Twenty seasons of swift arrivals
Swifts return to the same roofs within a few days of the same date each year, which makes them an unusually clean marker.
The first-sighting date is recorded only when two birds are seen together over the same street on two consecutive evenings. A single bird passing high is not counted; it is too easily a migrant still moving north.
Why the date moves so little
Arrival is cued by day length far to the south, not by conditions here. The spread across twenty seasons is eleven days, against nearly six weeks for the first chiffchaff. What does vary is the interval between arrival and the first confirmed nest, and that interval tracks insect abundance closely.
| Period | Median arrival | Arrival to first nest |
|---|---|---|
| 2006–2010 | 1 May | 9 days |
| 2011–2015 | 30 April | 11 days |
| 2016–2020 | 29 April | 14 days |
| 2021–2025 | 29 April | 17 days |
The arrival date has barely shifted. The interval has nearly doubled. If there is a signal in this record worth anyone's attention, it is the second column, not the first.