(Fulleret al.1995; Higuchi et al.1996; Meyburg and Meyburg, 1998), including
a pesticide hotspot (Woodbridge et al.in press), most avian radio-tagging has
involved VHF systems. Thus, VHF tagging has revealed the need to create reserves
that conserve unexpectedly large home range areas and to take account of unantici-
pated behavior in conspicuous species, for example, when they forage by night
(Evans et al. 1985). Tagging females is ideal for finding well-concealed nests or
recording survival of precocial broods (Chapter 3), while tiny tags in eggs or on the
young can indicate causes of loss (Willebrand and Marcström 1988). Tags are also
invaluable for recording survival (Chapter 5) and causes of death (Chapter 8) for
species that provide few ring returns due to rarity or remoteness, and especially to
monitor every individual in experiments or reintroduction programs (Chapter 12).
These are all applications where it is important to record the location and status
of animals at particular times. However, if it is simply a matter of recognizing indi-
viduals, cheaper techniques can be appropriate. Conspicuous birds can be marked
with wing-tags or color patterns or even recognized from photographs or DNA in
their young (Evans et al. 1999; Wink et al. 1999) and minimal survival estimated.
In line of sight, even insects can be tracked using tiny harmonic radar transpond-
ers (Riley et al. 1996). Long-distance movement routes can be recorded without
radio-tags, provided that animals can be recaptured to remove tags that store data.
Tags with photo-sensors record the time of dusk and dawn for each date, which
gives an approximation of latitude from the interval between them and of long-
itude from the absolute times (Wilson et al. 1992). These tags have cheaper and
lighter components than GPS tags, but are also much less accurate.
Nevertheless, all these other methods are vulnerable to bias from differential
detection or recovery, whereas data from radio-tagged animals can be recorded
systematically to a specified level of precision. This is important, because biased
information can be seriously misleading. An example is a case where humans were
thought to be slowing re-colonization by raptors. The majority of deaths recorded
with ring returns were from deliberate killing or impact with human artifacts.
However, radio-tracking in the same areas showed that deaths were much less fre-
quently caused by human activities, survival was better than estimated by ringing
and many adults were not breeding (Kenward et al. 2000). A focus on human
impacts, probably because dead birds are found most easily where human activ-
ities cause deaths (Newton 1979), had diverted attention from other factors that
constrain recolonization.
Radio-tagging can also be used to improve other techniques that are less costly,
and therefore often best for the volunteer effort that is growing so important in
conservation. If a bias can be quantified, correction factors may be applied, for
example, for birds missed during transect surveys that are recorded by their radios
144 |Radio-tagging