16．
A．positioned B．resolution C．suspected D．lightweight E．challenge
F．generated G．journal H．eventually I．accurate J．raised K．critically |
The Power of V
Birds of a feather may flock together，but why they fly together in V formations has never been known for certain．
Now，with the help of 14 northern bald ibises （欧洲秃鹰） equipped with（41）Dsensors on a 600-mile migration from Austria to Italy，researchers are suggesting that the explanation is one that was long （42）Cbut never proved：the formations helps the birds conserve energy．
Reporting in the （43）GNature，the scientists write that the ibises（44）Athemselves in spots that were aerodynamically optimal （空气动力学上最佳的）-allowing them to take advantage of swirls of upward-moving air（45）Fby the wings of the bird ahead．Since the lead bird got no lift advantage，the ibises regularly switched leaders．
"We've been wondering for years whether flapping birds can save energy by following each other in the right way，"said Geiffrey Spedding，a researcher at the University of Southern California，who was not involved in the study．"This word answers that question，and the answer is yes．"
The scientists，led by Jim Usherwood of the Royal Veterinary College in England，said a major （46）Ewas obtaining the data．The ibises hatched at Zoo Vienna in March 2011 and （47）Jas part of a conservation project aimed at reintroducing the （48）Kendangered species to its natural range in Europe．
Some of the study's authors served as human foster parents，taking the young birds on training flights in Salzburg，Austria．The humans rode in a paraplane，a type of lightweight aircraft，and the birds followed．Eventually，the foster parents taught the birds their 600-mile migration route from Salzburg to Orbetello，Italy，by flying alongside them．
The birds wore custom-made data loggers that allowed the researchers to track flapping，speed and direction．Weighing less than an ounce，the devices included a memory card and a GPS unit，among other things．It is（49）Ito about one foot and refreshes five times per second-the（50）Bnecessary to track the birds'positions in relation to one another．
The new study does not say how much energy the ibises saved through the V formation，but small gains could be useful over long migrations，experts say．
