best avoided on water birds in cold regions where they can load with ice. Patagial
tags are only for species that flap their wings slowly, and have been successful on
Californian Condors (Wallace et al.1994).
Tag sensors typically convey information by modulating the length of signal
pulses or the interval between them. Pulse interval modulation is most common,
because the resulting change in pulse rate is easy to detect by ear. Sensors are used
in many ways and can be coded with complex information. Thus, death can be
indicated by a sensor that is simply modulated by body temperature, or by
a change in signal rate that is triggered by absence of motion in an activity sensor
during a pre-set time period. Tags with microprocessors can be coded to insert
extra signal pulses. For example, they can vary the repetition rate of all signals to
indicate temperature and have an extra signal in every tenth interval if there has
been no activity.
Tags close to mass limits must be close to the center of lift, and are therefore
mounted with backpack or lumbar harnesses. Backpack harnesses, in which
a neck loop ahead of the wings is joined under the breast to a body loop behind
the wings (Figure 6.2(c)), are the most commonly used attachment on birds and
have thus provided many records of impacts on behavior or performance.
Assuming the use of safe harness material, such as Teflon ribbon, problems
may largely be due to poor fitting. Problems have been recorded mainly with
precocial birds and raptors, in which growth conditions and sexual dimorphism
146 |Radio-tagging
Table 6.1Limitations of tag attachment methods for birds, from low () to high
() in each category. Risk-assessment risk is from tests in 10 (2), 10 (1),
or no (0) published studies
Technique Handling Skill Rate of Risk: impact Other limitations
time requirement loss on wearer
Leg-mount ^1 Not for very small birds
Tail-mount ^1 Not for very small birds
Necklace ^1 Behavior, icing, crop-shape
Backpack: glue ^1 Detachment rate varies
greatly
body-loops ^2 Needs very careful fitting
wing-loops ^1 Needs very careful design
thigh-loops ^1 Based on 2 assessments
Patagial ^1 Only for large, slow fliers
Nasal-saddle ^0 Only for medium to large
birds
Implant ^1 Poor signal range, invasive