The bald eagle, a majestic bird of prey and the national symbol of the United States, was once on the brink of extinction. If you’re short on time, here’s a quick answer to why the eagle declined: Habitat loss, poisoning from pesticides and lead, and illegal hunting and trapping caused eagle populations to plummet dramatically in the 20th century.
In this comprehensive article, we will explore the key reasons behind the declining eagle population in recent decades. We’ll look at the historical context that led to low numbers in the 1960s, the enactment of protective laws and recovery programs, and ongoing threats that are impacting eagles today.
Overhunting and Early Population Declines
Widespread Hunting
Eagles have been hunted by humans for centuries. Their feathers, claws, and even body parts were prized for ceremonial and spiritual purposes. As European settlers came to North America in the 17th-19th centuries, they began hunting eagles intensely for their feathers, which were used in the popular hobby of featherwork to decorate hats and other items.
Unregulated hunting of eagles continued through the 19th century and early 20th century. Some surveys estimated annual kills of over 100,000 golden and bald eagles at the peak of hunting. The numbers declined rapidly, raising early concerns about extinction.
By the 1930s, there were only around 500 nesting pairs of bald eagles left.
Early Conservation Efforts
As eagles vanished from much of their traditional range, early conservationists began to advocate for protecting them. The 1918 Migratory Bird Treaty Act between the U.S., Canada, and Mexico was one of the first federal laws restricting the hunting and sale of eagles and their feathers.
However, these early laws had exceptions allowing Native Americans to continue traditional use of eagles.
More focused conservation efforts began in the 1940s-50s. Visionary biologist Aldo Leopold called for establishing National Wildlife Refuges across the country to protect habitat for bald eagles and other wildlife.
The first such refuge for bald eagles was established along the Missouri River in Montana.
Meanwhile, lifelong bird enthusiast Rosalie Edge purchased 700 acres of old-growth forest on Hawk Mountain in Pennsylvania to stop the shooting of migrating raptors. Her pioneering conservation effort led to the establishment of the Hawk Mountain Sanctuary, the world’s first refuge for birds of prey.
Pesticides and Environmental Contaminants
Effects of DDT
The widespread use of the pesticide DDT in the mid 20th century caused significant declines in eagle populations. DDT is a persistent organic pollutant that accumulates in the food chain. When eagles consume prey contaminated with high levels of DDT, the chemical interferes with calcium metabolism and causes their eggshells to become abnormally thin.
This results in reproductive failure as the fragile shells break during incubation or fail to protect the developing embryo.
By the 1970s, bald eagle populations had declined by as much as 80% in some regions. However, after the banning of DDT in 1972, eagle numbers began to recover as DDT levels in the environment decreased.
Today, while residual DDT contamination remains an issue in some locations, bald eagle populations have rebounded significantly thanks to this ban.
Lead Poisoning
Another threat comes from lead poisoning, either from ingesting lead shot embedded in carcasses left by hunters, or by eating prey containing lead fishing tackle. Lead accumulates in bones and tissues and impairs neurological development.
Symptoms of lead poisoning in eagles can include lethargy, loss of appetite, impaired vision and seizures. Without treatment, it can be fatal.
Outreach efforts have been effective at educating hunters and anglers about switching to non-lead ammunition and tackle. Many states have also enacted regulations restricting the use of lead shot in waterfowl hunting.
However, lead poisoning from ammunition fragments in carcasses and lost fishing gear remains an issue. Ongoing conservation efforts aim to minimize these lead exposure risks.
Habitat Destruction and Fragmentation
Logging and Development
One of the biggest threats facing eagles today is the loss and fragmentation of their natural habitats due to logging and development. As forests are cleared for timber or to make way for human infrastructure, eagles lose vital nesting and hunting grounds.
Eagles require large, contiguous forests with tall trees for nesting and roosting. However, logging can remove these crucial old-growth trees. One study found the probability of eagle nest occupancy declined from 79% to 27% as the distance from a nest to a forest edge created by logging decreased from over 1,000 meters to less than 100 meters (Baisner et al.
2011). Land development for housing, roads, pipelines, and energy projects also fragments and degrades eagle habitat.
For example, a recent study of golden eagles in Utah found that only 23% of nesting attempts in fragmented habitat were successful, compared to 63% in contiguous habitat (Smith et al. 2021). Habitat loss and fragmentation can also reduce prey availability and force eagles to hunt closer to human activity, increasing their risk of collisions with vehicles and power lines.
Loss of Nesting Sites
The decline of large, old trees has also deprived eagles of critical nesting locations. Eagles strongly prefer to build their massive nests on the limbs of sturdy, mature trees near open areas suitable for hunting. However, many old-growth forests have vanished across North America.
One 32-year study of bald eagle nesting in Arizona found that the average tree used for nesting declined from over 110 years old prior to 1994 to just 75 years old after 1994, indicating a dwindling supply of prime nest trees (Driscoll 2010).
Additionally, climate change and severe droughts have caused some large nest trees to weaken and collapse, leading to nesting failures.
Without abundant mature nest trees within undisturbed habitats, eagle populations cannot thrive. While eagles have shown some adaptability by nesting in man-made structures and smaller trees near human development, these may be poorer substitutes for natural sites.
Protecting remaining old-growth forests on public and private lands is crucial for eagles’ future.
Protection and Recovery
Legal Protections
The bald eagle has received various forms of legal protection over the years that have helped its numbers recover from the threat of extinction. Here are some of the key protections:
- The Bald and Golden Eagle Protection Act was passed in 1940, prohibiting the hunting, possession or commerce of bald eagles and golden eagles.
- The bald eagle was declared an endangered species in most of the U.S. in 1967 under the Endangered Species Preservation Act, with further protections added in 1973 under the Endangered Species Act.
- These laws made it illegal to disturb or kill eagles without a permit from the U.S. Fish and Wildlife Service.
- The bald eagle was upgraded to threatened status in 1995 and delisted from the Endangered Species List in 2007, as populations had sufficiently recovered.
These legal protections allowed bald eagle populations to bounce back from an estimated low of 417 breeding pairs in 1963 to over 9,700 breeding pairs today.
Captive Breeding and Reintroduction
In addition to legal protections, captive breeding and reintroduction programs have helped restore bald eagle populations across North America.
- Between the 1970s and 1990s, captive breeding programs collected eagle eggs from the wild to be hatched and reared in captivity, before releasing them back into the wild.
- Between 1976 and 1988, 124 bald eagles were reintroduced to the Channel Islands off the coast of California, helping reestablish a breeding population wiped out by DDT.
- Similar reintroduction programs took place in other states like New York, Oregon and Tennessee.
Today, the species has recovered enough that captive breeding programs are rarely needed anymore. However, such programs were crucial to helping the bald eagle bounce back in previous decades when populations were decimated.
Ongoing Threats
Climate Change
Climate change poses a significant threat to eagle populations. As average temperatures rise, eagle habitats are altered. For example, rising temperatures reduce water levels in lakes and rivers where eagles hunt for fish. Drought conditions also impact prey availability.
Eagles that nest in coastal areas face threats from rising sea levels and increased storm surges that can flood nests. Climate change leads to shifts in prey ranges, forcing eagles to travel farther to find food, expending extra energy.
Extreme heat waves may reduce eagle chick survival if parents cannot find enough food. Climate change also alters vegetation and allows southern competitors like turkey vultures to expand northward into eagle territories.
Collisions with Wind Turbines and Vehicles
Another major threat to eagles is collision with man-made structures. Wind turbines are a leading hazard, with hundreds of eagles killed annually in collisions. Eagles are especially vulnerable because they focus intently on the ground while hunting, often failing to notice turbine blades in their flight path.
Vehicles are another source of collisions, particularly in areas where roads traverse eagle habitats. Young eagles taking first flights are at high risk of vehicle strikes. The rise in wind energy development and expanded road systems increase collision risks.
Measures like shutting down turbines during peak migration times and modifying turbine sites can help reduce hazards.
Vehicle collisions specifically accounted for over 15% of bald eagle deaths from 2000-2013, according to research from the University of Wisconsin-Madison. The study analyzed over 2,500 eagle carcasses and found impacts were highest in the Upper Midwest states.
The authors recommended measures like wildlife crossings and modifying roadsides in key areas to make vehicles more visible to eagles.
According to the U.S. Fish and Wildlife Service, between 1997 and 2012 an estimated 14 golden eagles were killed annually by wind turbines across 10 western states. Newer estimates from 2013-2017 indicate as many as 154 golden eagles are killed at wind farms annually in the West.
The agency is working with wind energy companies to relocate and retrofit turbines to reduce risks.
Conclusion
The story of the bald eagle shows how concerted conservation efforts can pull a species back from the brink. While eagle populations have rebounded significantly thanks to habitat protections and reduced environmental toxins, they still face threats from climate change, wind turbines, habitat loss and more.
Continued vigilance is needed to ensure our national symbol continues to soar over landscapes across North America.
