Lore and indirect evidence from previous studies suggest that nocturnally migrating vertebrates (perhaps bats but mostly birds) sometimes fly widely dispersed from each other, but in flocks. The observations include stationary and scanning radars, recordings of flight calls, and watching the moon with telescopes. Direct observations of such flocks have been lacking. This article presents data from novel tracking of nocturnal aggregations of radar targets. Statistical analysis of straight, detailed flight paths supported the hypothesis that vertebrates, almost certainly birds, flying within about 200-300 m of each other fly parallel (in the same direction at the same speed) more often than do vertebrates flying farther apart. This inference was strengthened by comparisons with a partial control for wind and for small-scale atmospheric structure: namely, small nocturnal arthropods tracked by the identical method did not fly parallel. Radar data also indicated that birds flying together may have similar wing beats, suggesting taxonomic similarity between birds flying parallel. Possible functions include not only mutual benefits on the ground during migratory stopover (habitat use, avoidance of predators, and social feeding) but also in-flight sharing of information about orientation.