Bald eagles have made a stunning comeback in Maine since first going on the endangered species list in 1978, then being delisted in 2007. From the more than 600 nesting pairs in Maine, what happens to the young adult birds after they leave the nest but before they settle down and begin to mate?

That’s the subject of a study going on at the Biodiversity Research Institute in Gorham, and the subject of a talk next month by Chris Desorbo, the director of the institute’s raptor program.

Desorbo will speak about his ground-breaking research Feb. 25 at the Curtis Memorial Library in Brunswick.

“When eagles fledge they’re known to roam widely. It becomes almost impossible to study. They’re known to up and leave in a day and go 100 miles,” Desorbo said.

“The advent of satellite telemetry technology meant we could track individuals daily. It’s filling in a whole gap of information from when the birds fledge to when they settle somewhere and breed.”

While the bald eagle population in Maine has made a strong comeback, it could face unknown threats because little is known about the sub-adult age class, Desorbo said. While there are no immediate concerns driving the study, he said it makes sense to understand that age class to better protect bald eagles.


Banded sub-adult eagles were known to go to Canada and as far south as Virginia, but all the stops those birds made or if they would definitely return to Maine was unknown, Desorbo said.

“The sub-adult age class is a five-year interval and essentially represents a black hole in what was known of bald eagle ecology,” Desorbo said.

“That age class is particularly interesting because it’s hugely important to the stability of the population. That is the subset of the population that can dictate the overall trend of the population. If you can predict the sub-adult and the adult, you basically can keep that population trend moving in the right direction.”

To find out more about this age class, the Institute began using grant money to fund a telemetry study of bald eagles in 2005. Small satellite tracking devices were affixed to bald eagles using Teflon straps, like small backpacks running along the spine between the wings, Desorbo said.

More funding made more telemetry tracking devices possible. After affixing two bald eagles between 2005 and 2006, and adding four to the study between 2009 and 2010, the Institute used Maine Outdoors Heritage Fund grant money to add 15 bald eagles to the study between 2011 and 2013.

Currently, there are 15 active transmitters being used in the study.


Other satellite telemetry studies on bald eagles have been done in other states, but Desorbo said Maine’s is the largest in New England.

This summer for the first time, Desorbo will begin to break down the data that has been gathered since 2005, with as many as 60,000 stops recorded from 19 transmitters. What the data will reveal is unknown, but Desorbo said some story lines have become apparent.

One trend that has come clear is the existence of historic migration corridors the birds favor. Another trend shown by the data is how sub-adult eagles show “fidelity” or an attachment to the nest site where they hatched.

Of course, there are problems and complications even within nature.

“Honestly, if a sub-adult eagle that has progressed into adulthood wanders back to its natal nest, it’s likely to get its butt kicked if it goes there at the wrong time. When birds come to breeding age, they really have to find their own territory,” Desorbo said.

At this time there’s no additional funding being gathered for the study or plans to expand it. With each unit costing between $3,000 to $4,000, the study is an expensive one.


But bald eagles can live to be 25 or older. So in years to come, Desorbo said a lot more data could be gathered to help tell interesting stories about this age class of birds.

“They’re solar units so they recharge in the sun. And it’s pretty solid technology so theoretically, it could go a long time,” he said.

Deirdre Fleming can be reached at 791-6452 or at:

Twitter: FlemingPph

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