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 fitted with lightweight sensors on a 600-mile migration from Austria to Tuscany, researchers are suggesting that the explanation is one that was long suspected but never proved: The formation helps the birds conserve energy.

Reporting in the journal Nature, the scientists write that the ibises positioned themselves in spots that were aerodynamically optimal — allowing them to take advantage of swirls of upward-moving air generated by the wings of the bird ahead. (The lead bird gets no lift advantage; the ibises regularly switched leaders.)