Hundreds of bald eagles have begun their annual migration down the Mississippi River, soaring from bluffs and swooping for fish as towns from Minnesota to Missouri gear up for yearly celebrations.
But the festivals this winter are tempered by concerns about West Nile virus, which some biologists fear could thin an avian population that only recently was removed from the endangered species list.
Nationwide, six bald eagle deaths from West Nile have been reported this year, including one in the Springfield zoo. Observers say the deaths are far from an epidemic but enough to worry scientists, who in recent years have delighted in the bald eagle’s recovery.
“There aren’t that many bald eagles in the first place,” said research biologist Nicholas Komar, who works in the arbovirus disease branch of the U.S. Centers for Disease Control and Prevention in Ft. Collins, Colo. “I think it demonstrates the potential for a significant impact.”
Komar said the deaths that get reported to his office are probably the tip of the iceberg. Other bald eagles infected with West Nile probably died in remote areas and were eaten by prey or not found.
“It’s quite a concern,” said Terrence Ingram, executive director of the Eagle Nature Foundation in Apple River, Ill., who helps coordinate a bald eagle count every January. Ingram believes the number of young bald eagles will be down this year because of the virus, but he’s unsure what counters will find.
“We know of young ones being raised, but we don’t have any reports of them flying in Illinois,” he said.
Not everyone is worried about the impact of the virus on the eagles, and some biologists say initial reactions are overblown. Some observers along the Mississippi River expect a record number of bald eagles if a cold spell pushes the birds south from Canada.
Patrick Redig, a professor and director of the Raptor Center at the University of Minnesota, said he’s not concerned because his facility saw no bald eagles with West Nile this year.
And in Glen Haven, Wis., wildlife biologist Brett Mandernack said he has counted 3,200 bald eagles so far this season, about average for recent years.
“The numbers are still good,” Mandernack said.
The majestic birds have been on the rebound in the U.S. for nearly three decades since the federal government banned use of the pesticide DDT, which had pushed the national symbol to the brink of extinction.
By 1994, the population had recovered to the point that the U.S. Fish and Wildlife Service upgraded the bald eagle’s status from “endangered” to “threatened.” Five years later, Illinois followed suit, removing the bird from its endangered list.
Following the fish
Although bald eagles can be seen flying over all of Illinois, they have been particularly visible along the Mississippi, where eagles from Canada gather each winter in search of food. As waterways freeze to the north, the birds are pushed south, often following waterways because they offer an abundant supply of fish.
With the bald eagle population increasing, many towns along the Mississippi have tried to cash in with weekend celebrations, hoping to attract tourists for bird watches in winter months. Most celebrations are held in January.
Biologists also use the migration to make their annual counts of the birds, a process that was hamstrung last winter by relatively warm weather. Without a deep freeze, many bald eagles stay farther north, where waterways remain open and food is accessible.
Last winter Ingram’s group tallied 2,511 bald eagles in its 43rd annual midwinter bald eagle count, down from 3,101 in 2001, when conditions were better for migration.
Bald eagle counters are hoping for a cold snap this winter so they can get a better handle on what effect the West Nile virus has had on the population.
“The counts will start to give us an indication if anything meaningful is happening,” said Glen Kruse, endangered species program manager for the Illinois Department of Natural Resources. “The trend has been up consistently over the years.”
Kruse said he has not heard from anyone with expertise in bald eagles who has suggested there will be a population decline because of West Nile. Although the birds make an excellent target for carriers of the disease, mosquitoes, because they are large and often sedentary, there are other factors that could lessen the impact.
Mosquito season
A majority of bald eagles live farther north, where the mosquito season is shorter. And it is not clear whether West Nile has spread to parts of Canada where larger numbers of bald eagles roost.
Although the virus has been widespread in Illinois, most of the eagles that winter here arrive when the mosquito season is waning or has ended.
The West Nile virus killed 52 people in Illinois in 2002, according to the latest figures from the Illinois Department of Public Heath. Another 726 people were found to have the disease but survived.
The effects have been more disastrous on birds. Nationwide, birds from more than 150 species have been killed by the disease, from small songbirds to horned owls and other large raptors.
Downstate at the Henson Robinson Zoo in Springfield, a bald eagle died of West Nile on Sept. 4, one day before the zoo received a shipment of vaccines for the virus.
The 17-year-old bird, named Abe after Abraham Lincoln, had been at the zoo since 1988, after it was shot and injured by a hunter near Seattle. Zoo Director Talon Thornton said it showed no signs of illness before it died.
Another bald eagle in the exhibit, a female named Sushi, has shown no signs of the virus. Another bird at the zoo, a great horned owl, came down with West Nile but lived. Thornton said he hopes the zoo’s birds will have built immunity to West Nile by next year.
Biologists are cautiously optimistic about the national symbol.
“There’s just a lot of uncertainty,” Kruse said. “But I’m not aware of anything beyond watchfulness at this point.”
