Birds are impressive and agile fliers. To remain airborne, they must produce forward thrust and upward lift forces while overcoming gravitational and drag forces on their body and wings. Thrust drives a bird forward, while lift forces support weight and drag forces act against the wings’ flapping movements, increasing the energy requirements for flight. Yet, previous studies have shown that birds orient their wings differently during take-off and landing by inclining the wings steeply as they flap, raising the question: do lift and drag fulfill different purposes during the start and end of flights? Diana Chin and David Lentink at Stanford University, USA, sought to answer this question by investigating the take-offs and landings of Pacific parrotlets to understand how the forces produced by their wings allow them to lift off and land.

However, measuring aerodynamic forces produced by a freely flying bird is no easy feat, so Chin and Lentink developed a box called an aerodynamic force platform (AFP) that measured the birds’ flight forces by sensing the air displacement on its sides as the animals flew through it. Using the combination of these forces and simultaneously filmed high-speed videos of the birds’ wing movements, they were able to reconstruct the magnitude and direction of the drag and lift components of the force produced by the birds during short flights from one perch to another.

The videos showed that birds steeply incline their wing beats during take-off and landing. During take-off, the birds positioned their wings to direct the drag forces upward and support almost half of their body weight. However, when coming in to land, the parrotlets flapped their wings at an angle that shifted the lift forces backward to act like a brake during the final wing beats. Increasing drag forces to aid take off is energetically expensive, although their use of lift for braking probably saves energy when landing. Together, these findings flip the conventional understanding of the purposes of lift and drag on its head. It turns out drag might not be such a drag after all and lift can be a great brake.

It also turns out that these short parrotlet flights may show us how the ancient ancestors of the birds we know today first reached for the sky. Several theories could explain how these predecessors took their first flights; flight may have originated when they made gliding falls from trees or from running starts on the ground. Both behaviors require that would-be fliers overcome the challenge of wings that were not built to support their weight. The use of drag in modern parrotlets shows that while these wings may not have been able to produce enough lift, they could have used drag to generate the force required for take-off or to stay aloft longer. Our understanding of animal flight for both modern birds and their ancient counterparts is enhanced by considering how forces act on flapping wings.

D. D.
Birds repurpose the role of drag and lift to takeoff and land
Nat. Comm.