About Oystercatcher Tracking
Wildlife tracking is a varied field, with multiple techniques available to researchers, from radio telemetry in which field staff with antennas follow an animal’s signal, to geolocators that must be recovered from the animal to access their data, to GPS receivers that transmit their data automatically via cell phone towers. One of the biggest challenges is size—fitting the tracking technology and a battery into a package small enough that even a bird can carry it. The extra weight must not exceed 3% of their body weight, so as not to take too much extra energy to carry. For an oystercatcher, which weighs between about a pound to a pound and a half, a tracking transmitter must be pretty light: no more than the weight of about two U.S. quarters.
For example, the oystercatchers in the 2013 North Carolina tracking project carried small 9.5 gram units manufactured by Microwave Telemetry. These units are called solar-powered PTTs (platform transmitter terminals). The PTT consists of a small box about an inch long with an antenna affixed to it. Solar panels the size of your thumbnail sit on top of the PTTs and charge a battery inside. Typically, only the antenna (represented as a gray line above) is visible on tracked oystercatchers, sticking out from their backs unobtrusively.
Every couple of days, PTTs turn on and begin transmitting a signal to satellites orbiting 500 miles overhead. The satellites are part of the Argos System, a unique worldwide location and data collection system created in 1978 to study and protect the environment.
As an Argos satellite passes overhead, it measures the Doppler shift in the radio frequency of the PTT’s transmission and calculates its distance from the PTT. When three or more satellites simultaneously receive the radio signal, the system uses triangulation to pinpoint the PTT’s location. Depending on the angle of the satellite’s pass, cloud cover, signal strength, and other conditions, the location fix could be accurate to within 250 meters (less than 1,000 feet).
Other types of tracking technology exist as well. GPS and GSM trackers, which are usually heavier than PTTs, use GPS satellites and signals from cell phone towers respectively to pinpoint the location of animals they are tracking. Their accuracy can be within a few feet, making it possible to monitor fine-scale habitat use as well as larger movements such as migrations. Meanwhile, geolocators have a light sensor that measure the time between dawn and dusk to determine latitude and record the midpoint between dawn and dusk to determine longitude. Though they are not as accurate as the other trackers, they are so lightweight that even songbirds can carry them.
Over time, as technologies improve, tracking devices have gotten smaller, allowing a greater number of species to carry them. While many hundreds or even thousands of birds can be banded, time and expense means that fewer can be tracked with remote technology such as PTTs or geolocators. Combined, however, data from all techniques helps stitch together the story of oystercatchers–and many other species’–lives.