Light level geolocator

A light level geolocator, light-level logger or GLS (global location sensor) is essentially a lightweight, electronic archival tracking device, usually used in bird migration research to map migration routes, identify important staging areas, and sometimes provide additional ecological information. A geolocator periodically records ambient light level (solar irradiance) to determine location.

Animal tracking using light level data appears to have been first carried out on elephant seals. Although not described until 1992, the first device to be developed was reportedly in 1989 as an adaptation of a TDR (time-depth recorder) and called a geographic location, time-depth recorder (GLTDR) weighing 196g. Also in 1992 was a publication by a different group with a similar design (weight 113g) who called it a global location sensor (GLS) though no field use was mentioned.

The use of dedicated light level recorders for tracking birds was pioneered in the 1990s by engineers and scientists at the British Antarctic Survey (BAS) who first developed a device in an attempt to record the movements of juvenile wandering albatross during the many years between fledging and returning to their colony to breed. From albatrosses and other seabirds the use of geolocators has been extended to other migratory species, including waders, wildfowl, raptors and songbirds as designs have become gradually smaller and more energy efficient.

Light level geolocators primarily use an electronic light sensor to record light level and may also make other measurements to aid geolocation (e.g. temperature or water immersion). The smallest are archival types that do not use satellite or radio telemetry to offload data and recapture of the bird is necessary to obtain the data. The disadvantage of having to recapture is offset by the miniature size to which archival loggers can be made. By using low power design techniques and data compression they can record data for long periods of time.

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