Common Swift: The Summer Surge
The Summer Surge: Understanding the Influx of Prospecting Subadult Common Swifts in the UK
Each summer, the skies above the United Kingdom come alive with the frenetic energy of common swifts Apus apus, a species renowned for its extraordinary aerial lifestyle. Among the most captivating phenomena associated with these birds is the significant influx of prospecting subadult swifts that join breeding populations in Britain during the warmer months. This blog post delves into the behaviour, origins, and migration patterns of these subadult birds, with a focus on their summer influx and explores the conditions that facilitate their spectacular migrations. Drawing on recent scientific literature, we aim to provide a comprehensive and evidence-based overview of what is one of the spectacles of the summer.
Who Are the Prospecting Swifts?
Common swifts are migratory birds that spend most of their lives in flight, feeding, sleeping, and even mating on the wing. Subadult swifts, typically one-to three-year-old birds that are not yet sexually mature, are a significant component of the summer swift population in the UK. Unlike breeding adults, which arrive in late April to early May to nest and raise young, subadult swifts typically arrive later, often in late May to early June, in what is sometimes referred to as the "second wave" of migration. These younger birds are not yet ready to breed but play a critical role in the species’ life cycle by prospecting for future nesting sites and gaining experience in their breeding grounds.
Subadult swifts are often observed in large, dynamic groups, engaging in “screaming parties” high-speed, vocal gatherings around rooftops and potential nest sites, particularly in the evenings. These behaviors are thought to serve multiple purposes: scouting for suitable nesting locations, learning from established breeding pairs, and potentially forming pair bonds for future breeding seasons. At sites like Spurn Point in East Yorkshire, a renowned migration watchpoint, these birds can form impressive movements, sometimes numbering in the well into the thousands, especially during favourable conditions.
What Are Subadult Swifts Doing in the UK?
The primary reason for these young swifts being in the UK is prospecting, exploring potential nesting sites and familiarising themselves with the local environment. Swifts typically do not breed until they are three to four years old, so these younger birds spend their early summers investigating urban and rural areas where breeding colonies are established. They are often seen closely following breeding pairs, observing their behaviour, and occasionally attempting to enter nest sites, though they rarely breed during these visits.
This prospecting behavior is critical for their future reproductive success. Swifts are highly philopatric, meaning they return to the same nesting sites year after year, often for life. By visiting breeding colonies as subadults, these birds learn to recognise suitable nest sites, such as crevices in old buildings or specially designed swift bricks, which are increasingly vital due to the loss of traditional nesting sites in modern architecture. Studies suggest that subadults may also use these visits to assess local food availability, particularly flying insects like aphids, which are a primary food source.
At Spurn, and at watchpoints along the coast, the large gatherings of swifts are often composed of these non-breeding subadults, which are channelled along the coast, often foraging on clouds of insects that blow and become trapped there during westerly winds. These flocks can be particularly prominent in insect-rich areas near wetlands or coastal marshes, where swifts feed in large numbers, sometimes covering a radius of up to 100 km to exploit abundant food sources. Their presence in such numbers also suggests social learning, as subadults may follow older, more experienced birds to prime feeding and nesting locations.
On of many hundreds of Common Swifts channelling along the coast at Filey during a large migration in early July
Where Are These Subadult Swifts From?
Common swifts breeding in the UK, including subadults, undertake one of the most remarkable migrations in the avian world, wintering in sub-Saharan Africa, primarily in the Congo region and southeast Africa. Geolocator studies have revealed that swifts from northern European populations, including those in the UK, migrate to wintering grounds in the Sahel zone and further south, with some individuals visiting multiple countries across Africa during the non-breeding season. Subadult swifts follow similar migration routes as breeding adults but differ in their timing and behavior.
Unlike breeding adults, which return to the UK to raise young and depart by mid-July to early August, subadults often remain in Africa longer or arrive in the UK later, typically in late May or early June. This delayed arrival is partly due to their lack of immediate breeding responsibilities and may also reflect a strategy to avoid competition with established breeders for nesting sites. Some subadults may even remain in Africa for the entire summer, particularly one-year-olds, though most join the northward migration to gain experience in their future breeding grounds.
The chain migration pattern of common swifts, as described by Åkesson et al. (2020), is particularly fascinating. Southern European breeding populations winter furthest south in Africa, while northern populations, including those from the UK, winter further north in the Sahel zone. This pattern suggests that subadult swifts from UK colonies likely originate from wintering grounds in central or northern sub-Saharan Africa, following inherited migration routes that align with their breeding populations.
How Does Their Migration Differ from Breeding Birds?
The migration of subadult swifts differs from that of breeding adults in several key ways:
Timing of Arrival and Departure: Breeding adults arrive in the UK from late April to early May and depart by mid-July to early August, driven by the need to nest and raise young. Subadults, however, arrive later (late May to early June) and may linger longer in the UK, sometimes into late August, as they are not constrained by breeding schedules. Their later arrival is influenced by environmental cues, such as day length, and their departure is less urgent, allowing them to explore and prospect without the pressure of rearing chicks.
Migration Routes and Stopovers: Both subadults and breeding adults use low-pressure fronts to exploit favorable winds during migration, particularly tailwinds in spring that facilitate rapid travel. However, subadults may spend more time at stopover sites in Africa or southern Europe to refuel, as they are not under the same time constraints as breeders. For example, one tracked swift stopped for 15 days in Liberia before completing a 5,000 km journey to the UK in just five days, highlighting the importance of stopover sites for refuelling.
Behaviour During Migration: Subadults are less focused on reaching specific nest sites and more on exploring multiple locations. Their migration is characterised by greater flexibility, allowing them to join large feeding parties or prospecting groups, particularly in areas like Spurn, where insect abundance supports large flocks. Breeding adults, in contrast, prioritise returning to their established nests and are less likely to join these large, non-breeding flocks.
Wintering Behavior: While breeding adults rebuild fat reserves before departing for Africa, subadults may remain in flight for up to 10 months, feeding on aerial insects and using unihemispheric slow-wave sleep to rest without landing. This continuous flight capability allows subadults to explore a wider wintering range, potentially visiting multiple African countries before returning to the UK.
Conditions for Observing Large Migration Events
The large influxes of subadult swifts, particularly at migration hotspots along the Yorkshire coast, are heavily influenced by environmental conditions. Several factors create ideal conditions for observing these spectacular gatherings:
Weather Conditions: Swifts are highly sensitive to weather, particularly wind and temperature. Warm, stable weather with tailwinds or low-pressure fronts facilitates rapid migration and large gatherings, as swifts can exploit these conditions to travel long distances and feed efficiently. Conversely, cold, wet springs, as observed in 2024, can delay arrivals and reduce insect activity, leading to smaller flocks.
Insect Abundance: Swifts rely on flying insects, such as aphids, for food. Areas with high insect biomass, such as wetlands, coastal marshes, or river valleys, attract large numbers of swifts. Places rich in insect biomass like Hornsea Mere can be particularly good too.
Timing: The peak movement of subadult swifts occurs around late June and early July within a two week window.
Habitat and Location: Coastal sites like Spurn are ideal for observing large migrations due to their position along migratory flyways. Swifts often follow coastlines or congregate in areas with abundant food resources, making Spurn a prime location for spotting subadult flocks.
References
Åkesson, S., et al. (2020). Evolution of chain migration in an aerial insectivorous bird, the common swift Apus apus. Evolution, 74(10), 2377–2391. https://doi.org/10.1111/evo.14093
Finch, T., et al. (2022). Demography of Common Swifts (Apus apus) breeding in the UK associated with local weather but not aphid biomass. Ibis. https://doi.org/10.1111/ibi.13156
RSPB. (n.d.). Swift Bird Facts | Apus apus. Retrieved from www.rspb.org.uk
BTO. (n.d.). Swift. Retrieved from www.bto.org
The Guardian. (2025). Swifts’ decline: how can Britons help these remarkable birds? Retrieved from www.theguardian.com
BirdLife International. (2024). Migratory bird of the month: The Common Swift. Retrieved from www.birdlife.org
Woodland Trust. (2021). Swift (Apus apus). Retrieved from www.woodlandtrust.org.uk
BTO. (n.d.). Tracking Swifts. Retrieved from www.bto.org[](https://www.bto.org/our-work/science/research-areas/migration/swifts) (http://www.bto.org[](https://www.bto.org/our-work/science/research-areas/migration/swifts))
