Problem
Radio telemetry tags are often attached to animals for the purpose of understanding their behavior. Installation of instruments on an animal requires capture of the animal. For various reasons, it is not always a simple matter to re-capture the animal and retrieve the tag. In some cases the electronic tags are data loggers in addition to their radio telemetry ability, so recovery of the tag must be achieved in order to obtain the stored data. AO was asked to develop a device to release data loggers at a pre-programmed time. It must be light weight and waterproof for use on seabirds.
Solution
We had already designed a simple mechanical release mechanism that used a heater to melt a small segment of nylon monofilament. The first use of this tag-release mechanism was in retrieval of tags from sea lions. We also made a larger version to release moored submerged oceanographic instruments. We created a smaller and lighter weight version for retrieval of tags from Cormorants.
The mechanism had to be as lightweight as possible. It contained a very low power microprocessor that served as a timer and triggered the release. A small coin cell provided power for the processor and a larger battery delivered the energy for the melt line heater. The unit was encapsulated in urethane to make it waterproof. Power for the processor was switched by a tiny magnetic reed switch embedded in the casting. A magnet attached to the outside opened the switch after the unit was encapsulated to preserve battery life. After the device was attached to a harness on the bird the magnet was removed to start the timer. A LED provided a tell-tale to show the timer was running.
Results
Many of these releases were used in the field to recover data loggers. They were programmed to time out at night when the birds were roosting, making it a fairly simple task to recover the instruments the next day.
