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What are Light-level Geolocators?

  • a bird with a tracking device on its back is held in a human hand

    What time does the sun rise where you live? You probably know that the sun rises and sets at different times throughout the year. But did you know that it rises and sets at different times across the globe as well? For example, on the summer solstice, the sun rises at about 5:24 a.m. and sets at about 8:36 p.m. in New York City. In Washington, D.C., a city that is about 200 miles (330 kilometers) from New York, the sun rises at 5:43 a.m. and sets at 8:36 p.m. on the same day. This is because the Earth is tilted on its axis. Using the day of the year and our planet’s tilt and speed of turning, scientists can predict the sunrise and sunset time at any location on Earth. Explore the sunrise and sunset times where you live.

    Just as scientists can determine the sunrise and sunset time based on location, they can also use sunrise and sunset times to predict a location. Light-level geolocators are tracking devices that use daylight to estimate location. A light-level geolocater has a light sensor, an internal clock, a battery and a computer that stores a measurement of the amount of light that the sensor is exposed to. Recording light levels does not require a lot of electricity, so batteries for these devices can be very small and last longer than a year. Because of this, these devices are incredibly lightweight (just 0.3 grams) and can be placed on any bird that weighs more than 7 grams.

    How do scientists use light-level geolocators to study birds?

    When scientist use light-level geolocators to study bird movements, they need to travel to the locations where they can find the birds they are studying. These locations are often where birds breed in the summer or where they spend their winters. Researchers capture birds in mist nets—tall, long nets made of very fine threads that blend into the surroundings. Once a bird is caught, researchers place an aluminum band and a unique combination of colored plastic bands around its legs, so they can identify captured individuals. This is known as bird banding.

    Scientists then carefully position the tiny light-level geolocator on the bird’s lower back, using a harness that loops around the bird’s legs. Once the device is firmly attached, they release the bird, and the light-level geolocator starts collecting data. A year later, the scientist will return to the same location and attempt to re-capture the bird. Recapturing a bird is a challenging task. Only about one in every five birds that scientists tag is captured again the next year. If a bird is successfully re-caught, the device is removed and attached to a computer where scientists can download the data. By analyzing data collected and stored by the light-level geolocater, researchers can find out where a bird traveled.

Two pieces of a tracking device with small antennas called light-level geolocators

What are the pros and cons of using light-level geolocators?

Because they are lightweight and have a long battery life, light-level geolocators are a great option for scientists who study the long-distance movements of migratory birds. And, in many cases, light-level geolocators may be the only option for studying the movements of small migratory bird species. Other options are often too heavy, or the batteries do not last long enough. Because light-level geolocators cost less than $200, they are also cost effective, meaning they are a low price for how well they work.

Even though light-level geolocators have a lot of pros, they also have some cons. For example, the cost for one device is low, but because only one in five birds with tags is recaptured, the total cost of the study can be much higher. That means scientists spend money on devices that they will never be able to collect information from. Also, the location accuracy of these devices is not exact. This means that scientists can only get a general idea of where a tagged bird traveled. For example, researchers could learn that a bird spends its winters in southern Florida, but would not be able to pinpoint the exact city or town. Finally, because birds spend a lot of time under the shade of the tree canopy, it can be difficult to determine what the sunrise and sunset times were at the bird’s location.

Learn more about light-level geolocators.