By 1973, New Jersey's bald eagle population diminished to one known nest in the entire state.

It was hidden in Bear Swamp, an old-growth forest of American sweetgum and red maple trees in Cumberland County on the shores of Delaware Bay. At that time, biologists believed eagles could only survive in remote areas.

That's not the case, though, as the large predatory birds and national symbol are thriving in the Garden State, the most densely populated state in the country. They're even nesting on cell towers.

"You learn something new about them all the time," said Larissa Smith, a wildlife biologist with Conserve Wildlife Foundation in Trenton and co-author of the 2018 Bald Eagle Project, published jointly with the state's Division of Fish and Wildlife.

The annual project, released this month with data collected in the field by observers, reported 204 total nests, 185 were active with eggs.

"When I got involved there were 25 nesting pairs. The thought was that they needed to nest 1 ½ miles apart from each other, but that's not the case. It was also thought they couldn't nest in suburban areas, but they're nesting now closer to people than we thought," said Smith who began researching and monitoring eagles in 1982.

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Long-term data is incomplete, but David Wheeler, executive director of Conserve Wildlife Foundation, said the 2018 numbers are the highest number of nests and birds tallied in New Jersey since 1982, when the state began counting them.

"Their continuing recovery has been inspiring. The eagle is very adaptive," said Wheeler.

The main threat to bald eagles in New Jersey was the use of the pesticide dichloro-diphenyl-trichloroethane, commonly known as DDT. The pesticide killed insect pests, Wheeler said, but it also poisoned the food chain.

The DDT contamination made the eagles' eggshells too thin to withstand incubation.

After eggs failed to hatch in at least six consecutive years at Bear Swamp, biologists removed the egg for artificial incubation at Patuxent Wildlife Research Center in Maryland and fostered the young nestling back to the nest.

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Artificial incubation and fostering chicks continued with success until 1989, when the female of the pair did not return, and a new, younger female was able to hatch eggs without intervention.

The population began to rebound when DDT was banned in the United States in 1972.

The ban, combined with restoration and management efforts by the state's Endangered and Nongame Species Program, including the release of 60 young eagles to the state, resulted in population increases to 23 pairs by 2000, 48 pairs by 2005, 82 pairs by 2010, and 150 pairs by 2015.

Biologists are now wondering how many bald eagles New Jersey can house.

They're an apex predator, but they do have threats in the animal kingdom from nest robbing gray-horned owls and raccoons, said Smith. They're also territorial and will clash with each other.

"If the eagles are doing well then it's a sign the ecosystem is doing well because the food chain can support them," said Smith.

While the federally endangered designation was lifted on bald eagles in 2007, Smith said eagles are still considered endangered by the state of New Jersey.

Nesting season

Bald eagles start to pair up and lay eggs in late January. To view a nest, check out the live video feed at the Duke Farms Eagle Cam.

The Manasquan Reservoir Environmental Center in Howell also has an eagle scope to view nests.

Dan Radel: @danielradelapp; 732-643-4072; dradel@gannettnj.com