Bald Eagle population soars, disease source identified

Bald Eagles have been in the news this week. Here’s a summary.

Population soars

The U.S. Fish and Wildlife Service says in a new report that the Bald Eagle population in the lower 48 states has more than quadrupled since 2009. It estimates that the population has 316,700 individual eagles and 71,400 nesting pairs. These numbers do not include Canada, Alaska, and the southwestern U.S.

The bulk of the breeding range covers Canada and Alaska, but neither has a recent population estimate. A 2009 study estimated 70,500 eagles in Alaska, but no one know if that population is growing at the same rate as the birds in the lower 48 states. In any event, 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 neighboring areas of the Southwest is small — 74 breeding pairs at last report. It is monitored annually by the Arizona Game and Fish Department.

To estimate the eagle population in the lower 48 states, Migratory Bird Program pilot biologists and observers from many FWS regions, programs, and contract observers conducted aerial surveys in 2018 and 2019 over high-density eagle nesting areas to generate accurate estimates and count occupied nesting territories. To obtain information on the lower density eagle nesting areas, the agency worked with the Cornell Lab of Ornithology to use eBird relative abundance data to acquire information on the areas that were not practical to fly in aerial surveys.

“Working with Cornell to integrate data from our aerial surveys with eBird relative abundance data on Bald Eagles is one of the most impressive ways the Service has engaged with citizen science programs to date,” said Jerome Ford, assistant director for the Migratory Bird Program. “This critical information was imperative to accurately estimate 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, which included remarks by the new Secretary of the Interior Deb Haaland.

I asked Bill Route, a wildlife ecologist with Northwoods Wildlife Consulting and a retired National Park Service scientist who studies environmental contaminants in Bald Eagles, about the new report. He was not involved in the research but gave it a thumbs-up.  

The report “is an incredibly complex design and statistical estimation procedure. I have high confidence in the investigators, and dual count procedures with a randomized design that accounts for sampling bias are the gold standard.

“At its core, the procedure provides an unbiased estimate for management units across the U.S. such that the entire population can be compared through time,” he explains. “That simply was not possible with state-by-state efforts in the past. The increase in Bald Eagle numbers is evident here in the Great Lakes region, so the estimates the FWS obtained in this effort align with my estimates of increasing numbers of eagles and occupied territories in my study areas.

“I think this is testament to how well the ESA [Endangered Species Act], Clean Water, and Clean Air Acts have helped to clean up our waters. Now we just need to heed the lesson and continue to monitor and regulate new and existing chemicals to make sure another poisonous stew is not brewed.”

Eagle killer identified

Over two winters in the mid-1990s, more than 70 eagles died at a reservoir in Arkansas due to a neurological disease. The incidents left scientists searching for answers, and now 25 years later, the full story has emerged.

An article published by The Atlantic sums up the results of a new study into the disease:

“The birds died because of a specific algae that lives on a specific invasive water plant and makes a novel toxin, but only in the presence of specific pollutants. Everything had to go right—or wrong, really—for the mass deaths to happen. This complex chain of events reflects just how much humans have altered the natural landscape and in how many ways; unraveling it took one scientist the better part of her career.”

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 lead author of a paper published yesterday by the journal Science that reports that a new cyanobacterial neurotoxin called aetokthonotoxin caused the eagle-killing disease known as avian vacuolar myelinopathy (AVM).

The cyanobacterium grows on an invasive plant (Hydrilla verticillata) in man-made water bodies in Virginia, North Carolina, South Carolina, Georgia, Florida, Alabama, Louisiana, and Texas. Elevated amounts of bromide that came either from herbicides or from water treatment and power plants enabled the toxin to grow. American Coots, ducks, and other wildlife then consume the contaminated plants, which made them ill — and easy prey for eagles. The eagles were then sickened after eating infected prey. No one seems to know how many eagles have died in the last quarter century due to this disease.

And eagles aren’t the only raptors to be concerned about. Wilde says Peregrine Falcons have been seen hunting coots at a reservoir where the toxin occurs on the Georgia/South Carolina border, and Snail Kites have consumed invasive apple snails that contain the toxin.

The good news, The Atlantic notes, is that after the original incidents at the lake in Arkansas, officials took steps (with the help of a drought) that eliminated hydrilla, and now its Bald Eagles are not afflicted by AVM.

Bald Eagle photo gallery: From nestlings to full-grown fledglings