Bird migration demystified

Birds disappear in the fall. They reappear come spring. Where from? What happens to them in the meantime?

Aristotle thought small birds called redstarts became robins in winter. In his book "The History of Animals," written about 330 BC, migration was explained as magical transformation.

For all his genius, Aristotle nonetheless also believed swallows hibernated on the ocean floor in the winter. Other people of that era believed birds migrated to the moon and back.

Two thousand years later, in 1676, an Englishman named Francis Willughby got it right. Author of the book "Ornnithologia," he wrote that swallows in the fall went not into hibernation but south to the warmth of northern Africa.

Until recently, while we had solid understanding of migration north and south, questions lingered. Exactly where to? Precisely, where from?

Today, tiny electronic devices let the birds themselves answer the questions. This is real magic.

The tools are satellite transmitters and geolocator tags. Both are small, almost as light as a handful of feathers. Both can be attached to birds. Both collect information, handling it in different ways.

Researchers wanted information on loon migration routes and wintering range. The concern was loons wintering on the Gulf of Mexico, on often oily water. Satellite telemetry was one method used to follow the birds.

Blake Henke's California company, North Star Science and Technology, manufactures satellite telemetry devices.

The transmitters are either solar or battery powered, Henke told me in a telephone interview. The entire device can be as light as three grams, the weight of one penny. The rule of thumb is, the device should weigh no more than 3 percent of the bird's weight.

The transmitter sends a constant signal received by earth-orbit satellites. Data collected are sent from the satellite to earth. As the satellites move, signals received in effect draw a map.

Also being used in the loon study are geolocators. These are more sophisticated machines, albeit no larger and no heavier.

"The archival geolocator tags collect data on light levels, and use the data to calculate a daily location," Kenow wrote by e-mail. "Times of sunrise and sunset are inferred from light measurements; latitude and longitude are inferred from those estimates.

"The geolocator tags are programmed to collect daily location estimates of the loons for up to two years. The tags also measure and store temperature, which is collected every 30 minutes for about one year, and pressure data collected during daylight hours every 20 seconds for about two and a half months.

"The loons' dive depth can be interpreted from the pressure data. This documents feeding patterns."

The tags are attached with a leg band. The bird must be recaptured to recover the tag.

Carrol Henderson, Minnesota DNR nongame wildlife supervisor, said that of 96 geolocaters attached to loons from 2010 to present, 51 were recovered. Data were downloaded from 37.

Geolocators are widely used in bird studies. Information previously unavailable now can be collected.

Extraordinary, yes. But geolocators are no good for Pokémon Go.