While tags can refer to many different things, in this document we are talking about miniaturized radio transmitters that weigh as little as 0.15 g so they can be affixed to birds, bats, and even insects. Tags are made such that they each emit a slightly different radio signal allowing animals to be tracked individually by using a receiver.
There are several other ways animals can be tracked, each of which have their advantages and disadvantages. For a detailed summary of tracking technologies see - (insert references., e.g. Bridge et al.,). The first method of tracking was through banding/ringing which is still done to this day, but it is unreliable with a very low recapture rate. Colour banding can be very effective for tracking certain species where the search area is relatively small (e.g.: breeding passerines in a plot or migratory shorebirds confined to specific beaches).
For electronic tracking techniques, radar can be used to detect large movements of many birds at once without the need to directly affect anything, but individuals and species cannot be distinguished.
Passive integrated transponders (i.e.; PIT tags or Passive RFID) are a type of transmitter that does not require a battery and are instead powered by the presence of a strong magnetic field, similar to security devices found in items at retail stores. Due to the requirement of a strong magnetic field, these devices only operate in close proximity to a receiver which is useful for detecting movement within a narrow passage (i.e.; a burrow or river), but impractical for detecting aerial movements. Active RFID tags use a battery for the same effect and have a greater range, but typically only in the 10’s to 100’s of meters.
Light-level Geolocators (GLS tags) are a popular choice, first used by Dr. Bridget Stutchbury in Ovenbirds, offer a method for geopositioning small migratory wildlife without the need of any receivers. Instead, data is logged on a small memory chip which must be retrieved from the animal. This means GLS tags are only practical when the target individuals can reliably be re-trapped some time after they have been deployed. In addition, GLS tags use a combination of daylength and relative timing of sunrise and sunset to determine their geographic position which can have widely variable levels of precision making them most useful for detecting broad scale movements.
GPS and satellite tags both use GPS to record locations, but there are several different methods for data to be retrieved once it has been recorded. Certain GPS tags, like the PinPoint, function similarly to GLS tags, requiring physical retrieval of the tag to download data, but use more reliable satellite triangulation to gain position information.
The more advanced satellite tags will transmit data directly to a satellite which is then transmitted back to a base station on land, like Icarus tags. Although these tags can provide the high precision tracking data from anywhere on the globe without the need of re-trapping, it comes at the price of increased weight and cost making them unviable for small passerines, most bats, and insects.
Cellular Tracking Technologies has gone a step in between and uses a Cellular (Global System for Mobile Communication - GSM) tags to communicate GPS and other data to nearby cellular towers, offering lower cost data acquisition, but weight can is still an issue, making this technology not suitable for most passerines, bats, and insects.
The niche for radio transmitters like the ones we use in Motus fits where there are limits in the size or cost of tags. Their main restriction is that they can only be tracked where receivers exist and installing stations can be expensive. That’s why it’s so important to have a collaborative network of receivers for this type of technology.