The research by the University of Illinois and the Illinois Natural History Survey is said to the first to confirm birds fly together in loose flocks during nocturnal migrations.

Professor Ronald Larkin, who led the study with Illinois Natural History Survey ecologist Robert Szafoni, said previous research suggested although the birds were flying tens of feet apart, they kept together. However, he said the evidence was indirect.

Now the researchers have determined a significant proportion of the birds tracked were flying at the same altitude, speed and direction. Although some were quite far apart, they were traveling together, following their own course and not simply being blown along by the wind.

"To me, that's the marvelous thing," said Larkin, "that they're flying in social groups in the middle of the night in the middle of the air, over territory most of them have never been over before."

The research appears in the journal Integrative and Comparative Biology.