When swift fledglings depart the nest, there's no going back. They may not touch down again for another two years, and many swifts only perch to mate and raise young. According to Per Henningsson from Lund University, Sweden, the bird's aerial life style is reflected in their extreme build. With streamlined bodies and long slender wings, swifts are perfectly designed for life on the wing. `They are like no other bird' says Henningsson `which makes them very interesting'. What is more, little was known about the ultimate aeronaut's aerodynamics `which makes them an important piece of the puzzle to understand animal flight' adds Henningsson. Knowing that much can be learned about an animal's flight by analysing the complex fluid flows in their wake,Henningsson and Anders Hedenström teamed up with Geoff Spedding from the University of Southern California to make the first aerodynamic measurements of swifts during flapping flight(p. 717).
However, adult swifts are extremely sensitive birds that are notoriously difficult to keep in captivity. Fortunately, there was an alternative; to collect fledgling birds just before they left the nest. Capturing two juveniles, Henningsson returned to the lab before introducing them to the Lund University wind tunnel for their maiden flights. `They didn't know about real life' says Henningsson `so they just accepted the wind tunnel'. In fact, the birds flew so well that Henningsson was able to begin collecting data within a day of the fledglings' first flights.
Filming the birds from behind, Henningsson recorded 80 complete wing beats while the birds flew comfortably at three different speeds. As the birds'speeds increased, Henningsson noticed that their wing beat frequency dropped while the birds raised their wings higher at the start of every wing beat. The birds' muscles were shortening at a fixed rate, regardless of their flight speed `like an engine with one gear' says Henningsson.
Having analysed the birds' wing beat geometry, Henningsson moved on to visualise the air flows in the birds' wakes. Introducing a thin fog into the flight tunnel, Henningsson visualised the eddies and currents in narrow slices of the birds' wakes with a fine plane of laser light. Digitally tracking the trajectory of individual fog particles in the laser plane and combining individual wake slices, Henningsson could build up a complete picture of the wake to see how the birds remain aloft.
Analysing the reconstructed wake, the team could see that the wing generated both lift and thrust as the swift brought its wing down, just like any other flapping bird. But the aerodynamics of the wing beat's upstroke was completely different. Henningsson explains that most birds retract their wings during the upstroke to minimise the effect of drag, despite the loss of lift. However, the team could clearly see that the swift's wings remained extended,generating lift while reversing the thrust direction, resulting in an effective drag. The swifts got a smoother ride despite the incurred cost. And the wake structure was completely different from anything that had been seen,or modelled, before; the birds continually shed force-generating vortices during the course of each wing beat. Most remarkably, the swifts generated the highest lift-to-drag ratio that had ever been measured for birds during flapping flight.