About 40 years ago, the bald eagle was declared an endangered species in the United States and was a rare sighting in Illinois, the Chicago Tribune reported.
Now, bird-watchers throughout the state have flocked to places like Starved Rock State Park, where five or six eagles were spotted on Thursday.
John Knoble, an Army Corps of Engineers supervisory park ranger in charge of natural resource management along the Mississippi River from Wisconsin to Missouri, said colder weather in northern states forces the eagles to move south in search of open waters, such as the Mississippi and Illinois rivers in Illinois.
"The colder it is and the longer it stays cold, the more eagles you see," Knoble said.
"We had 300 birds in one location in Fulton, Ill., this past weekend," Knoble said. "There was a day when we had over 1,000 birds there."
Army Corps park ranger Bob Petruney said eagles arrived at the Starved Rock lock and dam in December and will likely depart in March, and that about 150 to 200 people have been showing up on Saturdays and Sundays to see the birds.
"Sometimes, especially on weekends, they are knocking down the door before 9, and they are still here at 5," Petruney said. "Eagle numbers are up. And I'd say visitor numbers are definitely twice what they were 10 years ago."
Joe Kath, endangered species manager with the Illinois Department of Natural Resources, said that while the numbers of eagles currently in Illinois is impressive, the population will likely continue to grow, as there are a large number of breeding pairs of bald eagles in the state.
"The midwinter count directs people toward the sites where they can see the eagles," he said. "When it comes to recovery, it's based on the number of breeding pairs."