Up In The Air: A Swift’s Tale
You might know the swift as a cute little bird that visits your garden and dashes around at speed, however these small avians are have a lot more metal than you give them credit for: A new study has shown that the Common Swift can remain airborne for incredible amounts of time.
With its light weight and streamlined body, the Common Swift is extremely agile, capable of capturing both food and nest material from the air mid flight. It is not surprising that many biologists agree that the Common Swift is ideally built for an aerial lifestyle.
Observation of their flight and migration habits had led scientists to suggest that the Common Swift remains airborne much of its time out of the breeding season. During this time it is thought that most individuals will migrate to Sub-saharan Africa, however roost sites and sightings are rare. So no-one was really sure what the Common Swift got up to during the winter months…
Nb. The swifts that some people find roosting around their home in the UK and Europe are generally juveniles roosting before migration due to poor weather conditions. However if you are in Northern America you could find either the North American swift (Chaetura pelagica) and Vaux’s swift (C. vuaxi) roosting during migration and even for the entire winter with some groups no longer migrating South.
However a new Swedish study has allowed an insight into the little studies life of the Common Swift during the non breeding winter season and found surprising results.
The team equipped 19 Common Swifts with micro-data loggers in the aim of recording the individual’s flight activity for 1-2 years. The data-loggers were composed of an accelerometer (years 1 and 2) and a light-level sensor for geolocation (during year 2)
Firstly the light data showed that the swifts either spent the duration of the winder in West Africa (Liberia, Ivory Coast and Ghana) or in Central Africa (Democratic Republic of Congo and Congo Brazzaville), spending the majority of their time between two main wintering areas. This correlated with the previous findings from studies of Swedish swifts.
The obtained data allowed them to observe that the swifts remained airborne for over 99% of the time during the 10 month non-breeding season. Meaning that some individuals never landed during the entire 10 month duration, equal to over 314 days, and most only landed on rare occasions with inactivity periods ranging from 2 hours up to 12 hours. These landings generally happened nocturnally.
Additionally the team remarked that the swifts followed a diurnal cycle of energy expenditure with a high flight activity around dawn and dusk followed by a period of gliding descent. This has been commonly observed and studied during the summer with some swifts known to reach altitudes over 2500 meters. However the reason for the ascents still remains obscure with some studies suggesting a link with navigation and not foraging as first thought.
Most studies have only been able to conjecture on whether the behaviour is continued during winter months. These findings allow confirmation that such behaviour does indeed occur throughout the annual cycle and not just during the summer and at least solve some of the enigma surrounding swift behaviour.
Observed energy expenditure, was less during the day compared to the night. It is thought that this is due to the swifts using thermals to glide and soar for long durations on time. This is very similar to the energy saving technique used by the wandering albatross which is known to be capable of being airborne for years at a time. However the direct effects of such a long time without landing are yet to be studied. Additionally the evolutionary benefits of such a strategy remain little understood.
The Common Swift has quite an extreme lifestyle with an aerial period in the wintering months that is 3.5 months longer than that of its relative, the Alpine Swift which has an already impressive airborne period of 6 months.
Other birds also have adopted this lifestyle:
For example, Great Frigate birds (Fregata minor) can stay in the air for over two months at a time and can sleep while airborne. And the wandering Albatross, with an up to 3.5 meter wing span, can be airborne for up to 5 years without landing. Their energy expenditure is so efficient that they use less energy in flight than they would sitting on dry land.
Hedenstrom, A. Norevik, G. Warfvinge, K. Andersson, A. Backman, J. and Akesson, S. (2016) ‘Annual 10-Month Aerial Life Phase in the Common Swift Apus apus’ Current Biology 26 : 1 – 5.
Find the full paper here:
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