Bird Ecology and Conservation A Handbook of Techniques

frequency, with signal coding to identify individuals. The high power require-
ment raises tag mass to at least 15 g, and tracking accuracy is (at best) several hun-
dred meters. However, locations can be recorded automatically at 10-m accuracy
by tags that receive UHF signals from the Navstar global positioning system
(GPS). Minimal tags of 30 g can store the locations, but must be recovered to
extract the data. Tags that can relay GPS data to satellites or mobile phone systems
are too large for most birds.
Although animals have been radio-tagged for more than 40 years, the tech-
niques are seldom used to their full potential. One problem is the considerable
knowledge, planning, and skill required to make best use of the tagging. Another
is the need to avoid adverse effects of tagging on the welfare of animals and the
quality of information. Great care is needed in obtaining suitable equipment, field
skills, and data. This chapter gives pointers for success, but wider reading is essen-
tial. Recent reviews are Fuller et al. (in press), or more extensively Kenward
(2001), with Millspaugh and Marzluff (2001) for analysis techniques.

6.2 Choice of techniques

6.2.1Constraints on radio tagging

Radio tagging is practical only if animals can carry large enough tags for long
enough to give the required data. Tags at 10% of body-mass have been used, but
any above 2–3% are liable to reduce survival, especially on birds. Except in the
smallest tags, mass is constrained mainly by the power supply, which limits the life
and power of the tag. Figure 6.1 shows typical transmission lives available for
VHF tags of increasing mass. The smallest tags rely on single silver-oxide cells,

142 |Radio-tagging

0

5

10

50

100

500

1000

0 0.5 1 5 10 50 100

Mass of tags in grams (log scale)

Life of tags in days (log scale)

Power cell: lithium

Power cell: silver

Fig. 6.1Tag transmission life expected from bird tags of different mass.