November 15, 2012 | Volume 42, No. 46

Tracking eagles by satellite: Birds to wear 'backpacks'

A new study is aimed at gathering more information on where the eagles that show up here in November come from, and where they go when they leave.

For years, the pat answer to that question has been “other parts of Southeast Alaska, British Columbia and as far south as Washington state.”

Science teacher Pam Randles, who studies the local eagle population, calls that explanation “a best guess” based on a study 15 years ago that tracked four birds leaving Haines.

“The hard data we have is pretty small,” Randles said.

Work by Rachel Wheat, a doctoral candidate from the University of California-Santa Cruz, may provide a more complete picture.

Wheat recently fitted five eagles along the Chilkat River with satellite beacons in “tiny backpacks” that will send continuous signals to researchers for three years. She’s hoping to tag up to 30 birds, depending on funding.

The information will indicate not only where birds from here are going, but how long and what route they take to get there, and where they stop.

Unlike songbirds and other birds that travel the same routes each year, typically on a north-south axis to distinct summer and winter ranges, eagles migrate in any direction, depending on food availability. If an eagle has access to food year-round, there’s no incentive for it to make a long migration between two points, she said.

In Haines, there’s only enough food to support a year-round population of about 200 birds.

Wheat’s study may show what factors determine when eagles leave the Chilkat Valley and how closely they track availability of salmon in other watersheds, providing insight into how they make their migration decisions.

New technology will give the experiment a boost. Eagles formerly were tracked by radio telemetry, requiring researchers using mobile antennas to track the birds. Solar-assisted satellite tags fitted between the wings of eagles will allow Wheat to follow the birds from California.

“We don’t have to be near the eagle to see where it is, and we can know where it is all the time,” Wheat said.

Information from the work may eventually be tied to salmon management, as information about upriver escapement of fish is linked to how eagles move through the landscape, Wheat said.

Wheat recently tagged two adult females, an adult male and two juveniles, a male and female. Satellite reports show the juvenile male tagged Nov. 4 has since moved to Admiralty Island. “We’re already getting information from the birds,” Wheat said.