Linking light and location
Geolocators are miniature tracking devices that record ambient light levels, water temperature and time. The light curve produced over the sunrise and sunset twilight period is unique to a given latitude, while the time of noon is specific to a given longitude. Collecting this data identifies the relative location of a tagged animal to within 200km. Using remotely sensed sea surface temperature, with knowledge of the species’ likely travel speed, we can further constrain this area to achieve a more accurate position.
Here you can see the raw daily light data produced from a geolocator attached to a Black-browed Albatross. Each 24 hour period is represented with a single vertical line, with black as night and white as day.
Over December/January this individual is foraging close to the colony and taking turns on the nest to incubate an egg. While nesting, the light sensor becomes obscured by the adult tucking the geolocator under its body when keeping the egg warm.
Due to inexperience, this adult stops incubating toward the end of January and abandons its nest. The days are now getting shorter, but still, our tracked individual is found foraging close to the nesting colony.
That is, until April, where it undertakes a long easterly migration. In the daily light data you see midday has shifted about eight hours indicating a large longitudinal shift.
Over the austral winter this animal is taking longer breaks between feeding events by sitting on the water, which is evident in our light data as short periods of darkness, where the geolocator is underwater and obscured.
In October, it embarks on another long migration, this time back to the breeding colony where again, it breeds and begins the incubation process.
It is at this time that researchers return to the colony and retrieve the valuable data recorded by the geolocator. Once the tag is retrieved, the data can be downloaded and processed to produce a track.