Animal tracking data helps us understand how individuals and populations move within local areas, migrate across oceans and continents and evolve across generations. This information is being used to address environmental challenges such as climate and land use change, biodiversity loss, invasive species, wildlife trafficking and the spread of infectious diseases.
Since the twentieth century, improved communication systems, shrinking battery sizes and other technological developments have led to a range of methods for tracking animals. Scientists have been systematically tracking individual animal movements since around 1900, when the first bird ringing (also known as bird banding) schemes were started. In the late 1950s, researchers began using radio transmitters to track wildlife. In the late 1970s, the Argos satellite system provided a new method for tracking animals globally. And in the early 1990s, the global positioning system (GPS) began to provide the potential to obtain high-resolution tracking data.
Choosing a tracking method involves trade-offs between size, price, and amount and ease of data collection. The ideal tag would be lightweight enough to be safely carried by the animal, cheap enough to put on many individuals, and able to transmit high-resolution data remotely so that the animal did not need to be captured again. In reality, a scientist must choose the best available method based on the size and movement patterns of the study animal, the study budget and the research questions they want to address. Below is a summary of commonly used methods for tracking animals using individual tags that are currently supported in Movebank.
A GPS tag calculates the location of an animal at specific time intervals using positions estimated by a network of satellites. These locations can be stored on-board the tag or transmitted to the user through a communication network (for example, Argos satellites or the GSM network) or through wireless downloads from a custom base station. These tags can provide high-resolution and accurate (within meters) location estimations for animals. However, these tags are relatively expensive and heavy, and so are usually limited to larger animals and require a large research budget if many animals will be tagged.
Argos Doppler tags (known as platform transmitter terminals, or PTTs) are electronic tags that send periodic signals to Argos transmitters on polar-orbiting satellites. Receiving stations located around the globe collect the data from the satellites and send it to a processing center, where location estimates are made by measuring the Doppler shift on the signals sent by the tag. The location estimates are less accurate (within kilometers) than those made with a GPS, but the tags can be cheaper and lighter than GPS units and can also be used to remotely transmit GPS locations if the tag is properly equipped, and allow for location measurements from anywhere on the globe. Find out more about the Argos System at argos-system.org.
Radio transmitters are electronic tags that emit a very high radio frequency signal that can be used to locate the animal. The researcher must track the signal using a receiver and directional antennae, which must typically be within a few kilometers or less of the animal to detect the signal. The signal can be tracked by foot, car or plane, or using one or more stationary receiver arrays. The tags are relatively lightweight, inexpensive and can have long battery lives. However, it can be labor-intensive to follow the animals with the receiver. This method can be used on small animals for populations that stay within a geographically restricted area. Some groups, such as the Motus Wildlife Tracking System, run networks of receiver stations that can track long-distance movements of animals too small to hold a PTT or GPS tag.
Acoustic tags emit sounds that are received by underwater hydrophone receivers. Using mobile receivers or stationary receiver arrays, researchers can monitor the presence, location and movements of fish and other animals in marine and freshwater systems. Monitoring long-distance movements requires communication between the tag owner (who knows the species and details of the animal the tag is deployed on) and the receiver owner (who knows the time and place at which the tag was detected). Collaborative groups and monitoring systems support regional (Great Lakes Acoustic Telemetry Observation System) and global (Ocean Tracking Network) animal tracking using acoustic telemetry.
Light-level loggers are tags that collect measurements of light levels. The tagged animal must be recaptured and the tag removed to access the data. The light level information is used to estimate sunrise and sunset times, which are used to estimate the movement of the animal. These tags can be lightweight and are relatively inexpensive, and provide the only available method for tracking movements of some smaller migrating animals as well as many marine species that spend most of their time below the ocean surface where they cannot be tracked by satellites or radio receivers. However, the location estimates can have very large errors, which vary depending on the time of year and location of the animal.
A band or ring is a physical tag with a unique code or number is attached to the animal. To record movement, the individual must be seen or caught again, and the number and location must be reported to the banding center. These tags contain no electronic components and are most commonly used on birds. They are lightweight, very inexpensive and can be attached by trained volunteers, meaning that large numbers of animals can be tagged. Because most of these animals are not caught again, however, only a small percentage of these tags result in movement data, and most of the movement data include only two locations per animal. Bird banding programs have been in operation since the early 1900s, and so large long-term datasets are available for this type of tracking.
Bio-logging, or biotelemetry, refers broadly to the collection of data using on-animal sensors. These sensors can collect a range of information about animals' location and movements as well as behavior (for example using accelerometers or cameras), physiology (for example by measuring heart rate or internal temperature) and surrounding environment (for example by monitoring salinity).