Geolocation technology has just solved a mystery: where to the sparrows go every winter?

At PRBO Conservation Science, four golden-crowned sparrows, which winter in California, were fitted with devices that measured the length of the day. With that information a team of biologists was able to track where the birds went in the summer.

PHOTOS: Lions Captured in the Wild by 'BeetleCams'

It turns out that they go to areas of Alaska's Katmai National Park, Wrangell-St. Elias National Park, and the Chugach National Forest. It's a long trip, anywhere from 1,600 to 2,400 miles from the starting point at the Point Reyes National Seashore. The birds make the trip north in about 29 days. One interesting piece of data was that they take twice as long to come south, about 53 days.

DNEWS VIDEO: GPS Shoe Hotfoots Your Location

To track the birds, the tags were attached using a harness, like a tiny backpack. Each tag measures the length of day against an internal clock.  Since the length of day at Point Reyes on a given date is known, one only needs to know how many hours of light there are and what time the sun came up to know where the bird was.

BLOG: Whale Migration Shockingly Straight

The reason for using the light detectors as opposed to GPS sensors was the weight. GPS locators were too big and heavy, even though they would offer better data. Nathaniel Seavy, research director, told Discovery News that the sensors they used were designed to be light and inconspicuous enough that it wouldn't affect the birds' flight.

Image: D. Humple / PRBO Conservation Science