which give only 1.5 V and therefore require two cells in series to reach the same
power as tags with lithium cells, at 3.0–3.7 V. Although solar cells can be used to
boost energy of rechargeable cells, the tags must be exposed to light (which may
result in increased drag) and eventually lose cell efficiency. Tags can also be
switched by microprocessors, to save power by transmitting only at desired
seasons or times of day. However, recent increases in tag efficiency can give similar
lives from good quality primary cells more reliably than with either micro-
processors, which can be vulnerable to static, or solar cells.
An important constraint on detection range is antenna efficiency. With a wave-
length of 2 m at 150 MHz (a common frequency for radio tags in Europe), an
“ideal length” quarter-wave antenna is 500 mm. This length decreases in pro-
portion to increasing frequency. A quarter wavelength is thus 434 mm at
173 MHz, as permitted in the United Kingdom, and 347 mm at 216 MHz, as
used in some other countries. In practise, antennas are usually shorter than the
ideal, with ground-plane or antenna loading systems to compensate. Efficiency
reduces as an inverse power function of length (though not appreciably down to
about 70% of the ideal) and also falls slightly with decreased width. As a result, a
transmitter that can be detected in line of sight at 40 km when coupled to an
antenna with efficient length and robust width (e.g. on a large bird) may not be
detectable much beyond 1 km with a 100 mm antenna on a small bird. Extra life
can be secured by decreasing the rate and duration of signal pulses, without appre-
ciable reduction in detection range from typical receivers until pulses are below
10 ms. See http://www.biotrack.co.uk for software to help select tags with optimal mass,
life, and range.
Another important consideration is cost. Automated tracking is most expen-
sive. A budget of US$5000 buys 1–2 tags for tracking by satellites with Doppler
or GPS-relay systems, about five tags for recovery with GPS data, or 20 of
the VHF tags with a receiver for manual tracking. Doppler-system tags are at their
best for migration studies (Chapter 7), for example, to identify important staging
or wintering sites. Although the tags are expensive, a small number on a rare
species can provide basic data with more detail and immediacy than ring (band)
returns. GPS tags are more accurate than Doppler-system tags for studies of habi-
tat and other resources (Chapter 11) and can be more cost-effective than VHF
tags for birds in dangerous or remote areas. However, GPS-relay tags may remain
suitable only for species with body-mass well above 1 kg. Ease of recapture will
continue to constrain the use of storage-only GPS tags.
6.2.2Applications and advantages
Despite some exciting tracking by satellite that has revealed unanticipated
bird movements ( Jouventin and Weimerskirch 1990) and migration events
Choice of techniques| 143