Migration is inherent to a substantial proportion of Earth’s wildlife. It has been extensively studied in marine species (fish, reptiles, sea mammals) and birds, but less is known about bat migration. Migratory bats travel closer to the surface than birds and are threatened by man-made structures, such as wind turbines. Knowledge about their migration and movement behaviour is of further concern, given the role that bats play as reservoirs for emerging diseases such as the recent COVID-19 .
Migratory animals use various strategies to find their way, including different types of compasses (Sun, polarized light, stars and geomagnetic compass) and cognitive maps . Most bats are nocturnal and so they migrate and hunt at night. This requires both an advanced obstacle avoidance ability and ability for long-distance navigation. For obstacle avoidance and short foraging trips over several hundreds of meters, bats use a natural sonar: they emit ultrasonic waves and sense their reflections from the environment . However, their migration flights are fast and straight and can extend to hundreds of kilometres, suggesting use of other navigational strategies. They have recently been shown to use the Sun’s position at dusk to calibrate their direction , but this does not explain how they are able to navigate during the long night flights. One possibility is geomagnetic navigation, since Earth’s magnetic field provides a global source of information . Bats have magnetite tissue structures and may therefore be able to detect changes in geomagnetic field , but how they use this information for navigation is unknown, which is what this project will explore.
Animal migration research has benefited from advances in tracking technologies that allow collection of high-resolution locational data (e.g. GPS loggers). However, bats are small (some weight less than 10g), which means that even the smallest currently available GPS trackers are still too heavy to place on individuals. One of the main ways to track migratory bats therefore remains radio-tagging (with tags of <1g in weight), where migrating bats are tracked either as they pass through a stationary array of antennas (Motus.org) or are followed on the ground or from an airplane [5,6]. Unlike high resolution GPS tracking however, radio-tagging data are sparse, temporally irregular and often only cover parts of the entire migration path.
A further complication making bat geomagnetic navigation difficult to study is that, to date, no method exists to connect radio-tagged data on bats – movements to real-time data on geomagnetic field. Earth’s magnetic field is a very dynamic system which responds to the influence of the solar wind (a constantly emitted stream of particles from the Sun), that can distort the field during geomagnetic storms. Such distortions may affect animal migratory navigation and have been linked to whale strandings. Knowing what geomagnetic conditions migratory animals experience on their journeys is therefore crucial to improve our understanding of how they use the field for navigation. While we developed a method that fuses real-time satellite geomagnetic data with high-resolution GPS tracking data , fusing geomagnetic data with sparse, irregular and sporadically missing radio-tagging bat data poses a significant data science challenge.