[email protected]) and other software (Chapter 5), and similar tech-
niques are suitable for recording survival of nests of radio-tagged birds, and hence
productivity (Chapter 3). For more details, and estimation of animal densities
with radio-tags, see Kenward (2001) and Millspaugh and Marzluff (2001).
Pilot work is advisable for studies of density, dispersal, survival, and breeding,
not least to check that tagging does not affect performance (Murray and Fuller
2000). There is something of a “Catch 22” in such tests, because power to detect
differences is low in tests with small samples. This has two implications. One is
that with a reporting rate of 5–10% (typical for ringing), it needs 10–20 times as
many marked birds as radio-tags for estimates with comparable confidence limits.
The second implication is that, even with the high reporting rate of radio-tags,
well over a hundred tags may be required to detect small differences in demo-
graphic rates. Therefore, on the one hand it can be difficult to detect not only
small impacts of radio tags but also the small differences in demographic rates that
often occur between stable and declining populations. On the other hand, if there
is a high re-sighting rate for colored markers, the cost of using radio tags merely to
estimate annual survival rates would not be justified.
In order to exploit the main advantages of radio-tags over other demographic
techniques, other preliminary tests will be needed. How frequently should ani-
mals be checked to reveal the seasonal details of survival and dispersal? When may
home range characteristics correlate most strongly with demographic factors?
What is the best time of day to check for presence or survival of tagged animals?
It can be at night, when living birds perch high enough to be detected at optimal
distances. How frequently should survival be checked if corpses are to be fit for
cause-of-death analysis (Chapter 8) at different times of year? If cause-of-death
is unimportant, thrice-yearly checks may suffice. One check can identify a
wintering area, as a home-range-sized circle round roosts. A second check during
incubation makes it easy to find nests, at least for females of single-brood species,
and a third check during brood-rearing identifies the successful breeders (and
prepares for tagging the next generation).
6.5 The future
The development of radio-tagging has depended on other technologies. Impro-
vements have depended mostly on consumer or military requirements. The
smallest raptor tags were provided initially for falconry, and it was these raptor
enthusiasts who obtained US military support to reduce Doppler tags to a size for
tracking migratory peregrine falcons. GPS units at wrist watch size have been
developed for consumer-electronics. Miniaturized sensors (including vision) and
156 |Radio-tagging