How Do We Track Eagles?
This study is made possible by some wonderful gadgets designed specially to track birds, called Platform Terminal Transmitters (PTTs). There are many different types of PTT but these ones are of the Solar GPS/Satellite variety. Here's what they look like:
Manufactured by Microwave Telemetry Inc., each PTT is pre-programmed to record a GPS fix (accurate to within 18 m) almost every hour of daylight, taking 15 fixes per day. I adjusted their recording to change as day length
changes throughout the year. This means that in winter, hourly fixes are
recorded from sunrise to sunset, capturing the whole day. But in summer, in
order to 'catch' the eagles roosting, they take fixes from before sunrise to
mid-morning, then after midday to well after sunset.
Along with a GPS fix, the units record other
parameters including altitude, speed and direction. About 80% of altitudes
logged are accurate to within 15 m, and the remainder are mostly accurate to
within 100 m, although we may get occasional outliers which are not 100%
accurate.
Data is recorded and stored in a micro-computer, then
every 3 days the PTT 'contacts' a satellite and transmits the information,
which I access via the internet. Solar panels on the unit's back keep it
charged, a fantastic development since many tracking studies in the past which used battery-powered devices with a limited lifespan.
PTTs are attached to each eagles' back with a
specially designed Teflon harness. The total weight of this outfit is less than
3% of the eagles' body weight, well below the 5% recommended by
international standards. To minimise impacts to our study birds, captive trials
were conducted on eagles to make sure the mounted PTTs did not interfere with or restrict the birds' movements. These trials, and the monitoring of PTT-tagged wild
eagles since June 2013, have shown the devices have negligible impact. The fact both adult eagles satellite-tagged in this study have produced chicks successfully while being tracked (Gidjee in 2013 and Wallu in 2015) provides further
evidence the PTTs are harmless. This study is approved by a State Government
Animal Ethics Committee, who oversee all aspects to ensure compliance with
Wildlife Conservation and Animal Welfare legislation.
