Evidence for widely dispersed birds migrating together at night

Lore and indirect evidence from previous studies suggest thatnocturnally migrating vertebrates (perhaps bats but mostly birds)sometimes fly widely dispersed from each other, but in flocks.The observations include stationary and scanning radars, recordingsof flight calls, and watching the moon with telescopes. Directobservations of such flocks have been lacking. This articlepresents data from novel tracking of nocturnal aggregationsof radar targets. Statistical analysis of straight, detailedflight paths supported the hypothesis that vertebrates, almostcertainly birds, flying within about 200–300 m of eachother fly parallel (in the same direction at the same speed)more often than do vertebrates flying farther apart. This inferencewas strengthened by comparisons with a partial control for windand for small-scale atmospheric structure: namely, small nocturnalarthropods tracked by the identical method did not fly parallel.Radar data also indicated that birds flying together may havesimilar wing beats, suggesting taxonomic similarity betweenbirds flying parallel. Possible functions include not only mutualbenefits on the ground during migratory stopover (habitat use,avoidance of predators, and social feeding) but also in-flightsharing of information about orientation.