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Iowa’s bald eagle counts return to normal in 2023 after bird flu outbreaks in 2022
Four fledglings spotted in one Iowa nest is unusually high
Erin Jordan
Dec. 20, 2023 6:00 am, Updated: Dec. 20, 2023 12:42 pm
Bald eagle surveys in Iowa in 2023 showed a return to normal after 2022, the worst nesting season for many years because of bird flu, the Iowa Department of Natural Resources reported.
Volunteers in January 2023 counted more than 2,900 eagles along 1,663.5 miles of Iowa waterways, which works out to almost two eagles per river mile surveyed, the DNR reported. The most eagles were spotted on the Mississippi and Des Moines rivers but the highest density (eagles per mile) was on the Iowa River.
The share of the overall number that is immature eagles indicates population health. Since the Iowa survey started in 1991, that proportion has hovered around 30 percent and this year was consistent with that at 32 percent.
“The 2022 nesting season was unprecedented, at least in the history of this survey (since 2010) in how bad it was and there was a lot of uncertainty about whether the flu mainly impacted the eaglets or if we lost a lot of breeding adults,” said Stephanie Shepherd, a wildlife diversity biologist with the Iowa DNR. “So it was a huge relief for things to return to normal, indicating that it either didn’t kill too many adult birds or if it did, there were at least enough ‘unattached’ adults in the population to fill in.”
Bird flu affects bald eagles
The H5N1 strain of the bird flu virus has affected both domestic poultry and wild birds in Iowa.
About 16 million birds in commercial and backyard flocks were killed in 2022 after the bird flu was detected in their ranks, the Iowa Capital Dispatch reported. This caused the state’s annual egg production to fall from 15 billion in 2021 to 12.2 billion in 2022.
Although some wild birds can have H5N1 without showing signs of illness, eagles are more susceptible to the virus’s impact on the nervous system, researchers at the University of Georgia told USA Today.
For the first time in the survey’s history, the number of young bald eagles fledged per nest in Iowa dropped below the threshold of one in 2022, the Iowa DNR reported. Only an average of .88 young were fledged per Iowa nest in 2022.
Recovery observed in summer survey
But when volunteers went out from late winter to summer of this year to observe bald eagle nests in Iowa, they found an average 1.77 fledglings per nest, one of the highest rates in recent years, the DNR reported.
Of 329 monitored bald eagle nest sites in Iowa, 66 percent were successful in fledging an eaglet, 12 percent failed and the rest had an unknown outcome.
Usually bald eagles lay only one to three eggs per season, but one volunteer observed one Iowa nest with four eaglets.
“Eagles are what is called a K-selected species — i.e. their strategy is to have long lives, and invest a lot of energy into a few young to ensure high survival rates,” Shepherd said in an email.
The opposite would be rabbits, which live short lives, but produce many young.
“Basically, the eagles expend a lot of energy raising their eaglets and when the young leave the nest they are the size of the adults, so you can imagine how much food they need, which the adults must hunt and bring to them,” she said. “As such, the most common number of young in an eagle nest is two. Three is uncommon and four is unheard of! For example, out of 137 territories only 12 produced three young. So to produce four is a very impressive feat!”
Considering the rate of successful nests, the overall number of known active bald eagle territories and the average number of young produced per nest, a possible 720 young eagles were fledged by Iowa nests in 2023, the DNR reported.
Bald eagle fun facts
Wingspan: 6 to 7.5 feet
Weight: 8 to 15 pounds
Height: 3 to 3.5 feet tall
Eyesight: Five to six times more powerful than humans
Diet: Fish, waterfowl and carrion
Nesting season in Iowa: January through August, with most activity from February through July
Nest size: Seven feet across and 12 feet deep, weighing more than 2 tons
Number of young: Usually one to three eggs
Coloring: Bald eagles don’t develop the characteristic white head and tail of an adult until they are 4 to 5 years old.
Life span: 15 to 25 years in the wild
Source: Iowa Department of Natural Resources
Help monitor bald eagles or report a nest
The DNR will be training some new monitors for the 2024 nesting season.
“Generally Linn, Johnson and Polk counties are well covered so anyone in those areas that wants to get involved may have to travel to get to an unmonitored nest,” Shepherd said. Interested Iowans should keep an eye on the DNR’s volunteer wildlife monitoring website where the agency will post training workshop dates.
The DNR keeps a database of bald eagle nests and wants to keep it as up to date as possible. So if you see a nest, report the information to the DNR through a form on the website. The agency would like to know the exact location, whether the nest is being used by eagles, if young are present and the date of your observation.
Comments: (319) 339-3157; erin.jordan@thegazette.com