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

Nocturnal migration has fascinated and puzzled ornithologists for
centuries. Today, using technologies from weather surveillance radar to
multi-sensor 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 three 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.