Tracking butterfly flight in the field from an unmanned aerial vehicle (UAV): a methodological proof of principle
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Abstract
Tracking and understanding the movements of animals in the wild is a fast-growing area of research, known as movement ecology . However, tracking small animals such as flying insects, which cannot easily carry an electronic tag, remains challenging as existing field methods are costly either in terms of equipment or tracking effort (e.g. VHF radio-tracking, scanning harmonic radar). Here we attempted to record the movements of free-flying butterflies from an unmanned aerial vehicle (UAV), maintaining a static position in the sky and recording video vertically downwards. With an appropriate flight height and image filtering algorithm, we recorded 166 flight tracks of Pieris butterflies ( P. brassicae and P. rapae ), with a median tracking length of 40 m (median flight duration 13 s), and a high temporal resolution of 30 positions per second. Average flight direction varied significantly over the course of the flying season, from a northward azimuth in June and early July, to a southward azimuth in September, congruent with a trans-generational migratory behaviour that has previously been documented by field observations or experiments in flight cages. In addition, UAV imagery unlocks the possibility to measure high-resolution flight movement patterns (e.g. path tortuosity and transverse oscillations), which will possibly help understand perceptual and locomotor mechanisms underlying spatial behaviour. We explore the technical details associated with UAV tracking methodology, and discuss its limitations, in particular the biases associated with a 2D projection of 3D flight movements, the limited spatial scale, and the difficulty to distinguish between visually similar species, such as P. brassicae and P. rapae .
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Understanding how animals move within and across landscapes is fundamental to behavioural ecology, conservation biology, and movement ecology. Tracking movement provides insights into migration and dispersal patterns, habitat preferences, intra- and interspecific interactions, etc. For long, movement recording was limited to indirect methods, such as Capture-Mark-Recapture. Despite being at the basis of an incredible amount of knowledge and developments in ecology, these methods do not inform on the movement path itself, just its beginning and end. Tracking individuals during their movement was really needed.
Over the years, researchers have developed a range of tracking methodologies, with technological innovations continually improving precision and efficiency (Trappes, 2023). While tracking large terrestrial and marine animals and …
Understanding how animals move within and across landscapes is fundamental to behavioural ecology, conservation biology, and movement ecology. Tracking movement provides insights into migration and dispersal patterns, habitat preferences, intra- and interspecific interactions, etc. For long, movement recording was limited to indirect methods, such as Capture-Mark-Recapture. Despite being at the basis of an incredible amount of knowledge and developments in ecology, these methods do not inform on the movement path itself, just its beginning and end. Tracking individuals during their movement was really needed.
Over the years, researchers have developed a range of tracking methodologies, with technological innovations continually improving precision and efficiency (Trappes, 2023). While tracking large terrestrial and marine animals and birds is now well-established using GPS telemetry and biologging, monitoring small flying insects remains a significant challenge due to their size, erratic flight patterns, and sensitivity to environmental disturbances. It is especially the case for butterflies due to their lightweight bodies and relative low flight power. Given the role butterflies play as model organisms in diverse areas of ecology, research to allow tracking their movement path is of prime interest.
I remember the many hours I spent, in a time (early 2000s) GPS technology was still quite imprecise, following butterflies for a distance, placing sticks at turning points and reconstructing afterwards the movement path by triangulating the distances of sticks to know location marks (Schtickzelle et al., 2007). It was quite effective but prohibitive in terms of resources. Later came GPS devices precise enough for an individual to run in the footsteps of a butterfly to record its path. Still, methods have been highly desirable that could track butterflies with some level of automation and from a distance. Experiments were performed with harmonic radar (attaching a passive transponder that reflects radar signals; Cant et al., 2005) but were never largely adopted given they required acquiring and positioning costly and heavy equipment and maintaining at all time a direct line of sight with the tracked butterfly.
Here comes this pioneering study by de Margerie and Monmasson (Margerie & Monmasson, 2025) who introduce an innovative approach using a consumer-level commercial drone to track butterfly flight, offering a promising solution for long-duration, high-resolution flight trajectory analysis in natural habitats. Their study is a proof of principle that a drone, hovering in a fixed position, can be used as a flying platform to capture high-resolution vertical imagery to precisely record butterfly flight movements. Images are then analysed to reconstruct the flight path, a point on which they developed innovative approaches in the study.
The study therefore represents a significant leap forward in butterfly flight tracking methodology, with technology that many labs could acquire and operate. Further research is needed to alleviate some of the current limitations before large-scale adoption to track butterfly movements in the field is within reach: e.g. the need for a very high contrast between the butterfly and the vegetation above which it flies (here white Pieris butterflies over a relatively homogeneous green crop field were filmed), the limits in spatiotemporal scale due to the fixed drone position and its short battery life, and some difficulties in image processing to reconstruct movement paths, in particular when several individuals would cross paths. Considering the fast progress in both the drone technology and image analysis techniques, such progress could however come faster than we might anticipate.
References
Cant E. T., Smith A. D., Reynolds D. R. & Osborne J. L. (2005). Tracking butterfly flight paths across the landscape with harmonic radar. Proceedings of the Royal Society of London B 272, 785–790. https://doi.org/10.1098/rspb.2004.3002
de Margerie E. & Monmasson K. (2025) Tracking butterfly flight in the field from an unmanned aerial vehicle (UAV): a methodological proof of principle. bioRxiv, ver.5 peer-reviewed and recommended by PCI Ecology https://doi.org/10.1101/2024.07.17.603869
Schtickzelle N., Joiris A., Van Dyck H., & Baguette M. (2007). Quantitative analysis of changes in movement behaviour within and outside habitat in a specialist butterfly. BMC Evolutionary Biology 7, 4. https://doi.org/10.1186/1471-2148-7-4
Trappes R. (2023). How tracking technology is transforming animal ecology: Epistemic values, interdisciplinarity, and technology-driven scientific change. Synthese 201, 128. https://doi.org/10.1007/s11229-023-04122-5
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