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Eagles' Gains at Herons' Expense?

April 15, 2009 - AUGUSTA -- The great blue heron, that slender marsh walker, may be a victim of the bald eagle's glorious return, state biologists say.

As the Maine Department of Inland Fisheries and Wildlife embarks on its first great blue heron survey in 13 years, biologists wonder whether the bird's apparent low numbers are a result of bald eagle predation.

The bald eagle's rebound in Maine is now legendary, with its state population increasing from 29 nesting pairs in 1972 to 477 nesting pairs today, according to the state.

Meanwhile, the number of coastal great blue heron colonies may have dropped from 42 in 1967 to as few as six today, said state bird biologist Danielle D'Auria.

"We are not sure that they have actually declined. There is evidence that they could have. The '95 survey showed a lot of large coastal colonies either dwindled or broken up entirely ... there is a big question mark in our head," D'Auria said.

"There is a lot of evidence that the eagles have expanded and pushed the herons out. They are a predator of the heron, especially the young," she said. "A lot of the coastal islands now have eagles on them, where they didn't 20 to 30 years ago."

Information has been gathered sporadically by the state during the past eight years, D'Auria said.

If the number of great blue herons in Maine has dropped, the state would need to look for clues as to why.

Great blue herons can fly up to four miles from their nests to feed. So herons seen along the coast may not necessarily indicate nesting colonies nearby.

"When you don't have continual data, you don't know. We don't know if there is a need for concern over the species or not," D'Auria said.

Whatever the reason for the bird's decline in Maine, it seems to have happened fast.

D'Auria said most of the 42 historical coastal colonies were still active in the mid-1980s, and many as late as 1996, when the last heron survey was conducted.

Biologists will look to see whether the habitat around colonies has changed. Predators such as bald eagles will also be considered.

But if bald eagle predation has affected Maine's heron population, there may not be a lot the state can do, D'Auria said.

"We don't like to step in when it's (natural predators)," she said.

There usually is an abundance of natural food such as alewives where bald eagles and herons coexist without the herons being affected, D'Auria said, but that doesn't arise often.

Historically, great blue herons were found as far north as Madawaska and St. Agate on the St. John River. D'Auria said those colonies are now gone.

The public is invited to help locate the state's heron colonies by identifying nesting sites – from a safe distance.

D'Auria has created a Heron Observation Network to organize volunteers. There are already 35 participants signed up, but as many as 100 are needed.

In addition, state biologists will conduct surveys with $12,000 in federal funding funding and a portion of the state's oil spill fund.

Locating the birds will not be quick or simple.

"A lot of people argue that they see them all the time," D'Auria said. "They are really good at fishing. But just because they are foraging does not mean they are all breeding, necessarily."

DEIRDRE FLEMING, Portland Press Herald, April 9, 2009


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