As we near Independence Day, images of the bald eagle are becoming more and more prevalent. This serves as an apt metaphor for the last 30 years: Once endangered, the bald eagle’s recovery is described as one of the greatest success stories among endangered species.

As this ScienCentral News video reports, the bald eagle is back. But is it back for good?

An eagle relocation program

The word "hacking" conjures up images of grubby college students trying to break into computer systems they shouldn’t be accessing. But hacking is also the term conservationists use to describe releasing fostered young birds into the wild to populate the surrounding area.

Hacking has proven to be the most successful population recovery technique for the bald eagle, especially in areas where the population was virtually eliminated. The New York State Department of Environmental Conservation began the first North American bald eagle hacking project in 1976, after seeing the state’s bald eagle population plummet from over 70 breeding pairs before 1950 to one unproductive pair in 1960. The New York State Bald Eagle Restoration Project hacked its first pair of eagles June 27^th, 1976, at the Montezuma National Wildlife Refuge. The female, Agnes, began breeding in the area in 1980 when she reached maturity at four years old. Agnes’ success heralded the success of the hacking program. The program reached its goal of 10 breeding pairs in 1989, after transplanting 198 nestling eagles from Alaska, Wisconsin, Michigan and Minnesota.

The term is borrowed from the sport of falconry, where trainers use food to teach birds to return to a board (known as a hack) when given their freedom. Similarly, conservationists raise eaglets in a hacking tower, built high above the ground like a typical nest, then release them, hoping they will return to the area where they were once given food to build their own nests.

Hacking towers are enclosed, with at least two sides allowing views of the outside world. Often a lake, source of the birds’ diet of fish, is visible to the young birds. Inside the structure conservationists build a nest just as adult eagles would.

Caretakers feed the birds without making contact so the eagles do not lose their fear of humans or rely on them for food and become unable to hunt in the wild. Sometimes keepers use adult eagle puppets to put food in the cages; sometimes an unmanned trolley enters the cage bearing food.

The youngest birds face a graver danger: If birds younger than 6 weeks sense humans in any way, they identify and "imprint" themselves as human rather than eagle. These eaglets not only will be unable to hunt for food in the wild, but they also will be reluctant to mate with other eagles, effectively sterile.

When the eagles reach about 12 weeks of age, the conservators open the hacking cages and the birds take their first flight, called fledging. Eagles return off and on to the hacking tower for food over the next few weeks, but soon take off to live independently. Often they return to the hacking area to breed, signaling success of the repopulation efforts.

Experts estimate that bald eagles numbered anywhere from 50,000 to 200,000 in the lower 48 states when the Second Continental Congress designated the bald eagle the national symbol in 1782. Due to loss of habitat, hunting, and DDT poisoning, eagle numbers dipped to fewer than 1,000 in the 1960s, when the bird went on the Endangered Species list. While experts disagree on the best way to count the mobile eagles, today eagles probably number at least 20,000, with about 5,000 adult breeding pairs.