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Forward flapping flight in birds

Birds represent the most diverse group of flying vertebrates spanning a size range from 2g to 15kg and a flight speed range of 0 to >30 m/s. How birds are able to fly across these ranges have been an ongoing research field for more than a hundred years. The key to understanding this lies in studying the aerodynamics of bird flight.

Flapping, or powered, flight is a relatively expensive mode of locomotion per unit time compared with for example walking, running or swimming, but the fast transport speed in flight means that the cost of transport is still lower. Therefore the ability to fly has been key to the success of birds as a taxonomical group because it allows for long distance movements which are simply not possible for an animal bound to the ground.

In the Animal Flight Lab we have been concerned with flapping flight in birds for quite some time. The wind tunnel was built in 1994 and the focus of the studies back then was primarily on flight performance and energetics of migrating birds flying long-haul flights in the wind tunnel. The first detailed aerodynamic study of a flying animal was a thrush nightingale in flapping flight in 2003 using particle image velocimetry and it was the start of the era of detailed aerodynamic studies. The evolution of the wake across speeds found in the nightingale, showed that birds do not use different so called 'gaits' at different flight speeds as in terrestrial locomotion which had previously been the persisting assumption for a long time. The result from the nightingale study instead showed that the wake changed gradually when accelerating from low speeds up to fast forward speeds, which was quite a revolutionary discovery at the time.

Since the introduction of the PIV technique in 2003 we have performed many studies on flapping flight covering a number of different bird species such as robins, flycatchers, blackcaps, house martins and swifts. Our studies have been focused on describing and explaining various parts of the mechanisms used by the animals in flapping flight at an ever increasing level of detail.

In our current work we continue to explore the forward flapping flight in birds, both by adding more species in order to perform comparative studies and by focusing on parts of the wakes (such as body wake or wingtip vortices) or particular aerodynamic mechanisms.

Here follows just two examples of our previous work to give a flavour of the type of research we do:

Swifts

Swifts (Apus apus) are renowned for their extremely aerial life style. From the moment the young swift leaves the nest it sets out on a very long journey. It will spend almost its entire lifetime in the air, day and night. The only time swifts land for a considerable time period is during breeding. They forage on the wing, collect nest materials on the wing, sleep on the wing and perform yearly migrations between Europe and Africa.

We have studied the aerodynamics of flying swifts in the wind tunnel. The results have shown that the flight style, both regarding kinematics and wake structure, differs from that of other bird species. The swift flies with relatively rigid wings, flexing them very little during the upstroke. This results in a different wake compared to previously studied birds, with more or less constant shedding of vortices into the wake throughout the complete wingbeat, both down and up, which suggests that the change in forces generated is smooth.

Plot showing the wake behind a swift at three different speeds
The wake of a swift in three different forward flight speeds, from above 6, 8 and 10 m/s.

Bird tails

One of our studies concerned the aerodynamic function of the bird’s tail. We conducted the first high-speed PIV measurements in the transverse plane behind free-flying blackcaps during forward flapping flight, with and without a tail. We found novel wake structures previously not shown in birds, including weak wing root vortices of opposite as well as the same sign as the wing tip vortices. However, we were unable to detect any differences in the wake pattern between birds with and without a tail and thus conclude that the birds do not use the tail aerodynamically during steady flight. The tail of birds has also been hypothesized to reduce the drag of the bird, by acting as a splitter plate, but contrary to expectations a bird with tail had slightly higher drag than one without. The function of the tail in birds during level steady flight thus remains unclear.

Transverse vector fields behind a bird flying with and without a tail
Vortex wake and induced velocities behind a blackcap with a tail (A-E) and without a tail (F-J).
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A swift in flight making a banked turn
A turning swift. Note that the head is kept horizontally even during this extreme maneuver.