House Martin numbers in the UK have declined by 39% over a 25 year period, but it is unclear why. This small hirundine is a summer visitor that spends the winter months in an unknown part of sub-Saharan Africa, returning to build their cup-shaped nests under the eaves of buildings during April and May. In the 2016 and 2017 breeding seasons, BTO ran the House Martin Nest Study which involved members of the public monitoring the breeding behaviour of House Martins nesting close to where they lived. The findings of this survey show that House Martins arrive earlier in the east of the UK and begin breeding earlier than birds nesting in the west, possibly a result of drier weather in the east. Birds using old nests from previous years or artificial nests also have greater breeding success than those that building nests from scratch. Furthermore, birds that build nests on PVC as opposed to brick, concrete or wood have much lower breeding success, with nests more likely to collapse on the PVC substrate. House Martins breeding in an agricultural environment are also less successful than those breeding in a suburban setting. The study also reported the first confirmed triple broods in the UK; whilst pairs of House Martins have been recorded undertaking three nesting attempts in a single summer further south in Europe, this had not previously been observed in the UK. The research, which identifies factors potentially influencing House Martin population trends, has direct conservation applications for this declining species. One simple recommendation from the study's results is to install artificial nesting cups to save birds around 10 days of nest building time when they return from migration, especially on buildings wtih plastic soffit. The results also suggest that old nests should not be cleared for similar reasons.

The Newsletter of the Wetland Bird Survey (WeBS). Read about why data received from WeBS counts are as important as ever, and WeBS' continued support to count waterbirds along the East Atlantic flyway.

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.