6. Radio-tagging
Robert Kenward
6.1 Introduction
Radio-tagging has been used for more than 40 years to reveal where animals are
(location), how they are (physiology, alive or dead), and what they are doing
(behavior). Whereas academic studies can be based on choosing a common and
conspicuous species that is easy to mark and watch, conservation is usually
focused on species that are rare, elusive, or living in remote areas. Radio tags are
often the only practical way to record the basic requirements of such species, such
as the areas and habitats they use for foraging and sheltering, or how individuals
interact for mating or when transmitting disease. Radio-tags can reveal the fate of
every valuable animal in a release scheme, and small samples of rare animals pro-
vide details of conditions along migration routes. Radio-tagging is therefore an
essential tool in conservation ecology.
The development of radio-tags was made possible by invention of the trans-
istor, leading to microelectronics and tiny power supplies that now permit tags of
300 mg. These Very High Frequency (VHF) tags can be detected at hundreds of
meters, or a few kilometers if antennas are long enough, by converting brief signal
pulses to audible beeps in very sensitive receivers. By tuning to different tag fre-
quencies, individual animals can be tracked on foot or from vehicles (including
aircraft) and located by triangulation. Accuracy is typically 10–100 m. Although
the smallest VHF tags function for only a week or so, 2 g tags can transmit signals
for months. At 20 g, tags can transmit VHF signals detectable at 1–100 km for
2–3 years.
A second type of radio-tag can record locations automatically anywhere on the
globe. Although the ultra high frequencies (UHF) of these tags penetrate vegeta-
tion poorly, high power is used to communicate with satellites. In the ARGOS
system, Doppler principles are applied to locate tags that transmit one very stable