Tuesday, November 20, 2012

Bald Eagles, Bad to the Bone

Bald eagles, which can live up to 30 years in the wild, are extremely territorial. In general, they mate for life. Many eagles are, in human terms, good spouses and parents—loyal to their mates and good providers for their young. But all eagles aren’t the same. There are eagles that cheat on their mates and birds that seem to want to hang around and loaf when they ought to be out on the river bringing home the bacon—or rather, the fish.

Bryan Watts, the director of the Center for Conservation Biology, says that it seems that as the eagle population goes up, the famous monogamous nature of the birds begins to go down.

“The thinking has always been that pair fidelity is high,” Watts said. “But my guess is that it’s not as high as people think. We have places like the upper James where density is just phenomenal and I have to believe that there is a lot more extra-pair ‘cheating’ going on.”

Eagle researchers even talk about “divorce rate” among the birds; Watts said the eagle divorce rate is thought to be around 15 percent. Both males and female eagles will cheat, he said.

“The most likely scenario is that Charlie’s off fishing somewhere and Joe comes in to mate with the female while she’s at the nest. Charlie comes back. He doesn’t know what’s happened but he ends up raising Joe’s chicks,” Watts explained.

Researchers are checking the stray rate, he said, by comparing DNA of feathers collected beneath nests in different years.

There also are instances of what humans call home invasions. Turrin notes that researchers use the term “nest intrusion” for events when one eagle forcibly interrupts the domestic tranquility of the resident pair of birds.

The nest-cam project aims to examine eagle nesting behavior—good and bad. Turrin had 12 study sites, including the live-streaming webcam loblolly nest. Each nest in the study had a camera set up as much as 100 meters away to monitor activity around the nest site. Turrin and other CCB researchers have been going through the hours and hours of eagle videos, logging various behaviors.

She explained that in addition to the population density per se, the number of single adult eagles causes some friction.

“There is a higher proportion of non-breeding adults because of the number of birds and the limited availability of nesting territories—and we are observing sneaky behaviors by young males. We’re seeing some nest intrusions,” she said. The female birds don’t seem to mind. In fact, some are exhibiting polyandrous tendencies, which Turrin points out is not beneficial to the breeding male because he might wind up investing his energy in raising chicks which aren’t even his. Or he invests his energy in “mate guarding” which takes him away from his other duties.

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