Habitat use and population dynamics of the Azure-Winged Magpie ...

tracked birds. Transmitters have been found to not work as effectively in canyons and dense
vegetation (Beaty & Tomkiewicz, 1990; Samuel & Fuller, 1994) because of radio waves
bouncing and deflecting off reflective surfaces such as canyon walls, tree trunks or foliage.
Furthermore, the problem of signal bounce is greater when transmitting at higher frequencies
(i.e., > 100 MHz; Macdonald & Amlaner, 1980 in Withey et al., 2001), which was the case in my
study (the VHF transmitter was transmitting at > 100MHz). On many occasions I could not detect
signals at all, and sometimes located signals from sites which were impossible for the birds to be
at that time. Often I would rely on listening for calls from the birds (they are quite loud when the
whole colony is calling and can be heard over 200 m away).
Attaching transmitters onto tail feathers appeared to be insecure because of snagging on
branches. The first recovered transmitter was high on a tree branch with a tail feather attached.
The long tail feathers (Fig. 2.1) could have made it more susceptible to snagging on branches.
Also, I may have deployed the transmitters when the birds were undergoing moult although I did
visually check the condition of the radio-tagged birds. Juvenile C. cyanus moult between late July
and mid-November whereas adults commence moult in May (de la Cruz et al., 1992). I have
observed individuals losing feathers when they were caught, but it is unknown if C. cyanus drop
their tail feathers when they are stressed or alarmed. Furthermore, these birds live in social
groups and practise allopreening (pers. obs.). Birds may have deliberately removed transmitters
from their own or others’ tails during preening.
Attached transmitters may have affected the behaviour of the birds, particularly by increasing
preening activity. However, it is unlikely that the transmitters had a significant or long-term
effect on the birds. The weight of the transmitter was about 3% of the bird’s body mass (adult
birds weigh around 80-100g, juveniles about 70g), within the proportional weight limit suggested
by Withey et al. (2001). Furthermore, the transmitters would be shed as the birds moult for
winter, therefore creating no long-term effects on the birds.
An improvement for radio-tracking these birds would be to use a transmitter that is secured to the
bird’s body (e.g. with a harness modified especially for this species). However, this requires
careful design and study before implementing, as some studies have shown that tagging can have
some detrimental effects on birds (Samuel & Fuller, 1994; Withey et al., 2001).VHF radio-
tracking is the most feasible method for this type of observational study and, because of
restrictions in technology there is no way to prevent interference in valleys. Radio-tracking would
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