Seeing in the dark: Using thermal imaging to directly observe nocturnal migration

Abstract Nocturnal migration has fascinated and puzzled ornithologists for centuries. Today, using technologies from weather surveillance radar to multisensor geolocators, we can study the continental magnitude of these flights and the intricate details of individual journeys. Yet, we still lack a way to directly observe migrants, hampering our ability to understand migration at the individual and species level. Combining recent advances in thermal-imaging optics and digital photography, we detected, illuminated, and identified nocturnally migrating birds at low altitudes (0–300 m). We describe insights this technique has to offer using ∼200 observation hours during 3 autumns (2020–2022) at Cape May, New Jersey, USA. Our novel approach allowed us to observe migratory behavior at night, identify thousands of nocturnally migrating birds, and begin quantifying the passage of silent individuals and silent species. Aside from a few highly vocal families—primarily thrushes, sparrows, and wood-warblers—the vast majority of nocturnally migrating birds passed silently or inaudibly over Cape May. With acoustic-only monitoring, all of these individuals and nearly two-thirds of families (62%) would have been missed. In our sample, social behavior was restricted to waterbirds, and no compact flocks of passerines were observed more than an hour after dusk, even for diurnally gregarious species. Because this labor-intensive technique cannot yet be automated, it does not lend itself to broad-scale migration monitoring. However, when integrated with acoustic monitoring, the local, fine-scale resolution of these data can complement and ground-truth both radar and acoustic studies. These portable, easily deployable technologies are particularly well-suited when nocturnal migrants are nearest the ground and may help to visualize how birds interact with wind turbines and other tall structures. By transforming the invisible into the visible, this method provides a new window into the study of nocturnal migration.