1. Fine-scale tracking of animal movement is important to understand the proximate mechanisms of animal behaviour. The reverse-GPS system-ATLAS-uses inexpensive (~€25), lightweight (<1 g) and low-power (~0.4 mJ/transmission) tags. Six systems are now operational worldwide and have successfully tracked over 50 species in various landscape types. The growing use of ATLAS to track animal movement motivates further refinement of best-practice application and an assessment of its accuracy.2. Here, we test the accuracy and precision of the largest ATLAS system, located in the Dutch Wadden Sea, using concurrent GPS measurements as a reference. This large-scale ATLAS system consists of 26 receivers and covers 1,326 km 2 of intertidal region, with almost no physical obstacles for radio signals, providing a useful baseline for other systems. We compared ATLAS and GPS location estimates for a route (mobile test) and 16 fixed locations (stationary test) on the Griend mudflat. Precision was estimated using standard deviation during the stationary tests. We also give examples of tracked red knots Calidris canutus islandica to illustrate the use of the system in tracking small shorebirds (~120 g).3. ATLAS-derived location estimates differed from GPS by a median of 4.2 m (stationary test) and 5.7 m (mobile test). Signals that were collected by more receiver stations were more accurate, although even three-receiver localisations were comparable with GPS localisations (~10 m difference). Receivers that detected 90% of the 1 Hz transmissions from our test tag were within 5 km of their furthest detection but height of both receiver and tag seemed to influence detection distance. The test tag (1 Hz) had a fix rate of >90% at 15 of 16 stationary sites. Tags on birds (1/6 Hz) on the Griend mudflat had a mean fix rate of 51%, yielding an average sampling rate of 0.085 Hz. Fix rates were higher in more central parts of the receiver array.4. ATLAS provides accurate, regional-scale tracking with which hundreds of relatively small-bodied species can be tracked simultaneously for long periods of time. Future ATLAS users should consider the height of receivers, their | 1991 Methods in Ecology and Evoluঞon BEARDSWORTH et al.

Movement is a fundamental aspect of life and tracking wild animals under natural conditions has become central to animal behaviour, ecology, and conservation science. Data from tracked animals have provided novel scientific insights on extreme migratory journeys, mechanisms of navigation, space use, and early warning signals of environmental change. Studying movement is therefore important, particularly in systems that may be vulnerable to anthropogenic effects. Technological advancements, and chiefly the development of GPS tags, have enabled animal tracking at high spatiotemporal resolution, yet trade-offs between cost, sampling frequency, tag weight and data retrieval limit the use of GPS tags to relatively few individuals and large species. A new ‘reverse-GPS’ wildlife tracking system, ATLAS, employs an array of receiver stations that detect and localise small (∼0.6 g without battery), low-cost (∼25 euro) tags by calculating differences in the arrival time of tag signal at minimally three stations. In this study, we introduce the Wadden Sea ATLAS system (WATLAS), implemented in the Dutch Wadden Sea, the Netherland’s only natural UNESCO World Heritage Site, yet affected by a suite of anthropogenic activities, such as commercial fishing, mining, shipping, as well as sea level rise. From July 2017 to July 2021, we tracked 821 red knots, 182 sanderlings, 33 bar-tailed godwits, and 6 common terns. With four examples, we illustrate how WATLAS opens-up possibilities for studying space-use, among-individual variation in movement, and intra-specific interactions, and inter-specific (community) space use in the wild. We additionally argue that WATLAS could provide a tool for impact assessment, and thus aid nature conservation and management of the globally important Wadden Sea ecosystem.

We describe the design and implementation of Vildehaye, a family of versatile, widely-applicable, and field-proven tags for wildlife sensing and radio tracking. The family includes 6 distinct hardware designs for tags, 3 add-on boards, a programming adapter, and base stations; modular firmware for tags and base stations (both standalone low-power embedded base stations and base stations tethered to a computer running Linux or Windows); and desktop software for programming and configuring tags, monitoring tags, and downloading and processing sensor data. The tags are versatile: they support multiple packet formats, data rates, and frequency bands; they can be configured for minimum mass (down to less than 1 g), making them applicable to a wide range of flying and terrestrial animals, or for inclusion of important sensors and large memories; they can transmit packets compatible with time-ofarrival transmitter-localization systems, tag identification and state packets, and they can reliably upload sensor data through their radio link. The system has been designed, upgraded, and maintained as an academic research project, but it has been extensively used by 5 different groups of ecologists in 4 countries over a period of 5 years. More than 7100 tags have been produced and most of these have been deployed. Production used 41 manufacturing runs. The tags have been used in studies that so far resulted in 9 scientific publications in ecology (including in Science). The paper describes innovative design aspects of Vildehaye, field-use experiences, and lessons from the design, implementation, and maintenance of the system. Both the hardware and software of the system are open.

Tracking animal movement is important for understanding how animals interact with their (changing) environment, and crucial for predicting and explaining how animals are affected by anthropogenic activities. The Wadden Sea is a UNESCO World Heritage Site and a region of global importance for millions of shorebirds. Due to climate change and anthropogenic activity, understanding and predicting movement and space-use in areas like the Wadden Sea is increasingly important. Monitoring and predicting animal movement, however, requires high-resolution tracking of many individuals. While high-resolution tracking has been made possible through GPS, trade-offs between tag weight and battery life limit its use to larger species. Here, we introduce WATLAS (the Wadden Sea deployment of the ATLAS tracking system) capable of monitoring the movements of hundreds of (small) birds simultaneously in the Dutch Wadden Sea. WATLAS employs an array of receiver stations that can detect and localize small, low-cost tags at fine spatial (metres) and temporal resolution (seconds). From 2017 to 2021, we tracked red knots, sanderlings, bar-tailed godwits, and common terns. We use parts of these data to give four use-cases revealing its performance and demonstrating how WATLAS can be used to study numerous aspects of animal behaviour, such as, space-use (both intra- and inter-specific), among-individual variation, and social networks across levels of organization: from individuals, to species, to populations, and even communities. After describing the WATLAS system, we first illustrate space-use of red knots across the study area and how the tidal environment affects their movement. Secondly, we show large among-individual differences in distances travelled per day, and thirdly illustrate how high-throughput WATLAS data allows calculating a proximity-based social network. Finally, we demonstrate that using WATLAS to monitor multiple species can reveal differential space use. For example, despite sanderlings and red knots roosting together, they foraged in different areas of the mudflats. The high-resolution tracking data collected by WATLAS offers many possibilities for research into the drivers of bird movement in the Wadden Sea. WATLAS could provide a tool for impact assessment, and thus aid nature conservation and management of the globally important Wadden Sea ecosystem.