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Flight performance in the wild

After all it is the stunning natural flight performance of many animals that we wish to understand, and to do so we theorize and make detailed studies in the wind tunnel. Experiments are made to refine and amend the flight mechanical theory, which is used to make predictions about flight behaviour. Aerodynamic principles are also fundamental to the development of optimal migration theory, which predicts how animals should migrate in the best of possible ways. Regarding flight behaviour an important question is if birds select their flight speed according to the predictions generated from flight mechanics.

We have several ways of measuring flight speeds of birds in the wild, such as target tracking radar and ornithodolite. With radar we get the trajectory of bird individuals or flocks during migration, but the drawback is that we rarely know the species being tracked (with a few exceptions). An ornithodolite consist of a range finder that records distance, elevation and azimuth angles to the target, and a computer reads the data. To determine the airspeed of the animal we also need to measure the wind, which is done by an anemometer for low altitude winds and by tracking ascending helium filled balloons for high altitude winds.

As an example, flight mechanical theory predicts that birds should fly at the minimum power speed (Vmp) when they want to minimize the energy expenditure per unit time (e.g. nocturnal roosting flight on the wings, song display flight), while the maximum range speed (Vmr) is the prescribed optimum when energy cost per unit distance is of importance (e.g. energy selected migration, commuting between roost and foraging area). For example, skylarks (Alauda arvensis) do select airspeed as predicted when song flying (Vmp) versus during migration (≥ Vmr).

Also swifts appear to select flight speeds according to context, such as migration versus nocturnal roosting flights, although the difference was rather small. However, during their characteristic flock display flights near their nesting colonies (“screaming parties”) they reach speeds up to 30 m/s, which is the highest recorded flight speed of any bird during muscle-powered flight.

By measuring airspeed of a number of species with the ornithodolite method it is also possible to investigate how speed scales with body size. According to flight mechanics flight speed should scale as (body mass)1/6, while real birds have been found to increase speed less steeply.

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The ornithodolite setup in the field
The ornithodolite set up in the fields to measure flight speeds of migrating birds on Öland.