Zoology: The stability and instability of bird flight

Bird flight can be both inherently stable and unstable according to a Nature paper, a finding that goes against the prevailing assumption that birds are evolving away from stability to improve manoeuvrability. The study sheds light on the evolution of flight and lays the foundation for a theoretical model of avian manoeuvrability.

Birds can alter their wing shape to accomplish some extraordinary aerial manoeuvres, but the dynamics of these rapidly changing movements are not well understood. To learn more, Christina Harvey and colleagues studied wing morphing capacity in 22 bird species. Wing morphing allows birds to change their roll and yaw inertia (rotations across the longitudinal and vertical axes), but these changes have little impact on the position of the centre of gravity, they found. Seventeen of the species were shown to be able to transition between stable and unstable flight.

Although previous studies have suggested that modern birds may be capable of stable flight, it is widely thought that birds evolved to be unstable in pitch (rotation around the wing-to-wing axis) to enhance manoeuvrability. This study suggests a new narrative, proposing that evolutionary pressures may be maintaining the ability to shift between stable and unstable configurations. It also provides a missing link for those wishing to develop a mathematical theory of avian flight manoeuvrability. A similar theory currently exists for aircraft, but it could not be applied to birds without the detailed understanding of inertial properties that this study now provides.