Bald eagle population increases, source of disease identified

A Bald Eagle in a forest in British Columbia. Photo by Tony Joyce

Bald eagles have been on the news this week. Here is a summary.

Population increases

The U.S. Fish and Wildlife Service reported in a new report that the population of bald eagles in the lower 48 states has more than quadrupled since 2009. It is estimated that the population has 316,700 individual eagles and 71,400 breeding pairs. These numbers do not include Canada, Alaska, and the US Southwest

Most of the breeding area covers Canada and Alaska, but there is no current population estimate. A 2009 study estimated 70,500 eagles in Alaska, but no one knows if that population is growing at the same rate as birds in the lower 48 states. In any case, it’s safe to say that the total population in North America is much larger than the numbers in the new FWS report.

The population in Arizona and the adjacent areas of the southwest is small – last 74 breeding pairs. It is monitored annually by the Arizona Game and Fish Department.

To estimate the eagle population in the lower 48 states, pilot biologists and migratory bird program observers from many FWS regions, programs, and contract observers conducted aerial surveys of high density eagle nesting areas in 2018 and 2019 to provide accurate estimates and count occupied nesting sites Areas. To obtain information about lower density eagle nesting areas, the agency worked with the Cornell Lab of Ornithology to use the eBird relative abundance data to provide information about the areas that were impractical for aerial photography in the air.

“Working with Cornell to integrate data from our aerial photography with eBird data on the relative abundance of bald eagles is one of the most impressive ways the service has looked at civic science programs,” said Jerome Ford, associate director of the program for Migratory birds. “This important information has been essential to fully assessing the bald eagle population in the contiguous United States and we look forward to working with Cornell in the future.”

You can download the technical report and the announcement of the report to the media, including remarks from the new Home Secretary Deb Haaland.

I asked Bill Route, a wildlife ecologist at Northwoods Wildlife Consulting and a retired National Park Service scientist who studies environmental pollutants in bald eagles, about the new report. He wasn’t involved in the research but gave a thumbs up.

The report “is an incredibly complex design and statistical estimation process. I have great faith in investigators, and dual counting, with a randomized design that accounts for sample bias, is the gold standard.

“At its core, the process provides an unbiased estimate for management units in the United States so that the entire population can be compared over time,” he explains. “That was simply not possible in the past with government efforts. The increase in the number of bald eagles is evident here in the Great Lakes region, so the estimates the FWS obtained from this effort are consistent with my estimates of increasing numbers of eagles and occupied territories in my study areas.

“I think this is proof of how good the ESA is [Endangered Species Act], Clean Water and Clean Air Acts have helped purify our water. Now all we need to do is follow the lesson and continue to monitor and regulate new and existing chemicals to ensure that no more toxic stew is brewed. “

An eagle grabs a fish on the Mississippi, Iowa. Photo by Jim Buescher

Eagle killer identified

In two winters in the mid-1990s, more than 70 eagles died of neurological disease on an Arkansas reservoir. The incidents left scientists looking for answers, and now, 25 years later, the full story has surfaced.

An article published by The Atlantic summarizes the results of a new study on the disease:

“The birds died of a certain alga that lives on a certain invasive aquatic plant and forms a new type of toxin, but only in the presence of certain pollutants. Everything had to go right or wrong for the mass extinctions to occur. This complex chain of events reflects how much man has changed the natural landscape in how many ways. One scientist has spent most of her career trying to figure it out. “

The scientist in question is Susan Wilde, an associate professor at the Warnell School of Forestry and Natural Resources at the University of Georgia. She is the lead author of a paper published yesterday by Science magazine that reports that a new cyanobacterial neurotoxin called aetocthonotoxin caused the eagle-fatal disease known as avian vacuolar myelinopathy (AVM).

The cyanobacterium grows on an invasive plant (Hydrilla verticillata) in artificial waters in Virginia, North Carolina, South Carolina, Georgia, Florida, Alabama, Louisiana, and Texas. Increased amounts of bromide, either from herbicides or from water treatment and power plants, allowed the toxin to grow. American coots, ducks, and other wildlife then consume the contaminated plants, making them sick – and easy prey for eagles. The eagles then got sick after eating infected prey. Nobody seems to know how many eagles have died from this disease in the past quarter century.

And eagles aren’t the only birds of prey to worry about. Peregrine falcons have been seen hunting coots in a reservoir where the toxin is found on the Georgia-South Carolina border, and slug dragons have consumed invasive apple snails that contain the toxin, according to Wilde.

The good news, according to The Atlantic, is that following the original incidents at the Arkansas lake, officials took steps (with the help of a drought) to eliminate Hydrilla, and now the bald eagles are not affected by AVM.

Bald eagle photo gallery: from nestling to fully grown young bird

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