Abstract
Population size assessments for nocturnal burrow-nesting seabirds are logistically challenging because these species are active in colonies only during darkness and often nest on remote islands where manual inspections of breeding burrows are not feasible. Many seabird species are highly vocal, and recent technological innovations now make it possible to record and quantify vocal activity in seabird colonies. Here we test the hypothesis that remotely recorded vocal activity in Cory’s shearwater (Calonectris borealis) breeding colonies in the North Atlantic increases with nest density, and combined this relationship with cliff habitat mapping to estimate the population size of Cory’s shearwaters on the island of Corvo (Azores). We deployed acoustic recording devices in 9 Cory’s shearwater colonies of known size to establish a relationship between vocal activity and local nest density (slope = 1.07, R2 = 0.86, p < 0.001). We used this relationship to predict the nest density in various cliff habitat types and produced a habitat map of breeding cliffs to extrapolate nest density around the island of Corvo. The mean predicted nest density on Corvo ranged from 6.6 (2.1–16.2) to 27.8 (19.5–36.4) nests/ha. Extrapolation of habitat-specific nest densities across the cliff area of Corvo resulted in an estimate of 6326 Cory’s shearwater nests (95% confidence interval: 3735–10,524). This population size estimate is similar to previous assessments, but is too imprecise to detect moderate changes in population size over time. While estimating absolute population size from acoustic recordings may not be sufficiently precise, the strong positive relationship that we found between local nest density and recorded calling rate indicates that passive acoustic monitoring may be useful to document relative changes in seabird populations over time. Nature Conservation 7: 1–13 (2014) doi: 10.3897/natureconservation.7.6890 http://www.pensoft.net/natureconservation Copyright Steffen Oppel et al. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. RESEARCH ARTICLE Launched to accelerate biodiversity conservation A peer-reviewed open-access journal
Highlights
Seabirds are globally the most threatened group of birds (Croxall et al 2012), and assessing the severity of threats or the effectiveness of conservation measures requires periodic assessments of population size to understand whether populations are declining or increasing
Estimating population trends remains challenging for many seabird species due to logistical constraints imposed by often inaccessible breeding locations on remote islands
The mean calling rate per 1-min recording ranged from 0.7 to 55.2 Cory’s shearwater calls at the nine recorders with known local nest density, and from 0–25.1 calls at the recorders placed at inaccessible cliff locations
Summary
Seabirds are globally the most threatened group of birds (Croxall et al 2012), and assessing the severity of threats or the effectiveness of conservation measures requires periodic assessments of population size to understand whether populations are declining or increasing. Estimating population trends remains challenging for many seabird species due to logistical constraints imposed by often inaccessible breeding locations on remote islands. Several seabird species nest in burrows or cavities, and are active in breeding colonies only during the hours of darkness. For these nocturnal burrow-nesting seabirds visual assessments of population size are not possible, and robust estimates of population size are usually based on marking a large number of birds (Sanz-Aguilar et al 2010; Sutherland and Dann 2012) or estimates of burrow occupancy (Pearson et al 2013). On many islands the marking of seabirds or inspection of burrows is logistically not feasible, because birds nest on inaccessible cliffs. The group of species for which assessments of population size are critical is one of the most difficult to monitor
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