Flight altitude selection increases orientation performance in high-flying nocturnal insect migrants

Many insects migrate at high altitudes where they utilize fast-flowing airstreams for long-distance transport. Nocturnal insect migrants typically exhibit a strongly unimodal distribution of flight headings (a phenomenon termed ‘common orientation’), and the mean heading is often aligned downwind. In addition, these nocturnal migrants are sometimes concentrated into shallow altitudinal zones (termed ‘layers’). The mechanism by which widely separated insects select and maintain common flight headings had until recently eluded explanation, but recent theoretical advances have shown that atmospheric turbulence might enable insects to perceive the downwind direction and orient accordingly. This theory predicts that common orientation downwind should be: (1) widespread in nocturnal insect migrants; (2) facilitated when insects are concentrated into layers; and (3) more pronounced in larger insects. We tested these ideas using radar observations of 647 independent nocturnal migration events, and found strong support for all three predictions: (1) common orientation occurred in 75–90% of events; (2) common orientation was more frequent, had significantly less scatter and was significantly closer to downwind when insects migrated in layers; and (3) large insects exhibited significantly tighter orientation than ‘medium-sized’ insects. Our results provide robust evidence that wind-related common orientation is mediated by detection of atmospheric turbulence.