Using radio telemetry, we can track a bird's movements along their migration route. The Motus project is an international collaboration started by Bird Studies Canada, based on the attachment of radio transmitters to birds and the tracking of those birds using receiver towers to record their movement.
Nanotags are small lightweight radio transmitters that are attached to birds. Upon activation, the nanotags emit a radio signal. This signal is received by Motus towers with a receiver and antennae that are specifically tuned to the same frequency as the nanotags. Using an array of Motus towers along the migration route, we can observe where the birds are moving, at what time, and for how long.
Geolocators are another small device used to track bird migrations, using a different technique. Whereas a nanotag uses radio telemetry to determine a geographical location, a geolocator detects and records ambient light levels. A record of light levels can be used to determine location of the bird. The bird's latitude can be determined by day length, and longitude by the mid-point between dawn and dusk on a given day. Along with other data collected by the sensors, these two metrics can reliably provide a bird's location as it migrates.