JWM: Vehicles, lead poisoning cause Michigan bald eagle deaths

An analysis of more than three decades of necropsy data show that vehicle collisions and lead poisoning are the main causes of death for Michigan bald eagles.

The Michigan Department of Natural Resources has been recording data on necropsy diagnoses for bald eagles (Haliaeetus leucocephalus) since 1986 as part of its larger bald eagle monitoring project.

“Not many states are doing that,” said Kendall Simon, a fish and wildlife biologist for the U.S. Fish and Wildlife Service and lead author of the study in the Journal of Wildlife Management. Simon, a TWS member, had been a part of this research since she was a graduate student at the University of Maryland.

After reviewing the necropsy data, Simon and her colleagues found trauma was the biggest contributor to bald eagle mortality, specifically vehicle collisions. “The population is increasing so much, we believe eagles are now using non-aquatic habitat closer to humans rather than along remote waterways,” she said, so instead of eating primarily fish, they are seeking other food sources, including roadkill.

Collisions occurred mostly in the fall when white-tailed deer (Odocoileus virginianus) are more active due to breeding and hunting season, as well as in February when waterways are frozen and there’s less aquatic prey available.

Simon and her colleagues also saw a spike in lead poisoning around hunting season, and again in the spring, when unretrieved deer carcasses become visible after snowmelt. Eagles are seriously affected by lead toxicosis after inadvertently consuming lead ammunition in those carcasses.

Knowing what times of year these deaths are more likely to occur can help counties or municipalities with mitigation, Simon said. “If they only have a certain amount of resources to contribute to mitigation,” she said, “they can target certain months.”

To mitigate bald eagle deaths, Simon and her colleagues suggest moving carcasses from the roadway and transitioning to non-lead ammunition. “I hope the number one takeaway from this will be the need to switch from lead-based ammunition to copper,” she said. “This affects a variety of scavengers, from crows, to vultures, to coyotes. It affects so many species other than bald eagles.”

Other less significant factors, such as diseases like West Nile virus and botulism, also affect the species, researchers found. “As climate change becomes more and more of an issue, that will be a big driver because disease vectors thrive in warmer climates,” Simon said.

This article features research that was published in a TWS peer-reviewed journal. Individual online access to all TWS journal articles is a benefit of membership. Join TWS now to read the latest in wildlife research.

Header Image: Since 1986, bald eagles have been primarily dying from vehicle collisions and lead ammunition.
©Joan Zeller Bonin