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

Thomas B. Johnson; Douglas Gochfeld; Melissa C. Roach; Gautam A. Apte; Ross Gallardy; Samuel C. Wilson; Andrew Marden; Daniel J. Irons; Andrew Farnsworth; Cameron L. Rutt

doi:10.1093/ornithology/ukaf074

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 similar to 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. Nocturnal migration has puzzled ornithologists for centuries, largely because we cannot observe migratory birds at night. Currently, the only way to identify these migrants is if they emit (audible) species-specific call notes as they pass overhead. We used thermal imaging optics to locate nocturnal migrants, which we then illuminated, photographed, and identified to species. This technique enabled us to identify thousands of nocturnal migrants, the vast majority of which were silent or inaudible (aside from a few groups of vocal birds). We recommend pairing this observational method with acoustic monitoring, thereby allowing us to collect higher-resolution local data about individual species. La migraci & oacute;n nocturna ha fascinado y desconcertado a los ornit & oacute;logos durante siglos. Actualmente, mediante tecnolog & iacute;as que van desde el radar meteorol & oacute;gico de vigilancia hasta geolocalizadores multi-sensoriales, podemos estudiar tanto la magnitud continental de estos vuelos como los detalles intrincados de los viajes individuales. Sin embargo, a & uacute;n carecemos de un modo para observar directamente a los migrantes, lo que limita nuestra capacidad para comprender la migraci & oacute;n a nivel individual y de especie. Combinando avances recientes en & oacute;ptica de imagen t & eacute;rmica y fotograf & iacute;a digital, detectamos, iluminamos e identificamos aves migratorias nocturnas a baja altura (0-300 m). Describimos los aportes que ofrece esta t & eacute;cnica utilizando similar to 200 horas de observaci & oacute;n durante 3 oto & ntilde;os (2020-2022) en Cape May, Nueva Jersey, EE UU. Nuestro enfoque novedoso nos permiti & oacute; observar el comportamiento migratorio nocturno, identificar miles de aves en migraci & oacute;n nocturna, y comenzar a cuantificar el paso de individuos silenciosos y de especies silenciosas. Excepto por unas pocas familias altamente vocales-principalmente zorzales, chingolos y reinitas-la gran mayor & iacute;a de las aves migratorias nocturnas pas & oacute; en silencio o de manera inaudible sobre Cape May. Con monitoreo exclusivamente ac & uacute;stico, todos estos individuos y casi dos tercios de las familias (62%) habr & iacute;an pasado desapercibidos. En nuestra muestra, el comportamiento social se restringi & oacute; a aves acu & aacute;ticas, y no se observaron bandadas compactas de paserinos m & aacute;s de una hora despu & eacute;s del crep & uacute;sculo, incluso en especies gregarias diurnas. Debido a que esta t & eacute;cnica intensiva en trabajo a & uacute;n no puede automatizarse, no se adapta al monitoreo de la migraci & oacute;n a gran escala. No obstante, cuando se integra con el monitoreo ac & uacute;stico, la resoluci & oacute;n local de escala fina de estos datos puede complementar y validar tanto los estudios con radar como los ac & uacute;sticos. Estas tecnolog & iacute;as port & aacute;tiles de f & aacute;cil despliegue son especialmente & uacute;tiles cuando los migrantes nocturnos vuelan cerca del suelo y pueden ayudar a visualizar c & oacute;mo interact & uacute;an las aves con aerogeneradores y otras estructuras altas. Al transformar lo invisible en visible, este m & eacute;todo proporciona una nueva ventana para el estudio de la migraci & oacute;n nocturna.

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

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

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