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.
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 Vogelanzahl verursachen, was zu Verzerrungen in den daraus gefolgerten Bestandsentwicklungen führt. Wir bewerteten das Ausmaß möglicher Verzerrungen anhand der britischen Brutvogelerfassung „Breeding Bird Survey “, indem wir den Erfassungstermin, die Phänologie der Arten und die erkennbaren Verzerrungen der Bestandsentwicklungen zwischen 1994–98 und 2013–17 verglichen. Um für großräumige geographische Einflüsse zu kontrollieren, konzentrierten wir uns auf eine Auswahl von 888 untersuchten 1-km-Quadraten in Südostengland. Erfassungstermine wurden signifikant früher angesetzt, im Durchschnitt um 2–4 Tage vorverlegt. Wir bestimmten den saisonalen Verlauf der Häufigkeit für 68 Vogelarten. Nachdem diese standardisiert worden sind, um langfristige Häufigkeitstrends zu beheben, wurden die medianen Erfassungstermine durchschnittlich um 0,82 Tage vorgezogen. Auf Artniveau betrug die Veränderung mehrheitlich ± 2 Tage und nur bei fünf Arten zeigte sich eine signifikante Vorverlegung des medianen Erfassungstermins. Die phänologischen Veränderungen der Arten allein bewirken bereits Veränderungen von einer 8%igen Abnahme bis zu einer 21%igen Zunahme in den Bestandsentwicklungen, obwohl die Mehrheit ± 2% beträgt. Auswirkungen von ähnlichen Ausmaßen sind erkennbar, wenn man die Erfassungstermine ebenfalls sich ändern lässt, auch wenn verschiedene Arten davon betroffen sind. Kleine Anpassungen des statistischen Modells, das zur Erstellung von Bestandsentwicklungen verwendet wird, können für die Änderungen der Erfassungstermine kontrollieren, aber ohne zusätzliche Begehungen oder der Verwendung von Daten aus anderen Quellen können wir derzeit für die saisonale Variation in der Erfassbarkeit nicht kontrollieren. Obwohl die hier gezeigten durchschnittlichen Auswirkungen gering sind, könnten die Verzerrungen für einige Arten zunehmend bedeutender werden. Wir empfehlen den Organisatoren von Biodiversitätsmonitoringvorhaben zu prüfen, inwiefern ihre Methoden gegenüber einer Variation in der Phänologie der Arten und der Zeitabfolge der Erfassung robust sind.
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Data availability
The BBS data is available for research and conservation purposes on request by contacting: datarequests@bto.org.
Code availability
Code is available via GitHub (https://github.com/BritishTrustForOrnithology/Phenological_mismatch_J_Ornithol).
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Acknowledgements
We thank the thousands of volunteers who have taken part in the Breeding Bird Survey. The BTO/JNCC/RSPB Breeding Bird Survey is a partnership jointly funded by the BTO, RSPB and JNCC, with fieldwork conducted by volunteers. We are grateful to Eva Šilarová and Petr Voříšek for the information provided. We thank two anonymous reviewers whose comments improved the paper.
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The research was funded by a partnership of the British Trust for Ornithology, the Royal Society for the Protection of Birds, and the Joint Nature Conservation Committee.
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SH organises the Breeding Bird Survey. DM and SG conceived the idea, DM undertook all analyses and DM and SG wrote the paper.
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Communicated by T. Gottschalk.
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Massimino, D., Harris, S.J. & Gillings, S. Phenological mismatch between breeding birds and their surveyors and implications for estimating population trends. J Ornithol 162, 143–154 (2021). https://doi.org/10.1007/s10336-020-01821-5
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DOI: https://doi.org/10.1007/s10336-020-01821-5