title: Vocal activity as a low cost and scalable index of seabird colony size.
abstract: Although wildlife conservation actions have increased globally in number and complexity, the lack of scalable, cost-effective monitoring methods limits adaptive management and the evaluation of conservation efficacy. Automated sensors and computer-aided analyses provide a scalable and increasingly cost-effective tool for conservation monitoring. A key assumption of automated acoustic monitoring of birds is that measures of acoustic activity at colony sites are correlated with the relative abundance of nesting birds. We tested this assumption for nesting Forster's terns (Sterna forsteri) in San Francisco Bay for 2 breeding seasons. Sensors recorded ambient sound at 7 colonies that had 15-111 nests in 2009 and 2010. Colonies were spaced at least 250 m apart and ranged from 36 to 2,571 m(2) . We used spectrogram cross-correlation to automate the detection of tern calls from recordings. We calculated mean seasonal call rate and compared it with mean active nest count at each colony. Acoustic activity explained 71% of the variation in nest abundance between breeding sites and 88% of the change in colony size between years. These results validate a primary assumption of acoustic indices; that is, for terns, acoustic activity is correlated to relative abundance, a fundamental step toward designing rigorous and scalable acoustic monitoring programs to measure the effectiveness of conservation actions for colonial birds and other acoustically active wildlife.
authors.affiliation: Department of Ecology and Evolutionary Biology, Center for Ocean Health, University of California Santa Cruz, 100 Shaffer Road, Santa Cruz, CA 95060, U.S.A.
authors.forename: Abraham L
authors.initials: AL
authors.lastname: Borker
authors.forename: Matthew W
authors.initials: MW
authors.lastname: McKown
authors.forename: Joshua T
authors.initials: JT
authors.lastname: Ackerman
authors.forename: Collin A
authors.initials: CA
authors.lastname: Eagles-Smith
authors.forename: Bernie R
authors.initials: BR
authors.lastname: Tershy
authors.forename: Donald A
authors.initials: DA
authors.lastname: Croll
mesh_terms: D000818:Animals; D046091:Charadriiformes / Q000502:physiology*; D003247:Conservation of Natural Resources / Q000379:methods*; D011156:Population Density; D012495:San Francisco; D014828:Vocalization, Animal
keywords: Aves marinas; adaptive management; bioacoustics; bioacústica; manejo adaptativo; monitoreo; monitoring; population indices; seabirds; índices de población