The majestic flight of birds of prey, wings spread wide and eyes fixed on a potential meal, is indeed a thing of awe. Smaller birds impress with their manoeuvring capabilities: hummingbirds, famously, manage to fly backwards and sideways. But there are other birds that have also mastered hovering flight, with their bodies seemingly suspended in mid air as they rapidly flap their wings.

Jian-Yuan Su and colleagues have now uncovered a phenomenon that makes the feat seem even more remarkable, at least from a mechanics point of view: the most stable point of the bird's motion while hovering is not at its centre of gravity, as might be expected, but at eye level (Phys. Rev. E in the press). This does, on second thoughts, make sense, as a bird on the look out for food would benefit from a blur-free view of its environment.

Credit: © JECKY CHUANG

To work out how hovering birds might manage to stabilize their position such that their eyes are at the centre of motion, Su et al. studied a passerine known as the Japanese White-eye (Zosterops japonicus, pictured). They trained eight birds to perform their hovering flight inside a chamber monitored by two high-speed cameras, each recording 1,000 frames per second — fast enough to finely resolve the flapping motion at some 24 Hz.

The recordings reveal that the bird's body swings up and down quite substantially during hovering. During downstroke, the extended wings generate a downward air jet that pushes the bird up, whereas the upstroke, during which the wings are retracted, is aerodynamically inactive; therefore the bird drops under gravity to its original position. (Hummingbirds, by contrast, produce a continuous lift throughout the wingbeat cycle.)

The point of action of the lifting force does not, however, coincide with the centre of mass of the passerine; instead, its position is slightly dorsal. This means that the wing motion causes simultaneously a translational uplift and a rotation of the bird's body. And these two motions turn out to be so finely balanced that the eyes remain nearly stationary — their displacement is less than a tenth of that experienced by the tip of the tail.

As well as using the aerodynamic mechanisms proposed by Su et al., the bird can further stabilize its head and eyes by muscular and skeletal motion. This should enable it not only to keep a level head, but also to maintain its gaze — and keep its eyes on the prize.