As a bird takes to the skies, it might have to manoeuvre around a slalom course, avoiding obstacles, other birds and escaping from predators. However how they do it isn't that well known, partly because many birds such as pigeons struggle to learn how to fly around corners in the lab, and refuse to fly in wind tunnels. Fortunately, as Ty Hedrick explains, members of the parrot family are quick learners so he travelled to Australia to study the brightly coloured rose-breasted cockatoo, which can master flapping around a corner in the lab in only 30 minutes(p. 1897). Together with his colleague Andrew Biewener, Hedrick says that the first question they wanted to answer was, `how do they do it?'.
First, the team trained the birds to fly down a 7 m long `L' shaped tunnel,with a tight 90° corner 3 m down. Interested to know how the flight muscles were active as they flew around corners, they anaesthetised the birds before inserting small EMG wires into the two large chest muscles, the pectoralis and the supracoracoideus, which are the powerful `flight engine'. They also inserted wires into some of the many small wing muscles, before feeding all the EMG wires into a cable attached to the birds' backs which they carried behind them as they flew along. The team also placed markers on key joints so that they could monitor the movements of the cockatoos' wings during flight using high speed cameras.
When they looked at the birds' movements in more detail they found that they turned around the tight bend by rolling, much like an aeroplane does when it turns. `Even for this type of turn, the differences between the wings were very subtle,' Hedrick explains. Teasing apart the wing movements in more detail, they found that the wing on the outside of the turn worked harder at the beginning of the turn while the inside wing flapped harder at the end of the turn to get the bird straight again. This was caused by the birds slightly altering their wing movements which changed the forces that each wing generated and therefore changed their orientation. At the beginning of the turn the outside wing swept through a larger arc and rotated at the shoulder to meet the air at a steeper angle than the inside wing, which was held at a shallower, flatter angle. On the second half of the turn, the wings switched roles with the inside wing moving through a larger arc and meeting the air at a steeper angle, generating more force than the outside wing to get the bird straight again.
Looking at the muscle activity in more detail, Hedrick says that `there was not a turning muscle'. Instead, there were subtle changes in the activity of all the muscles to help the bird get around the corner. So rather than changing the activity of the flight engine in a big way, cockatoos use small changes in their wings to turn, much like an aeroplane uses wing flaps and a rudder.
Having described how the wings and muscles were working during turning,Hedrick and Biewener teamed up with Jim Usherwood to work out how they were doing it (p. 1912). Because they knew the mass of each bird and their exact trajectory and speed,they could calculate the forces the cockatoos generated as they turned. They wanted to know how the birds used changes in the inertia of the wings, and changes in the aerodynamic forces to turn. To do this they used a mathematical model which estimates forces acting on the wing and takes into account the fact that the wing is not uniform in size, shape or movement. `We use models to test how we think wings work,' says Hedrick. So, if their model predicted similar forces to the ones they had calculated, then they would know that the model was doing its job well.
They found that the inertia of the wings and changing the balance of the forces were equally important over shorter timescales, for example during part of the upstroke or downstroke, while aerodynamic adjustments were more important over a whole wing beat. However, just like with the muscles, the team found that there were many small adjustments that were contributing to the overall effect. The model came quite close to estimating the forces that they had calculated, such as lift and torque.
Hedrick explains that the results show that turning is not that simple, but that they were pleasantly surprised that the model worked as well as it did to predict the forces acting on the wing. `This show that our understanding of aerodynamic factors is better than expected,' he says. `It's good to know that we are on the right track!'