Morphing wings help birds control gliding

With an articulated skeleton under muscular control, birds are able to change the amount their wing feathers overlap and accordingly alter their wings’ shape and size, independent of whether the wings are flapping. To learn more, researchers in the Netherlands did windtunnel studies of the common swift (Apus apus), which spends most of its time in the air—even roosting on the wing—and has an extensive gliding repertoire. They found that wings fully extended outward are optimal for slow glides, lazy turns, and low metabolic cost. Indeed, swifts actually roost at a sedate 8–10 m/s with their wings outstretched. Wings swept backward toward the tail (see the figure) improved not only gliding at higher speeds but also agility. In particular, swept wings can bear a higher load and thus can tolerate the increased centripetal acceleration during high-speed turns at up to 30 m/s. The researchers used their data to develop a semiempirical model that correlated well with radar measurements on roosting swifts in the field. Wing morphing could help control the flight of future aircraft. (D. Lentink et al., Nature 446 , 1082, 2007 .)