Phenological mismatch between breeding birds and their surveyors and implications for estimating population trends

Several studies in recent decades, including those led by BTO, have demonstrated that many birds are migrating or breeding earlier as the climate changes. These so-called phenological shifts could have implications for monitoring, if people counting birds also change the dates on which they make surveys in ways that affect their likelihood of detecting birds. New BTO research has sought to investigate this, using data from the BTO/JNCC/RSPB Breeding Bird Survey (BBS). Data collected in South-East England were compared between two different five-year periods: 1994–98 and 2013–17. SE England was chosen because this is the region of the UK in which the largest sample of BBS squares is available, and the study's authors wanted to eliminate any regional biases in their findings.

The research showed that BBS volunteers advanced the dates of their visits by up to four days between 1994–98 and 2013–17. By contrast, the detection patterns of most species had changed less, generally by no more than two days, and were both advanced and delayed. For some species this means that surveys may better hit the peak of activity than they used to, whilst for others, surveys may now tend to miss the peak of activity. Although possible biases introduced to BBS results and population trends derived from these data were mostly small, they could have consequences, especially for the classification of Red-listed species. For example, the decline in Kestrel numbers becomes great enough for this species to qualify for the Red List if survey timing is accounted for.

BBS monitors around 120 bird species using a single method that is designed to be generally applicable to a wide range of species. The authors note that whilst it might be tempting to shift surveys progressively earlier, not all species are breeding earlier, with some now breeding later, meaning it is impossible to track every species. They suggest that surveyors should maintain fixed survey dates as much as possible, and instead visit timing should be factored into the analysis of BBS data. Overall, the research demonstrates the importance of accounting for accelerating phenological change in the design of surveys for monitoring wildlife and the statistical models used to analyse the resulting data, especially given current climate change scenarios.

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