In the Alaskan wilderness, bald eagles showed signs they were stressed out when people camped near their nests, raising concerns among scientists that more people vacationing in the wild could ultimately mean fewer eagles.
Biologists floating the Gulkana River in Alaska between 1990 and 1992 found that when they camped as close as 100 yards from a nest, the eagles fed themselves and their young less, spent less time preening their feathers and maintaining their nests, and spent more time screeching -- all signs of stress.The eagles seemed to get more used to having people around after 24 hours, exhibiting fewer signs of stress, but return visits by the biologists triggered renewed signs of agitation by the birds.
"Human activity near nests caused clear and consistent changes in behaviors of breeding eagles, suggesting that frequent human activities near nests could adversely affect nestling survival, and therefore reproductive success," authors Robert J. Steidl and Robert G. Anthony wrote in the current issue of Ecological Applications, a quarterly journal of the Ecological Society of America.
Steidl, an assistant professor of wildlife ecology at the University of Arizona, and Anthony, professor of wildlife ecology at Oregon State University, said the work offered insights into the effects of stress.
"Mostly when we study the effects of humans on organisms, we focus on the changes to habitat and how they influence organisms," said Steidl. "What we managed to show in a reasonably strong way is more subtle impacts can have impacts on organisms."
Widespread use of the pesticide DDT after World War II caused eagle populations in the lower 48 states to plummet, landing the national bird on the endangered species list. The pesticide thinned eggshells, making them break before the young could hatch. Populations in Alaska were always healthy enough that they did not warrant listing.
Since DDT was banned in 1972, bald eagle numbers have rebounded to more than 5,700 breeding pairs in the lower 48 states, prompting Fish and Wildlife to propose taking the national bird off the endangered species list. A decision is due this summer.
The behavioral changes observed by the study were significant. Preening dropped 53 percent per day, sleeping 56 percent and nest maintenance 50 percent. Feeding by the eagles themselves and of their chicks went down 30 percent. Screeching went up 307 percent.
Anthony said he did not think the eagles were being adequately protected from the subtle harms of people getting too close to them, especially if they no longer were protected by the Endangered Species Act.
Increasing recreational use is producing more conflicts with eagles. A proposal to build a ski resort on Pelican Butte, in Winema National Forest in Oregon, has raised concerns for 100 bald eagles that roost nearby in winter and 15 nesting sites along Upper Klamath Lake.
Responding to a draft environmental impact statement, biologists Patty Buetner and her husband, Mark Buetner, wrote that if the ski area is built, human activity will increase. Other nests have failed as development encroached, and the roost would likely be abandoned for a less desirable place.
Fish and Wildlife has considered the effects of disturbance on bald eagles, but their numbers appear to be resilient enough to overcome it, said Jody Millar, the agency's coordinator for bald eagle recovery.
While delisting would mean bald eagles were no longer protected by the Endangered Species Act, protection guidelines under the Bald and Golden Eagle Protection Act and the Migratory Bird Treaty Act would continue to prohibit harassment by people, she added.
The study was done for the U.S. Bureau of Land Management in Alaska. Anthony said he and Steidl recommended BLM prohibit people rafting the Gulkana from camping within 400 to 500 yards of eagle nests.
Elijah Waters, wildlife biologist on the BLM's Glenallen District, said he was not aware of any steps taken to protect eagle nests on the Gulkana as a result of the study, but added that the agency is studying the effects of increased use of the river, which can be reached by car from Anchorage in a few hours.