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.

We used pilot data collected in 2001–2004 to compare the power of radar and audiovisual survey approaches to detect trends in breeding population size and differences in trends between populations of marbled murrelets (Brachyramphus marmoratus) in northwestern California. Radar counts of murrelets were almost triple in inland Reserves than in Conservation Areas, and audiovisual counts were 7 times greater. Variation in counts was statistically significant among survey sites but not among years. Although annual variation in radar counts was not statistically significant (P = 0.13), mean radar counts more than tripled from 2001 to 2002, a difference that we considered to be biologically significant, and the radar counts reflected a large increase in the proportion of breeders between these 2 years as determined by radiotelemetry in another study (Acord et al. 2004). Audiovisual counts were much more variable than radar counts overall (CV = 1.10 versus 0.41) and within survey sites (CV = 0.94 versus 0.23). As a result, approximately twice the audiovisual survey effort was needed to detect trends with reasonable power (80%). Power to detect trends in murrelet breeding populations was most sensitive to the duration of the monitoring program and the magnitude of the trend; only relatively modest gains in power were realized by increasing the number of surveys or sites. A monitoring program designed to detect differences in trends between breeding murrelet populations required greater survey effort than a program designed to detect overall trends. Despite the fact that gains in power to detect trends from using radar were offset by the cost of purchasing radar equipment, we advocate the use of radar over audiovisual surveys for monitoring murrelet breeding populations because radar reflected changes in breeding effort whereas audiovisual surveys did not. We also advocate the use of radar because it may provide an estimate of the number of breeding individuals in certain situations, it can be used under poor viewing conditions, it samples larger areas, and it detects a higher proportion of inland‐flying murrelets.

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Habitat Conservation Plans (HCPs) commonly facilitate habitat conservation on private land in the United States, yet the effectiveness of individual HCPs is rarely evaluated. Here, we assess the effectiveness of a high-profile HCP created by a lumber company to protect old-growth forests used for breeding by Marbled Murrelets (Brachyramphus marmoratus) on private land. We used 17 years of HCP-monitoring data to compare trends in murrelet occupancy and inland counts between private HCP areas and public reference areas over time. Based on occupancy models applied to audio-visual survey data, average occupancy was higher in public reference areas (0.85; 85% confidence intervals [CI]: 0.79–0.90) than in private HCP areas (0.46; 85% CI: 0.38–0.54). Numerically, trends in occupancy were slightly positive in public areas ( = 1.01; 85% CI: 0.94–1.08) and slightly negative in private areas ( = 0.97; 85% CI: 0.87–1.06), but CI did not preclude stable occupancy on both ownerships. Based on generalized linear mixed models applied to inland radar survey data, murrelet counts in private HCP areas (least-squares [LS] mean = 8.7; 85% CI: 6.2–12.2) were lower than those in public reference areas (LS mean = 14.8; 85% CI: 10.1–21.7), but CI overlapped. Murrelet counts declined by 12–17% annually on both ownerships over the study period based on the top model, but a closely competing interactive model suggested more rapid declines in public reference (14–20%) than in private HCP (10–15%) areas. Both models indicated that murrelet counts were negatively related to sea surface temperature, suggesting that warm ocean conditions negatively affect murrelet breeding effort. Collectively, these results suggest that while HCP habitat may be lower quality than public reference areas, the HCP has likely not exacerbated ongoing declines of murrelets in the region. This work highlights the importance of including reference areas when evaluating conservation policies.