Jesse Conklin, a PhD student at Massey University, describes how geolocators work to keep track of godwits. He explains that they are basically light sensors that record light levels – giving indications of day and night. The scientists use the information to work out the bird’s location.
The elegance of the geolocation idea is a thing of beauty. It’s based on the idea that day length is different at each latitude and the time of day is different at the longitude, and that is such a simple idea. But all it is is a light sensor and a storage disc to put data on, and what it does is it records light levels all throughout the day and year that it is deployed, and what it picks up is every sunset and every sunrise.
So the data you get basically says dark, dark, dark, light, light, light, dark, dark, dark, light, light, and the length of those light and dark periods tell you what latitude the unit is at. And when you deploy it, you have it keyed to the local time. So when those days shift – the midnight and noon shift – you know that the unit has changed longitude, it’s moving with the time zones.
So what happens is, I put one of these on a godwit, it’s here, it migrates away, it comes back and then I recapture it. It has a record of all the sunsets and sunrises during the time that it was gone, so let’s say it has a year of data on it. I take that data logger when I capture the bird, take it off and then hook it up to a computer and it reads the information, and then I can send that into some software that is given to me by the people who made the geolocators, and I can figure that out of when all the sunsets and sunrises were, and I can… from that, I can calculate all the locations of the bird throughout the entire year.
It’s not as precise as things like satellite telemetry or GPS units – you can actually get down to the 1 metre or 10 metres of where the bird is – because this is just using day length. Day length is affected by things like weather, it’s not nearly as precise, but I can tell where a bird is to, in the best cases, maybe 30 or 50 kilometres and in the worst cases, 100 to 150 kilometres, and that is close enough for all the work I am doing.
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