Phenological mismatch between breeding birds and their surveyors and implications for estimating population trends
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ORIGINAL ARTICLE
Phenological mismatch between breeding birds and their surveyors and implications for estimating population trends Dario Massimino1 · Sarah J. Harris1 · Simon Gillings1 Received: 3 April 2020 / Revised: 4 August 2020 / Accepted: 17 September 2020 © Deutsche Ornithologen-Gesellschaft e.V. 2020
Abstract Citizen science monitoring of common birds often involves volunteers visiting selected survey locations twice per year, with interannual differences in the number of birds detected used to infer population trends. Two processes, changes in the timing of migration and breeding in response to climate change, and changes in the timing of surveys by volunteers, may cause variation in the number of birds detected, leading to biases in inferred population trends. We assessed the magnitude of potential biases using the UK Breeding Bird Survey, comparing survey timing, species phenologies and apparent trend biases between 1994–98 and 2013–17. To control for large-scale geographic effects, we focussed on a subset of 888 surveyed 1 km squares in South-East England. Survey dates became significantly earlier, advancing by 2–4 days on average. We calculated seasonal patterns of bird abundance for 68 species. After standardising these to remove long-term abundance trends, median detection dates were advanced by 0.82 days on average. At the species level, the majority of changes were ± 2 days and only five species showed a significant advancement in median detection date. However, species’ phenological changes alone are capable of inducing between an 8% suppression and 21% enhancement of species’ trends, although the majority are ± 2%. Effects of a similar magnitude are apparent if survey timings are also allowed to change, although different species are affected. Small modifications to the statistical model used to generate trends can control for changes in survey timing, but without additional survey visits, or using data from other sources, we cannot currently control for seasonal variation in detectability. Although the average effects shown here are small, biases could become increasingly important for some species, and we recommend organisers of biodiversity monitoring schemes assess whether their methods are resistant to variations in species phenology and survey timing. Keywords Monitoring · Phenological change · Citizen science · Population trends Zusammenfassung Phänologische Abweichung zwischen Brutvögeln und deren Kartierern und ihre Auswirkungen auf die Schätzung von Bestandsentwicklungen „Citizen Science “-Monitoring von häufigen Vogelarten umfasst oft Freiwillige, die ausgewählte Zählgebiete zweimal jährlich besuchen, wobei die zwischenjährlichen Unterschiede in der erfassten Vogelanzahl genutzt werden, um auf Bestandsentwicklungen schließen zu können. Zwei Prozesse, zum einen die Veränderungen im zeitlichen Ablauf von Zug und Brut in Anpassung an den Klimawandel und zum anderen die Veränderungen in der Zeitabfolge der Erfassungen durch Freiwillige, können Schwankungen in der erfassten Vogelan
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