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.

The highly aerial Common Swift Apus apus, which spends the non‐breeding period on the wing, has been found to exhibit a rarely‐found chain migration pattern.

Newly published research, carried out by staff at BTO Scotland, has investigated the response to wintering waterbirds to drones, and shown that they can be easily scared into flight by drone use. Drone use has increased sharply in recent years, facilitated by mass production and much-reduced retail prices. The mass proliferation of drones and the likelihood of commercial and recreational drone use taking place in proximity to wildlife creates a new and potentially significant source of disturbance to wild birds. Such disturbance causes birds to waste energy and reduces their feeding time. In extreme cases, birds might stop using an area altogether, and be forced to feed elsewhere, where feeding opportunities may be poorer or the risk of predation higher. This could be particularly harmful during the cold winter months. BTO scientists flew a commercially available quadcopter drone towards waterbird flocks in coastal, freshwater and arable farmland habitats. While one researcher flew the drone at a standardised speed and height towards the flock, another observed the flocks through a telescope to record any responses to the drone as it approached, including alarm calls, signs of heightened alert levels and taking flight. The results showed that larger flocks were more likely to take flight than smaller flocks, and large flocks also took flight at a greater distance from the drone than smaller flocks. This is probably because the larger the flock, the more likely there is to be a sensitive individual present – in almost all cases, once one bird had responded to the drone, the rest of the flock followed. Habitat type also had a strong effect on birds' responses to drones. Birds at inland lochs, which were already subject to lots of human activity, were very unlikely to respond to drone presence, while birds at coastal sites were more likely to respond. Birds in arable farmland were particularly sensitive – flocks feeding in this habitat are probably more susceptible to disturbance because of the need to be vigilant to potential predators. Britain hosts internationally important flocks of waterbirds outside the breeding season. While it has been thought that drones could be useful in monitoring their numbers, the disturbance caused by such monitoring would have to be carefully evaluated. If drone use were to become more frequent at important sites for our wintering waterbirds, and birds did not become accustomed to this novel form of disturbance, then the resulting increases in energy expenditure and stress would be likely to negatively affect their population. The results of this research could be used to help inform guidance and regulations on drone use in proximity to birds and other wildlife.