In recent decades an additional study method has become available, namely the day-by-day tracking of radio-tagged birds on their journeys. Initially, aircraft were used to follow the tagged birds (Chapter 8; for various thrushes see Cochran et al. 1967; for raptors see Hunt et al. 1992; for cranes see Kuyt 1992). But since the mid-1980s, however, tracking has been made much easier by use of satellite-based receivers (Chapter 8). The transmitters, called platform transmitter terminals (PTTs), can be tracked individually and automatically by the Argos satellite system. This is a joint French-American venture, originally designed to locate objects on earth such as floating weather stations and buoys. It is based on satellites that continually circle the globe over the poles, and is capable of detecting signals from anywhere on earth, with an accuracy of 150-3000 m depending on the angle of the satellite pass and the quality of the PTT signal. The satellites then transmit the information to a ground station. By measuring the Doppler shift of the emitted signal, the system can measure the exact distance between the transmitter and the satellite, and knowing the parameters of the satellite orbit, the system can also calculate the exact coordinates of the transmitter. The method is expensive, but the data provided are some of the best available on the movements of migratory birds. As the accuracy of each reading is known, the less reliable ones can be discarded if necessary.

The use of PTTs enables large birds to be monitored day by day on their journeys, and to be followed all the way from their breeding grounds to their winter quarters, and back again, regardless of where in the world they move (Chapter 8). With this new method, field work on bird migration is advancing in new directions, providing information on migration routes and progress, stop-over location and durations, flight speeds, wind and weather effects and orientation abilities.

In one of the earliest radio-tracking studies, six male Wandering Albatrosses Diomedea exulans had satellite transmitters attached to them at their nests on the Island of Crozet, midway between South Africa and Antarctica (Jouventin & Weimerskirch 1990). Four of the six birds were followed for about a month as they wandered around the ocean looking for food. One albatross covered a distance of

10 427 km over 27 days. The satellite located it 314 times and its maximum flight velocity between location points was 63 km per hour. Another albatross flew a total of 15 200 km during 33 days. The satellite located it 385 times, and its maximum flight velocity between location points was 81 km per hour. On one day this albatross covered a total of 936 km. For some years, satellite tracking provided the only way to obtain such information, especially in birds that cover such huge distances over the open sea. Additional information gained from the satellite-based tracking of albatrosses and other birds is discussed in Chapters 8 and 17.

Because of their weight, PTTs could until recently be carried safely only by birds weighing at least 1 kg (since reduced to 600 g), which excludes the majority of bird species (ideally the transmitters should not exceed 3-4% of the bird's weight). Most studies have involved swans and geese, raptors, cranes, storks, pelicans and albatrosses. Each transmitter provides information for a period of months or years, until the battery or transmitter fails. It gives immediate information on the daily movements of individuals and, if necessary, also on aspects of the physiology of the wearer and on the conditions of the environment through which the bird passes. Various devices, such as intermittent transmission, can be used to lengthen the life of a battery-powered transmitter, but as yet few battery-operated PTTs have lasted longer than a year. However, from 1995 solar-powered transmitters became available which, unlike battery-powered ones, could in theory last for many years. The current world record holder is a White Stork Ciconia ciconia, so far tracked (with periodic transmitter changes) over a 10-year period on six outward and six return journeys between its nesting place in Germany and its wintering places in different parts of Africa. In some years, this adult female wintered at localities within a few degrees of the equator in East Africa (about 7000 km from its nesting place) and in other years it wintered at places in southern Africa (about

11 000 km from its nesting places) (Chapter 8; Berthold et al. 2004). Only radio-tracking has so far revealed that individual storks have wintered in widely separated places in different years. For other species, satellite-based radio-tracking has also revealed previously unknown breeding or wintering areas (Meyburg et al. 1998, Ueta et al. 2002).

Other kinds of electronic and data-storage tags can now be used to track migrating birds on a worldwide scale. Geolocation systems (GLS) are based on continual measurements by photosensors of the ambient light intensity to record the geographical coordinates (latitude from daylength and longitude from absolute times of dawn and dusk), while global positioning systems (GPS) receive data from satellites for calculating the position of the bird. When first introduced, both systems required the recapture of the birds to recover the tags (attached to leg rings) and accumulated data. This was not difficult with seabirds, for example, returning annually to the same nest sites (e.g. Croxall et al. 2005). However, recent developments to link GLS and GPS to satellite transmitters now allow the data stored on the bird to be retrieved without the need for recapture, and some current solar-powered models can operate over periods exceeding ten years, providing that light levels are sufficient to generate the necessary power. GPS operate through a network of satellites launched by the United States Department of Defense. The bird is equipped with a GPS receiver, which collects locations at pre-set intervals (say every hour) from the GPS satellite network. These data can then be relayed to ground-based Argos processing centres, again at pre-set intervals (say every few days) (Seegar et al. 1999). Because the locations determined in this way are accurate to within 20 m, the method can be used to gain precise assessments of a bird's home range at different seasons, as well as its migration routes as often as required. Used in conjunction with high-power satellite images or aerial photographs of the ground, a bird can be placed accurately within a landscape situated thousands of kilometres from the observer who is seated comfortably at home in front of a PC. Other sensors can be added to a PTT in order to measure other environmental variables, such as altitude of flight or ambient temperature, but they also add weight.

To be of most value, radio-tags and sensors should ideally have no effect on the flight or other behaviour of the wearer. In practice, the capture and handling of birds (for whatever purpose) is likely to cause some temporary stress, and there could be an energy cost to carrying any extra load. However, birds are normally able to compensate for such effects, so that they are not obvious to the observer, and the weight of the attachment can be trivial compared to the weight of a meal or internal body reserves. Attempts to test the effects of tags on the flight performance of birds have either used experimental approaches (such as comparing the flight or energy consumption of tagged and untagged birds flown in wind tunnels, e.g. Holliday et al. 1988), or have compared the migratory timing and progress of radio-tagged birds with those of untagged birds, where this was measured (Beekman et al. 2002, Igual et al. 2005). So far as I am aware, no such study has revealed significant impacts of radio-tags or sensors on the flight performance of migrants, providing these items weighed less than 3-4% of the birds themselves. Michener & Walcott (1966) could detect no differences in the flight performance of homing pigeons Columba livia carrying transmitters weighing as much as 15% of pigeon body weight, although some effects were detected in a later study of homing pigeons by Gessaman & Nagy (1988). Hence, although we can generally assume that the data on migrations obtained by radio-tagging have not been significantly distorted by the effect of the tag on the wearer, this may not necessarily have been true of all such studies (for further discussion see Kenward 2001, Phillips et al. 2003).

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