Anyone watching the autumn sky knows that migrating birds fly in a $\mathrm{V}$ formation, but scientists have long debated why. A new study of ibises finds  that these big-winged birds carefully position theirwingtips and sync their flapping, presumably to catch the preceding bird's updraft-and save energy during flight.There are two reasons birds might fly in a V formation: It may make flight easier, or they're  simply following the leader. Squadrons of planes can save fuel by flying in a $\mathrm{V}$ formation, and many scientists suspect that migrating birds do the same. Models that treated flapping birds like fixed-wing airplanes estimate that they save energy by draftingoff each other, but currents created by airplanes are far more stable than the oscillating eddies coming off of a bird. "Air gets pretty unpredictable behind a flapping wing," says James Usherwood, a locomotor biomechanist at the Royal Veterinary College at theUniversity of London in Hatfield, where the research took place.The study, published in Nature, took advantage of an existing project to reintroduce endangered northern bald ibises (Geronticus eremita) to Europe.Scientists used a microlight plane to show hand-raised birds their ancestral migration route from Austria to Italy. A flock of 14 juveniles carried data loggers specially built by Usherwood and his lab. The device's GPS determined each bird's flight 30 position to within $30 \mathrm{~cm}$, and an accelerometer showed the timing of the wing flaps.Just as aerodynamic estimates would predict, the birds positioned themselves to fly just behind and to the side of the bird in front, timing their wing beats35 to catch the uplifting eddies. When a bird flew directly behind another, the timing of the flapping reversed so that it could minimize the effects of the downdraft coming off the back of the bird's body. "We didn't think this was possible," Usherwood 40 says, considering that the feat requires careful flight and incredible awareness of one's neighbors. "Perhaps these big V formation birds can be thought of quite like an airplane with wings that go up and down." 45 The findings likely apply to other long-winged birds, such as pelicans, storks, and geese, Usherwood says. Smaller birds create more complex wakes that would make drafting too difficult. The researchers did not attempt to calculate the bird's energy savings50 because the necessary physiological measurements would be too invasive for an endangered species. Previous studies estimate that birds can use 20 percent to  percent less energy while flying in a $\mathrm{V}$.55 "From a behavioral perspective it's really a breakthrough," says David Lentink, a mechanical engineer at Stanford University in Palo Alto, California, who was not involved in the work. "Showing that birds care about syncing their wing 60 beats is definitely an important insight that we didn't have before."Scientists do not know how the birds find that aerodynamic sweet spot, but they suspect that the animals align themselves either by sight or65 by sensing air currents through their feathers.Alternatively, they may move around until they find the location with the least resistance. In future studies, the researchers will switch to more common birds, such as pigeons or geese. They plan to70 investigate how the animals decide who sets the course and the pace, and whether a mistake made by the leader can ripple through the rest of the flock to cause traffic jams."It's a pretty impressive piece of work as it is, but 75 it does suggest that there's a lot more to learn," says Ty Hedrick, a biologist at the University of North Carolina, Chapel Hill, who studies flight aerodynamics in birds and insects. However they do it, he says, "birds are awfully good hang-glider 80 pilots."Q: What can reasonably be inferred about the reason Usherwood used northern bald ibises as the subjects of his study? Answer Choices: (A)The ibises were well acquainted with their migration route. (B)Usherwood knew the ibises were familiar with carrying data loggers during migration. (C)The ibises have a body design that is similar to that of a modern airplane. (D)The ibises were easily accessible for Usherwood and his team to track and observe.
A: Among A through D, the answer is
(A)The ibises were well acquainted with their migration route.
(B)Usherwood knew the ibises were familiar with carrying data loggers during migration.
(C)The ibises have a body design that is similar to that of a modern airplane.
(D)The ibises were easily accessible for Usherwood and his team to track and observe.
Answer: 3