NY Times article
here.

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.)

Thanks to the Mad Scientist.