Learn about the bald eagle
Having faced near extinction in the 1970s, the bald eagle represents one of the most well-known examples of species recovery, leading to its removal from the IUCN red list (International Union for the Conservation of Nature). Historically abundant throughout the United States, the bald eagle became increasingly threatened due to the decline of its prey, as well as habitat degradation and destruction. In addition to this, these regal birds were not only a target for trophy hunters, but were disproportionally feared as a threat to livestock, and consequently illegally shot, trapped, and poisoned. Through this culling, population numbers dwindled until their federal protection in 1940 by the Bald Eagle Protection Act. Bald eagles are opportunistic feeders and thus forage on a wide variety of prey with fish, birds, reptiles, amphibians, and small mammals being some examples. They are also known to snatch prey from other predators and scavenge for carrion which may include larger game animals. Occasionally, when prey is caught from the water’s surface, the eagle is unable to lift itself and the prey back up into the air. In this event the eagle is able swim to shore with the prey held firmly in its talons. The bald eagle itself has few natural enemies, however its young are preyed on by other birds, and by predators such as foxes and wolves if their nests are within their reach.
Bald eagle fledglings are particularly vulnerable in their first year, with disease, lack of food, climatic conditions, and human induced factors such as collisions among their threats. Mortality rate during this time is almost 50%.
Fledglings’ feathers are a dark brown color unlike those of older, more mature individuals. It takes between four and 5 years to gain their distinctive white head and tail feathers and during this transition they closely resemble golden eagles. The bald eagle is North America’s sea eagle and thus can be found near coastlines, lakes, rivers, and estuaries as they require an open body of water for foraging as well as large trees or cliffs to nest in, and perch on. A strong nest site fidelity means that these eagles return to their breeding grounds and nesting territories, usually only leaving when seasonally necessary to follow food supplies.
As a top predator, eagles help in keeping the ecosystem in balance
by controlling the species populations of their prey in lower trophic levels. In discarding carcasses of their prey, they also enable nutrient transport between aquatic and terrestrial ecosystems, facilitating fertilization. The decline or loss of this species can provoke trophic cascades whereby the whole ecosystem can be affected through complex predator-prey interactions.
The position of these raptors high up in the food chain however, also makes them highly susceptible to environmental toxins that accumulate through the food chain via a process called biomagnification.
An example of this is the insecticide DDT (dichloro-diphenyl-trichloroethane) which was used widely after World War II. It was highly praised at the time for its efficiency in pest management, including the control of insect-transmitted diseases such as malaria and typhus. DDT belongs to a group of chlorinated hydrocarbons which are highly persistent in the environment and bioaccumulate within food chains. The effects of which took an unexpected toll on the environment, as the toxic chemical leaked into waterways and worked its way up into the food chain. This unfortunately means that top predators, such as the bald eagle, accumulate these toxins from their prey. The shells of bald eagle eggs were thinner and more fragile, resulting in breakages during incubation and offspring more likely to be killed by the weight of their parents even before hatching. This lead bald eagle population numbers to decline drastically to roughly 500 individuals in the wild. Bald eagles were not the only species affected by the insecticide – populations of a wide variety of other wildlife were decimated as a result. Ospreys, brown pelicans, robins, and peregrine falcons were among the unlucky species affected by this. After recognizing these effects, DDT was banned in 1972 in America, followed by Canada one year later. Population numbers had already dropped to the extent however that the bald eagle was officially declared endangered, and protected by the Endangered Species Act in 1978. In the years that followed,
conservation initiatives including habitat protection and restoration, captive breeding and reintroduction programs, as well as strict law enforcements, successfully helped rebuild their populations.
The species recovered and could be taken off the endangered species list by 2007. Despite this, many chemicals remain, and are continuously distributed in our environment. This includes trace amounts of DDT as well as other similar substances both old and new. Lead poisoning has in the past, and continues today, to threaten bald eagles when feeding on waterfowl shot down with lead shot. Raptors in particular are highly susceptible to environmental pollution, which is why they can provide early indication of reduced ecosystem health. Continuous monitoring and modelling of bald eagle populations and their effect on other trophic levels remains essential in controlling their long-term development and in detecting possible changes. While we were able to help this species recover, we might not be able to do so every time it is needed, especially considering the struggle biodiversity faces today. We are very quick to release waste substances into our environment, without giving it enough consideration.
It is easy to upset the natural balance of the environment, while conversely it is incredibly difficult to foresee effects in all aspects and interactions within the ecosystem.
Species Fun Fact
A bald eagle’s nest can weigh several hundred kilograms. They usually mate for life with the couple returning to the same nest each year, adding more material rather than building a new one. Considering they can live for up to 30 years in the wild, their nests can grow extremely heavy. The biggest bald eagle nest was found in Florida, weighing almost three tons!
For reference, or to find out more please explore the following:
Harvey, C.J., Good, T.P. and Pearson, S.F., 2012. Top–down influence of resident and overwintering bald eagles (Haliaeetus leucocephalus) in a model marine ecosystem. Canadian journal of zoology, 90(7), pp.903-914.
All information was collected and up to date in 2020.