Swifts typically leave the UK in late summer and early autumn to migrate to Africa for the winter. The exact timing of their departure varies slightly each year, but generally occurs between late July and early September.
Peak Departure Times
According to data from the British Trust for Ornithology (BTO), the peak departure times for swifts leaving the UK are:
- Late July to early August – Large numbers begin leaving the UK from breeding sites.
- Mid-August – Peak departure period when the bulk of swifts depart.
- Late August to early September – Remaining stragglers leave the UK.
Within this broad timeframe, the exact timing in a given year depends on weather conditions and availability of insect prey. On average, most swifts will have left the UK by mid-September at the latest.
Differences Between Sites
There can be regional and local differences in departure times across the UK. Swifts nesting further north tend to leave earlier, while those in southern parts of the UK may remain a bit longer into September. The altitude and habitat of nest sites can also influence timings.
As an example, data collected by the BTO shows the following average departure periods for four UK locations:
Location | Average Departure Period |
---|---|
Aberdeen, Scotland | Mid July to late August |
Derby, England | Late July to mid September |
Cardiff, Wales | Early August to early September |
Belfast, N. Ireland | Mid August to early September |
As you can see, the more northern breeding populations in Aberdeen and Belfast depart earlier on average compared to those further south in Cardiff and Derby.
What Triggers Departure?
What signals swifts to begin their epic migration back to Africa each year? Some key factors believed to trigger their departure include:
- Food availability – Swifts are aerial insectivores and rely on catching insects like flies, aphids and beetles for food. As insect numbers decline in late summer, it becomes harder for swifts to feed efficiently, spurring departure.
- Weather – Cooler temperatures, rainier weather and storms in late summer make sustaining a diet of aerial insects more difficult for swifts.
- Breeding cycle – By late summer, swifts have finished breeding and rearing young, so no longer need to remain at nest sites.
- Internal rhythm – Swifts may have an innate rhythm and changes in hormone levels that dictate migration timing.
It is likely a combination of these external and internal signals that tells swifts it is time to embark on their long migration. Swifts may gather in large pre-migratory flocks before departing the UK en masse when conditions are right.
Notable Departure Dates
While swift departure dates vary annually, some notable late departure dates for swifts in the UK include:
- September 14, 1991 – Late departure date from Oxfordshire
- September 30, 1992 – Exceptionally late birds noted in Scotland
- October 3, 2013 – Very late swift seen flying over Minsmere, Suffolk
These ultra-late records are highly unusual and likely involve sick or injured birds that were unable to migrate with the rest of the population. Generally, very few if any swifts remain in the UK beyond mid-September in a typical year.
Arrival and Departure
The departure of swifts from the UK mirrors their arrival times. Swifts are summer migrants, spending winter in Africa after breeding in the UK. Key dates in their annual cycle include:
- April to May – Arrival back in the UK from Africa
- May to August – Breeding season in the UK
- July to September – Departure from the UK back to Africa
- September to March – Overwintering in Africa
The swifts’ short summer residency in the UK is punctuated by epic migrations of over 5,000 miles between the UK and southern Africa each year.
Threats and Conservation
Swift populations in the UK and across Europe have declined significantly in recent decades. Major threats facing swifts include:
- Loss of nesting sites as old buildings are demolished or modernized.
- Reduced insect prey abundance from pesticide use and habitat loss.
- Climate change mismatch between breeding season and peak insect populations.
Conservation efforts for swifts include providing artificial nest boxes, retaining natural nest site access in buildings, and reducing pesticide usage. Tracking swifts’ migration timing and causes of mortality can also inform conservation strategies.
Summary
In summary:
- Most swifts leave the UK between late July and early September.
- Peak departure occurs in mid-August when the bulk of swifts migrate.
- Northern breeders leave earlier than southern birds.
- Declining food, weather changes and breeding cycle trigger departure.
- Rare stragglers may remain into early October.
- Arrival mirrors departure, forming a summer residency in the UK.
- Conservation is needed to protect declining swift populations.
Tracking swift departure provides insights into bird migration patterns and cues. Their remarkable migrations highlight the need for international cooperation in migratory bird conservation.