===Radio transmitters and satellite-tracking=== Where detailed information is needed on an individuals' movements, scientists can fit tiny radio transmitters to birds. For small species the transmitter is carried as a 'backpack' fitted over the wing bases, and for larger species it may be attached to a tail feather or temporary leg band. Both types usually have a tiny (10cm) flexible aerial to improve signal reception. Two field receivers (reading distance and direction) are needed to establish the bird's position using triangulation. Transmitters may be recovered by recapturing the bird or designed to drop off. The technique is useful for tracing individuals during landscape-level movements particularly in dense vegetation (such as tropical forests) and for shy or difficult-to-spot species, because birds can be located from a distance without visual confirmation. The use of satellite transmitters for bird movements is currently restricted by transmitter size - to species larger than about 400g. They may be attached to migratory birds (geese and swans are popular subjects) or other species undergoing longer-distance flights. Individuals may be tracked by satellites for immense distances, for the lifetime of the transmitter battery. As with wing tags, the transmitters may be designed to drop off when the bird moults; or they may be recovered by recapturing the bird.
