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A light level geolocator, light-level logger or GLS (global location sensor) is essentially a lightweight, electronic
archival
An archive is an accumulation of historical records or materials – in any medium – or the physical facility in which they are located.
Archives contain primary source documents that have accumulated over the course of an individual or ...
tracking
Tracking may refer to:
Science and technology Computing
* Tracking, in computer graphics, in match moving (insertion of graphics into footage)
* Tracking, composing music with music tracker software
* Eye tracking, measuring the position of t ...
device, usually used in
bird migration
Bird migration is the regular seasonal movement, often north and south along a flyway, between breeding and wintering grounds. Many species of bird migrate. Migration carries high costs in predation and mortality, including from hunting by ...
research to map migration routes, identify important staging areas, and sometimes provide additional ecological information. A geolocator periodically records ambient light level (solar
irradiance In radiometry, irradiance is the radiant flux ''received'' by a ''surface'' per unit area. The SI unit of irradiance is the watt per square metre (W⋅m−2). The CGS unit erg per square centimetre per second (erg⋅cm−2⋅s−1) is often used ...
) to determine location.
History
Mathematical formulas for calculating global positions based on sunset and sunrise were first described by Gemma Frisius in 1530. Due to the lack of accurate clocks, his theoretical work could not be applied for two centuries before John Harrison produced a sufficiently accurate clock.
Animal tracking using light level data appears to have been first carried out on
elephant seal
Elephant seals are very large, oceangoing earless seals in the genus ''Mirounga''. Both species, the northern elephant seal (''M. angustirostris'') and the southern elephant seal (''M. leonina''), were hunted to the brink of extinction for oi ...
s. 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 engineer, Vsevolod Afanasyev, and scientists at the
British Antarctic Survey (BAS) who first developed a device in an attempt to record the movements of juvenile
wandering albatross
The wandering albatross, snowy albatross, white-winged albatross or goonie (''Diomedea exulans'') is a large seabird from the family Diomedeidae, which has a circumpolar range in the Southern Ocean. It was the last species of albatross to be desc ...
during the many years between fledging and returning to their colony to breed.
From albatrosses and other
seabird
Seabirds (also known as marine birds) are birds that are adapted to life within the marine environment. While seabirds vary greatly in lifestyle, behaviour and physiology, they often exhibit striking convergent evolution, as the same enviro ...
s the use of geolocators has been extended to other migratory species, including
wader
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Waders or shorebirds are birds of the order Charadriiformes commonly found wikt:wade#Etymology 1, wading along shorelines and mudflat ...
s,
wildfowl
The Anatidae are the biological family of water birds that includes ducks, geese, and swans. The family has a cosmopolitan distribution, occurring on all the world's continents except Antarctica. These birds are adapted for swimming, floating on ...
,
raptor
Raptor or RAPTOR may refer to:
Animals
The word "raptor" refers to several groups of bird-like dinosaurs which primarily capture and subdue/kill prey with their talons.
* Raptor (bird) or bird of prey, a bird that primarily hunts and feeds on ...
s and
songbird
A songbird is a bird belonging to the suborder Passeri of the perching birds (Passeriformes). Another name that is sometimes seen as the scientific or vernacular name is Oscines, from Latin ''oscen'', "songbird". The Passeriformes contains 5000 ...
s as designs have become gradually smaller and more energy efficient.
Methods
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
Telemetry is the in situ data collection, collection of measurements or other data at remote points and their automatic data transmission, transmission to receiving equipment (telecommunication) for monitoring. The word is derived from the Gr ...
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
In information theory, data compression, source coding, or bit-rate reduction is the process of encoding information using fewer bits than the original representation. Any particular compression is either lossy or lossless. Lossless compression ...
they can record data for long periods of time.
Recording light levels over time produces data that can be used to calculate latitude and longitude readings of a bird's long-distance movements. The traditionally used 'threshold analysis method' requires only twilight data accurately time-stamped. Typically, daylight length (the time between dawn and dusk) is used to determine latitude, while the mid-time between a dawn and dusk is used to determine longitude. In this way, two position fixes can be obtained daily. Other analysis techniques can include analysis of the dawn and dusk curve (rate of change of light), or use the noon light level to attempt cloud compensation. The location data so derived is not as accurate as that from
GPS
The Global Positioning System (GPS), originally Navstar GPS, is a Radionavigation-satellite service, satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of t ...
or PTT (platform transmitter terminal) tracking involving satellites, but the devices can be made considerably lighter and cheaper. Other sensors, such as for recording temperature, or whether the logger is wet or dry, may be used in conjunction with the light-level logging in order to provide further ecological information. The devices may be attached to the bird being tracked by a harness, or to the
band
Band or BAND may refer to:
Places
*Bánd, a village in Hungary
*Band, Iran, a village in Urmia County, West Azerbaijan Province, Iran
* Band, Mureș, a commune in Romania
*Band-e Majid Khan, a village in Bukan County, West Azerbaijan Province, I ...
on the bird's leg. The weights of geolocators range from 0.3g upwards, with a battery life of 6 months to 5 years.
The main accuracy limitation of light level geolocation is due to the uncertainty in the amount of attenuation of the ambient light level at any particular time. Light attenuation can have many causes e.g. cloud, feathers, foliage, topography. Because of this, the quality of the resulting location calculations varies with species, tag attachment technique, habitat and behaviour.
References
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Ornithological equipment and methods
Bird migration
Bird conservation
Geopositioning