Abstract
Monitoring programs are key to determine bird population trends and to assess environmental policies, and therefore are central to conservation biology. The European approach commonly used to estimate bird population trends (TRends and Indices for Monitoring data, hereafter TRIM) has proved useful to fulfil this task, yet it fails to account for imperfect detection and assumes constant detectability across years. We tested the role of detectability for population trend estimation in an open Mediterranean farmland context, which is a dynamic landscape likely to undergo yearly changes in detectability, by using data of 30 bird species over a nine-year study period. We evaluated species-specific population trends under the TRIM approach and hierarchical distance sampling models (hereafter HDS) that estimate true abundance by accounting for imperfect detection. When comparing both methods, 13 species presented differences in population trend estimates between TRIM and HDS models. Moreover, detectability was not constant across the bird community: observer and year affected detection, and these effects varied among species. Our study highlights the importance of accounting for imperfect detection in bird monitoring programs to ensure reliable trend estimates, providing a first insight for an open farmland bird community. Aside from trend estimates, our HDS model may prove useful as a tool to obtain site-specific abundance estimates (for intance, within Special Protection Areas) and trend probabilities of bird populations.
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Acknowledgements
We acknowledge the field assistance of Joan Estrada, Sergi Sales, Joan Castelló, Marc Anton, Arnau Bonan, Xavi Larruy, Albert Petit, Ferran González, Juan Bécares, Ferran Broto, Xavier Riera and David Guixé. We acknowledge Nuria Pou for the extensive support in data handling, and Cyril Milleret and Marc Kéry for providing useful feedback on the study design.
Funding
Departament d’Agricultura, Ramaderia, Pesca i Alimentació and Infraestructures de la Generalitat de Catalunya S.A.U. have funded a relevant part of the project. This work has been partially supported by the Generalitat de Catalunya through a FI predoctoral contract to Ana Sanz-Pérez (2018FI_B1_00196).
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R.S, D.G., F.S-P, and A.S-P conceived and designed the study, with G. B’s suggestions. A.S-P and R.S implemented the analysis. A.S-P wrote the manuscript with the help of R.S, D.G., F.S-P and G.B. All authors contributed to subsequent drafts and gave final approval for publication. D.G coordinated fieldwork and G.B secured funding.
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Communicated by Melvin Gumal.
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Sanz-Pérez, A., Sollmann, R., Sardà-Palomera, F. et al. The role of detectability on bird population trend estimates in an open farmland landscape. Biodivers Conserv 29, 1747–1765 (2020). https://doi.org/10.1007/s10531-020-01948-0
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DOI: https://doi.org/10.1007/s10531-020-01948-0