The bald eagle population in the lower 48 states made such a strong recovery from its low point in 1963 that the bird was "delisted" as a federally endangered and threatened species in 2007.

But the eagle still is protected by law, and governmental agencies track population via two ongoing, nationwide counts that would alert them if there were an unusual change.

The counts are the Midwinter Bald Eagle Survey done under the auspices of the U.S. Army Corps of Engineers and tallies compiled by the U.S. Fish & Wildlife Service that focus on the number of nests occupied by breeding pairs, an indicator of replacement birds, Wade Eakle, of the Corps office in San Francisco, said.

The most recent summary of the Midwinter counts between 1986 and 2010 showed that, nationwide, bald eagle counts increased .6 percent per year over the 25-year period, he said.

In the early years, the increase was steeper, but leveled off as eagles became more numerous and reached the carrying capacity, or the maximum amount of available food and shelter, in their winter habitats. This is called "density-dependent population regulation."

Data for the years 2011-15 is expected to be available in the spring, Eakle said, adding he expects previous trends to continue. That would mean a continued increase of bald eagles in the northeast and northwest parts of the country and an ever-increasing decline in the southwest.

Reasons for the decline might include a changing climate in which the birds no longer travel so far south and less suitable habitat because of increasing human populations, he said.

In Iowa, the Midwinter count is tallied by the state's Department of Natural Resources and in Illinois by the state's Audubon Society. The DNR and society turn the numbers into the Corps.

The Midwinter count isn't perfect. As the report for 1986-2010 notes, "due to weather and staffing limitations, not all standard routes were surveyed every year." Some states did not begin standard surveys until the mid-1990s, while others have stopped. "The number of states participating each year has ranged from 25-42 and the number of standard survey routes per state ranged from 1-86."

The other count that keeps tabs on eagles is conducted under Fish & Wildlife, which is required to monitor at five-year intervals all recovered species that have been taken off the federal threatened/endangered list.

Publication of the agency's first survey was delayed until 2016, and work continues on its second with publication hoped-for in 2019, said Ryan Anthony, bald eagle biologist for Illinois, Iowa and Missouri.

But, "the population is doing just great," he said of eagles. "It has exploded. We are expecting it to continue to go up until 2020 and then peak out as it reaches its carrying capacity. That's the projection."

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