Abstract The GPS recorder consists of a GPS receiver board, a logging facility, an antenna, a power supply, a DC-DC converter and a casing. Currently, it has a weight of 33 g. The recorder works reliably with a sampling rate of 1/s and with an operation time of about 3 h, providing time-indexed data on geographic positions and ground speed. The data are downloaded when the animal is recaptured. Prototypes were tested on homing pigeons. The records of complete flight paths with surprising details illustrate the potential of this new method that can be used on a variety of medium-sized and large vertebrates.
Considerable efforts have been made to track birds in order to find out more about their migrating, foraging and homing behaviour. The principal methods for recording flight paths of birds have been radio tracking (e.g. Kenward 1987) and tracking birds with the satellite positioning system ARGOS (Fuller et al. 1995; Jouventin and Weimerskirch 1990). ARGOS so far offers a low resolution in time and space and is thus unsuitable for tracking animals that move over distances of several 10 km in a few hours. Radio tracking has a limited range only and requires a lot of manpower and technical expenditure for measurements (Kenward 1987; Wolff et al. 1997).
Pigeons have also been tracked by aircraft (Griffin 1952; Michener and Walcott 1967; Wagner 1974), which is even more time consuming and expensive. Bramanti and Dall-Antonia (1988) developed a compass recorder which records the compass bearings of the longitudinal axis of the bird's body. However, flight routes have to be reconstructed by assuming an average flight speed and without knowing whether the bird was flying or resting, resulting merely in a more or less accurate approximation of the flight path. In contrast to the methods mentioned above, GPS, the Global Positioning System, offers a combination of advantages: (1) fixes can be taken once a second with a high accuracy; 95% have a positional error less than B50 m and 99% an error of less than B150 m; (2) there are effectively no limits in spatial range.
We have recently built a miniature flight recorder based on GPS that is light enough to be fitted to large and medium-sized birds with a weight of more than 0.5 kg. Our present prototype weighs 33 g. The device comprises a hybrid GPS receiver with integral storage, an additional microprocessor, a lithium battery and a DC-DC converter. It collects time-indexed records of latitude, longitude and ground speed at a rate of up to one per second. The information on ground speed can be used to determine where and how long a bird interrupted its flight. The recorder can store approximately 90,000 positions. With the presently used battery and when taking fixes at the rate indicated, it has an operational time of 3 h. To download the data, the bird has to be recaptured.
We tested the recorder at Obermölen, about 30 km north of Frankfurt am Main. The data showed complete flight paths and details of the behaviour en route, for instance initial loops flown immediately after release, breaks and different routes chosen around a hill near the release site (Fig. 1). The routes chosen for homing showed a great variability with deviations to the east of the direct route (Fig. 2), which is in accordance with tendencies of visual vanishing bearings observed in previous experiments at the same site (see Wiltschko 1993).
Fig. 1 Map of topography of the release site at Obermörlen with the initial flight paths of four pigeons carrying GPS devices; A is the release site (Auflaßort in German). Map after a topographical map 1: 50,000, reprint with permission of the Hessisches Landesvermessungsamt, No.: 99-3-119)
Fig. 2 Examples of homing flight trajectories. The direct route from the release site to the home loft at Frankfurt am Main is marked by a dashed line
These first results illustrate the potential of the GPSbased technique to yield a wealth of data on the behaviour of animals en route. Our new flight recorder will help to close a gap in biotelemetry, so that now a large number of new species can be tracked over medium distances, if they can be recaptured. At present, our GPS recorder is suitable for animals with a weight of above 0.5 kg, but we hope to further decrease the weight in the future. The GPS recorder should find a wide range of applications in behavioural and conservational studies.
This project has been supported by the Deutsche Forschungsgemeinschaft DFG (grant to W.W.) and the Arbeitskreis Telemetrie (grant to K.v.H.). We thank S. Werffeli and C. Buergi, ETH Zuerich, Switzerland, and J. Riley and A. Edwards, Great Malvern, England, for useful comments, and the companies Murata, Bosch, Varta and Eagle Picher for supplying samples at no cost. This study was performed in conformity with the German law for the protection of animals.
Note added in proof. Since 1 May 2000, the U.S. Department of Defense has discontinued the selective availability (S/A) which was a deliberate degradation of the accuracy of GPS. This improves the recorder's accuracy by a factor of 10 so that we can expect an error of only B5 m in 95% of the fixes.
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K. von Hünerbein, H.-J. Hamann, W. Wiltschko
Fachbereich Biologie, Zoologie, J.W.-Goethe Universität
Frankfurt, Siesmayerstrasse 70, 60323 Frankfurt am Main,
Rüter EPV-Systeme GmbH, Sandtrift 87, 32425 Minden,