UC Santa Cruz

Although wildlife conservation actions have increased globally in both number and complexity, the dearth of scalable, cost-effective monitoring hampers effective intervention to understand conservation efficacy. Automated sensors and computer aided analysis have expanded the tools available for conservation monitoring. For threatened and elusive seabirds, acoustic monitoring presents a cost-effective, scalable alternative to traditional monitoring methods that are limited by remote or inaccessible locations requiring skilled field personnel. A key assumption in monitoring population trends acoustically is that measures of acoustic activity are correlated with the relative nesting abundance of seabirds at colony sites. Here we tested the efficacy of acoustic activity as an index of seabird nesting abundance at colonies. Sensors recorded ambient noise at Forster's Tern (Sterna forsteri) breeding colonies in San Francisco Bay for two breeding seasons. We used an automated method (spectrogram cross-correlation) to detect and count tern advertisement calls from recordings. We calculated mean seasonal call rate and compared it to the mean active nest count at colonies. Our results show that acoustic activity was a reliable index of colony size among breeding sites and an accurate index of change in colony size between years (adj. R2=0.94, n=5, p<.01). Acoustic activity was a strong enough index to detect population differences anticipated by effective conservation actions. Acoustic activity was a more precise index when comparing within sites between years, than among different sites, likely due to differences in the sound environments of the small islands. Quantifying the relationship between acoustic activity and relative abundance is a fundamental step in designing effective and scalable acoustic monitoring programs to monitor the effectiveness of conservation actions for seabirds and other vocalizing wildlife.