ascentswift Why do Common Swifts fly at high altitude twice daily?

to roost at lower altitudes. One function of the ascending to high altitudes at suggested for these transient ascents is the dawn as well as dusk. Identifying suitable identification of the most suitable altitude altitudes for roosting thus cannot be the for roosting based on current weather sole function of these ascents. conditions. So what is the function of these high- However, a number of observations of altitude forays? One possibility is that the Common flight behaviour have been swifts are getting their bearings and es- inconsistent with the notion of them find- tablishing their position in the landscape. ing the optimal roosting altitude during Birds are known to make use of several these ascents. Most significantly, swifts sources of visual information – both ter- have occasionally been reported showing restrial and celestial – for navigation, and this behaviour at dawn as well as dusk, twilight is a good time for them to orien- suggesting a function unrelated to roost- tate themselves since distant landmarks as

COMMON SWIFT peter ryan ing. To investigate this phenomenon more well as stars are visible. Flying at altitude greatly increases the area of the Earth’s xtreme flyers of the world, So what is the function surface visible to birds, and some passer- Common Swifts apus spend ines are known to show comparable be- of these high-altitude almost their entire lives on the wing. haviour during long-distance migrations. EThey feed, mate and sleep in the air, land- forays? One possibility These ascents may also provide the ing only to breed. Many aspects of their is that the swifts are swifts with opportunities to assess cur- behaviour and ecology remain poorly un- getting their bearings rent and future weather conditions. The derstood, but the availability of increasing- and establishing their availability of the flying insects on which ly sophisticated radar systems has opened swifts feed is strongly dependent on vari- position many doors to ornithologists. Recent stud- ables such as temperature, wind and rain- ies using this technology have provided fall, and information gained by traversing hitherto-impossible insights into the aerial thoroughly, the researchers used a Dop- a range of altitudes could, in theory, help lifestyle of these remarkable birds. pler weather radar station in the Nether- birds track areas of favourable weather One study, reported in the journal Ani- lands to track swifts’ flight altitudes. The and high insect availability. mal Behaviour in 2013, provides intrigu- resolution was such that swifts could be No doubt future studies will resolve this ing new information on flight patterns. differentiated from other flying birds on question, and along the way provide even The authors, a team of Dutch and Swed- the basis of their wing-beat frequencies, more surprises about the airborne lives of ish scientists, were particularly interested measured by the rapid changes in their ra- swifts. in the swifts’ behaviour around sunset, dar cross-sections as they flapped. Andrew McKechnie when the birds form social flocks, known As expected, the swifts typically climbed as ‘screaming parties’, before ascending to around 2 500 metres above the ground Reference: Dokter, A.M. et al. 2013. ‘Twi- to higher altitudes. Previous work has in the twilight after sunset, before descend- light ascents by Common Swifts, Apus revealed that the swifts climb as high as ing to lower altitudes to roost. But this apus, at dawn and dusk: acquisition of ori- 2 500 metres above the ground during behaviour was then repeated before sun- entation cues?’ Behaviour 85(3): these twilight ascents, before descending rise, confirming anecdotal observations 545–552.

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