White Storks

Technical information

Since about 1991, satellite tracking has been used as a new tool to study the migration of the White Stork through Europe and Africa. Most of the early projects involved storks following the western route out of western Europe, through Spain, across the Straits of Gibralar, into Africa. Storks following the eastern route were tagged in the Czech Republic, Germany and Poland.

The largest project to date was that of the Vogelwarte Radolfzell (Max Planck Institute), in collaboration with the SPNI (Society for the Protection of Nature in Israel). In this project, 75 White Storks have been tagged with satellite transmitters; 54 were tagged in Germany, 11 in Poland and 10 in Israel. As the satellite tracking technology has improved, a variety of satellite tags has been used. The first tags weighed 90 g, and were battery powered. The weight has decreased steadily, and a solar-powered tag weighing 35 g has been the preferred system since 1997. A White Stork weighs about 3-4 kg, so that the tag adds about 1% to the weight; this is less than the percentage weight of the average pair of shoes on a person.

The transmitter is attached to the stork's back using 3.5 mm diameter nylon string. Careful observations made of migrating storks show that the progress and behaviour of the tagged birds does not differ from those of untagged birds. The solar transmitter should, in theory, work for several years.

The transmitters emit an impulse signal every 53 seconds for 300 msecs. Each signal contains information about the transmitter number, the battery level, temperature and activity. The signal is captured 850 km above the earth by NASA satellites of the TIROS series. These satellites circle the earth every 102 minutes, and make just over 14 orbits round the earth per day. A bird is "visible" to a satellite as it passes overhead for a maximum of 15 minutes. With 53 seconds between signals, a satellite can capture a maximum of 17 impulses during a single orbit.

When the satellite passes over the ARGOS ground-station in Toulouse, France, the information is downloaded. The ARGOS computers locate the position on the earth from which the impulses were made. It does this using the "Doppler" effect - because the satellite is moving, the impulses do not reach it at exactly the 53 second intervals at which they were made. The quality of the fixes depends on the battery power, weather conditions and the position of the antennae on the bird relative to the orbiting satellite. ARGOS places all the information on the company website. Access to the information is controlled by a system of passwords, so researchers only have access to their own satellite tags.

The activity signal that comes from the bird is a count of the total number of movements a bird has made. When a bird dies, this counter suddenly stops increasing, and the same number is transmitted for days or even weeks. In practice, it is fairly easy to distinguish situations in which the bird has died from those in which the transmitter has failed due to technical problems.