for counting murrelets in forested areas because detection probabilities were much higher for radar than audiovisual surveys, radar is less likely to detect the same group of murrelets multiple times, and radar surveys can be conducted in poor viewing and hearing conditions.
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Vol. 70 • No. 2