Telemetry for migratory bats – a feasibility study

In recent years, research into the occurrence of bats at the Dutch North Sea has shown that there is regular seasonal migration over sea. However, so far, little is known about their migration ecology, the fatality risks at offshore wind turbines, and the number of individuals migrating over sea. Since the Dutch government wants to boost the further development of wind energy production in the southern North Sea, the Ministry of Economic Affairs commissioned to Rijkswaterstaat a Wind at Sea Ecological Programme (in Dutch: Wozep). This study, as part of the Wozep-project Behaviour and Collision Risk of Bats (Bats_2), investigates how telemetry can be applied to gain insight into migratory movements of bats over land and over sea and individual bat behaviour near and in offshore wind farms. To find out whether it is wise to continue and further develop telemetry research in the context of the Wozep programme, we first identified potential telemetry methods for small bats based on a desk study and selected the most promising method for the application of telemetry. Members of the team attended an international workshop on telemetry in Lund (Sweden) to gather practical technical knowledge, gain insight in data-management standards, and increase their international network. Several field tests were conducted to test the equipment and explore the possibilities of tracking. Finally, suitable locations for bat trapping in bat boxes and for the use of a Heligoland trap were identified. There are several options to track bats with radio telemetry during their migration at the coast and over sea. However, for long-term monitoring of multiple individuals, establishing a grid of stationary receivers is the only feasible option. Eight field tests were carried out to test the performance of the technical infrastructure. Each of the tests was unique and set up to test the signal strength/detection range for a certain type of antenna or a certain transmitter/receiver constellation. We showed that detections over at least 6 km are possible, and likely more than 10 km can be achieved. Precondition is that the receiving stations must be installed at high structures (lighthouses, buildings, masts) or hills, installing them a few meters off ground level will lead to detection ranges just over one km. Furthermore we explored the possibilities of calculating movement tracks with a setup of several receivers using different estimation methods. These experiments indicated that a relatively high accuracy (of c. 100 m) can be reached estimating the location when signals of different receiving stations are combined. It is likely that the accuracy can be improved by estimating the bearings of the received signals based on the signal strength of different antennas. Even further improvement seems possible by assessing the (likely) flight route with a state-space model. There are plenty of locations with bat boxes, especially in the province of Noord-Holland, where potentially hundreds of bats can be captured during migration stopovers, though it is not known how many of these are likely to be migrants. In addition, we identified four locations where actively migrating bats can potentially be captured with an Heligoland trap. In conclusion, we are confident that telemetry can be successfully applied to study migratory movements of bats over land and over sea and individual bat behaviour near and in offshore wind farms. Joining a wildlife tracking system like Motus (Canada) in order to enlarge the data collection, is highly recommended. Motus-members can get detections from both

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