Tags using GPS technology are still too large and power hungry to be suitable for tracking small flying animals such as many birds. The latest Platform Transmitter Terminals (PTTs) operating using more traditional technology are, however, small enough to allow birds down to almost 100g to be tracked. For the Cuckoo tracking project, we have used PTTs weighing just under 5g that have been supplied by Microwave Telemetry Inc. . These operate via the Argos satellite system. Numerous recent technological advances have enabled the company to miniaturise their PTTs – the device we are using has a solar panel that allows a smaller battery to constantly recharge, for instance, both reducing the size of the unit and extending its operational life span.
The PTTs transmit basic data about their functioning that is picked up by the Argos satellites as they pass overhead. Due to the Doppler effect, a shift in wavelength of these radio transmissions occurs as the satellite passes overhead and this allows the ground-based part of the satellite system to calculate the positions of the PTT and the bird carrying it. This is not as accurate as GPS – typically the best locations are accurate to within 500m, whereas GPS has an accuracy of a few metres. It is, however, very accurate on a global and regional scale and provides a far more reliable and detailed track than other, slightly smaller, tracking devices such as geolocators.
The PTTs we are using are scheduled to transmit for 10 hours at a time before going into ‘sleep’ mode for 48 hours to allow the solar panel to re-charge the battery. This duty cycle should continue until the tag fails or the bird dies, enabling us to obtain unbiased information about what happens to each bird tagged.