DETROIT (AP) — America’s iconic national bird, the bald eagle, has made an impressive comeback from its days as an endangered species. But its leading threats remain rooted in human activity, the most comprehensive study ever of bald eagle mortality in Michigan finds.

Researchers with the U.S. Fish and Wildlife Service, in conjunction with Michigan State University, the Michigan Department of Natural Resources and others, reviewed a huge trove of bald eagle mortality data collected from DNR necropsies — surgical examinations of eagle carcasses to determine causes of death — from 1986 to 2017. Almost 1,500 eagles’ causes of death were reviewed, according to the Detroit Free Press.

The results: The leading killer of bald eagles was vehicular trauma — being hit by cars. Second on the list was lead poisoning, related to eagles ingesting lead ammunition fragments from hunter-shot animals, or lead sinkers from fishermen.

“The bald eagle population in Michigan has made a tremendous comeback since the ban or phaseout of DDT and PCBs,” said U.S. Fish and Wildlife Service biologist Kendall Simon, a lead author on the study.

DDT was a widely used synthetic pesticide from the 1940s until 1972, when the federal government banned its use over its impact on the environment and threats to human health. Polychlorinated biphenyls, or PCBs, were primarily used as insulating fluids in heavy-duty electrical equipment in power plants, industries, and large buildings from the late 1920s until the late 1970s, when their use was phased out, also over environmental and public health concerns.

In 1961, bald eagle populations in Michigan were at their lowest point, only 52 breeding pairs. But the chemical bans and protection on the federal endangered species list prompted a major turnaround. The federal government removed the birds from the threatened and endangered species list in 2007. The last full aerial survey of bald eagles in Michigan, conducted in 2017, found 835 breeding pairs.

But with that progress over the latter 20th century came a great deal of human development, Simon said.

“The good habitat along the waterways is being filled,” she said. “New breeding pairs have to settle, breed and nest in lower-quality habitat. It’s usually inland, not along the water where they can fish.”

That leaves the bald eagles looking for alternative sources of food. And one of those main sources is roadkill, Simon said. The data showed vehicular trauma deaths increased in the fall, coinciding with deer hunting season and the rut, a breeding period for deer when they are more active and less careful. That’s also when most car-deer accidents happen.

“Eagles, especially bigger females, are a little clumsy taking off” from a roadkill carcass, Simon said. “It’s such a big bird, it takes them longer than people would think.”

The mortality data also showed lead poisoning was greatest in bald eagles in late winter and early spring months — when waterways are often iced over, hampering fishing, and the carcasses of deer shot by hunters but not recovered, preserved in the snow over winter, become a supplemental food source.

James Sikarskie, a professor emeritus retired last year from Michigan State University’s College of Veterinary Medicine — and Simon’s stepfather — is a co-author on the Michigan bald eagle cause of death study. Sikarskie worked with bald eagles for 44 years at the university, and as a wildlife clinician.

“If they eat enough lead, it will kill them, just like kids with lead paint,” he said. “Lead poisoning causes damage to the liver and kidneys, and the treatment to draw the toxin out, chelation, is also traumatic on them.”

The federal government banned the use of lead shot in waterfowl hunting nationwide in 1991. Lead ammunition is banned for some other types of hunting, in particular states and areas. But only California bans its use for all hunting statewide.

“All of the (Michigan) DNR’s hunting digests recommend the use of non-lead shot and bullets, so we encourage voluntary hunter support for that practice across the state,” agency spokesman Ed Golder said. “This is largely a hunter preference issue. One of the factors for hunters is cost. Non-lead ammunition typically costs more than lead ammunition, although copper bullets in many people’s opinions perform better than lead.”

The study, published in April in The Journal of Wildlife Management, recommends moving road-killed animal carcasses to the far edge of rights of way, and a further transition from lead ammunition and fishing tackle to nontoxic alternatives.