Bald Eagle: Builder of the Biggest Tree Nests

The bald eagle is the national bird of the United States, a large sea eagle native to North America, and one of the most thoroughly studied raptors in the world. Despite its name, Haliaeetus leucocephalus is not actually bald.

The head and tail feathers of an adult are pure white, and the word "bald" in the species' English name comes from an older sense of the word meaning "white-headed" rather than "hairless." Everything else about the bird is dark chocolate brown, which makes the contrast striking at distance and has kept the species at the centre of American iconography for more than two centuries.

This entry covers the species in full: taxonomy, anatomy, vision, flight, hunting, diet, reproduction, nest structure, lifespan, conservation history and the long list of strange details that separate the real bird from its national-symbol mythology. It is a reference article, not a summary, so expect specifics: wingspans, dive speeds, nest weights, population counts, dates and records.

Etymology and Classification

The scientific name Haliaeetus leucocephalus was given to the species by Carl Linnaeus in 1766. It combines Greek roots: hali (sea), aetos (eagle), leukos (white), and kephale (head). The literal translation is "white-headed sea eagle," which is exactly what the bird is.

Linnaeus placed the bald eagle in the genus Haliaeetus alongside the white-tailed eagle of Eurasia and several related species, and molecular work has confirmed the grouping. The bald eagle's closest living relative is the white-tailed eagle ( Haliaeetus albicilla ), with which it forms a species pair that fills the same ecological niche in Eurasia.

There are two recognised subspecies, which may be the same species differing in size according to Bergmann's rule:

• Haliaeetus leucocephalus leucocephalus : the southern subspecies, found in the southern United States and the Baja California peninsula.
• Haliaeetus leucocephalus washingtoniensis : the northern subspecies, larger-bodied, found in the northern United States, Canada and Alaska.

The bald eagle was adopted as the national emblem of the United States on 20 June 1782, when the Second Continental Congress approved the design of the Great Seal. The bird appears on the seal, on the presidential flag, on military insignia and on US currency including the one-dollar bill and the quarter.

Benjamin Franklin objected to the choice in a well-known 1784 letter to his daughter, arguing that the bald eagle was "a bird of bad moral character" because it scavenges carrion and steals fish from ospreys. Franklin preferred the wild turkey.

Size and Physical Description

The bald eagle is a large raptor but not the largest. Its body measures 70-102 cm from beak to tail. Wingspan ranges from 1.8 to 2.3 metres, with exceptional Alaskan females reaching 2.4 metres. Weight varies from 3 to 6.3 kg and tracks latitude, with larger birds further from the equator (Bergmann's rule). Females are about 25% larger than males, a pattern called reversed sexual dimorphism that is standard among hawks, eagles and falcons.

Males:

• Average weight: about 4.1 kg

Females:

• Average weight: up to about 5.6 kg

Chicks at hatching:

• Chicks hatch covered in down

Adult plumage is unmistakable: chocolate brown body and wings, white head, white tail, bright yellow beak, yellow legs and yellow feet with black talons. The iris is bright yellow. Juveniles, however, look nothing like the adults.

For the first four to five years of life they wear a mottled brown and white plumage with a dark head, which causes constant confusion with golden eagles and with other large Haliaeetus species. Full adult plumage appears around the fifth year (rarely the fourth), when the bird reaches sexual maturity.

The beak is proportionally massive, a curved yellow hook used for tearing flesh rather than for killing. The killing work is done by the talons. Bald eagle grip strength is roughly ten times that of an adult human hand. The rear talon (hallux) is especially long and is driven into prey to pin it down while the front talons hold.

Vision and Sensory World

Bald eagles have exceptionally sharp eyesight. They can spot prey from great distances and track small fish moving just below the surface of a lake from high above.

Several anatomical features drive this performance:

• Two foveae per eye. The fovea is the retinal region of highest visual acuity. Humans have one per eye. Bald eagles have two, a central fovea for straight-ahead detail and a temporal fovea angled forward and downward. The temporal fovea functions like a built-in telephoto lens, giving the bird the ability to track a distant object even while cruising across a wide field of view.

Bald eagles hear roughly in the human range, with no exceptional adaptation for extreme frequencies. Smell is limited and plays almost no role in hunting, which is an important distinction from vultures and some other carrion-eating birds.

Flight and Dive Performance

Bald eagles are soaring birds. They reach 56-70 km/h in gliding and flapping flight, riding thermals and updrafts and about 48 km/h while carrying fish. Their dive speed is 120-160 km/h, though they seldom dive vertically. This is slower than a peregrine falcon's stoop.

Flight data:

Metric	Value
Gliding and flapping speed	56-70 km/h
Speed while carrying fish	about 48 km/h
Stoop / dive speed	120-160 km/h

Wings are broad and long with deeply slotted primary feathers, the "fingers" at the tip, which give the bird fine control during slow flight and reduce induced drag at low speed. The broad wing plan favours efficiency in soaring over raw speed, which is why bald eagles spend most of their active hours on thermal updrafts along coastlines, ridgelines and river valleys.

A bald eagle that hooks a fish too heavy to lift may be dragged into the water. Bald eagles can swim and often drag the catch ashore with their talons, though some eagles drown or succumb to hypothermia.

Hunting and Diet

Bald eagles are opportunistic carnivores with a strong preference for fish. Fish make up most of the diet in many populations, with the exact proportion varying by season, region and local prey abundance: across 20 food habit studies fish made up 56% of the diet of nesting eagles, and in the Columbia River Estuary in Oregon about 90%. In the Pacific Northwest, spawning trout and salmon provide most of the diet from late summer through autumn.

Primary prey:

• Fish. Include salmon, trout, herring, shad, catfish, carp and smaller species. They are taken either live from the water surface or scavenged from spawning die-offs.
• Waterfowl. Ducks, geese, coots, gulls. Eagles may attack birds in flight, up to the size of Canada geese.
• Small mammals. Rabbits, muskrats, prairie dogs, ground squirrels and occasionally young of larger mammals.
• Reptiles. Turtles and snakes in warmer latitudes.
• Carrion. Road-killed deer, livestock carcasses, beached fish and washed-up marine mammals.

Hunting techniques:

1. Perch-and-wait. The bird sits on a tall tree or cliff near water and watches. When a fish or small bird moves into striking range, the eagle drops into a stoop and snatches the prey in its talons at the end of the dive.
2. Low soaring. The eagle cruises over open water, watching for fish near the surface. When it spots prey it banks into a shallow dive and extends its feet to snatch the fish, often without fully submerging.
3. Kleptoparasitism. Bald eagles are unusually willing to steal food from other birds. Bald eagles steal prey from other predators, and ospreys are a well-known target: the eagle harasses the osprey in mid-air until it drops the fish.
4. Cooperative harassment. Paired eagles sometimes hunt large prey such as jackrabbits or herons together, with one bird driving prey while the other strikes. This is less common than solo hunting.

Eagles in Washington need about 489 grams of fish per day to survive. When food is abundant, an eagle can store up to 1 kg of food in its crop, which allows it to fast for several days if food becomes unavailable.

Life Cycle and Reproduction

Bald eagles pair for life, with divorce occurring mainly after repeated breeding failure. Pair bonding is reinforced with spectacular aerial displays. The best known is the cartwheel display: the pair flies high, locks talons mid-air and falls toward the ground in a tumbling cartwheel, breaking apart only seconds before impact. The display appears during courtship and is also used to resolve territorial disputes between rival birds.

Breeding cycle (varies by latitude):

• Egg laying: typically 1-3 eggs, most commonly 2
• Incubation: about 35 days

Both parents incubate. Hatching is asynchronous, which means the first chick is several days older and larger than the second. In lean years the older chick may kill the younger, a pattern known as siblicide common among eagles.

Chicks develop quickly on a diet of fish and small mammals delivered by both parents. After fledging, young birds depend on their parents for a period while they learn to hunt. Many young eagles fail in their first winter, with starvation, collisions and electrocution as the main killers.

Sexual maturity arrives at age 4 or 5, when the white head and tail appear.

The Largest Tree Nests in the World

The bald eagle builds the largest tree nests of any bird on Earth. Nests can reach up to 2.5 metres across, 4 metres deep and about 1 tonne in weight, the largest tree nests recorded for any animal species. Pairs return to the same nest year after year, adding fresh sticks, grass, moss and softer material for lining. Over decades the structure grows into an enormous platform that can dominate its host tree.

Nest records and averages:

Metric	Value
Maximum typical diameter	up to 2.5 m
Maximum typical depth	up to 4 m
Maximum typical weight	about 1,000 kg
Largest verified (diameter)	2.9 m
Largest verified (depth)	6.1 m
Year of record nest	1963
Location of record nest	Florida
Typical host tree	Tall conifer or hardwood, including cottonwood

Nest trees are chosen for height, structural strength and visibility. Bald eagles prefer the tallest tree in the area with an unobstructed approach and good sightlines. In treeless regions they will nest on cliff ledges, on tall man-made structures, or, rarely, on the ground. Excessive nest weight eventually causes structural failure: some storied nests have collapsed their host trees during storms, and birds build a replacement nearby.

Territories around active nests are defended against other eagles. Territorial disputes are noisy and can include the cartwheel display, chases and occasional talon-locked fights to exhaustion.

Distribution, Populations, and Migration

Bald eagles are native only to North America. Their range includes most of Canada and Alaska, all of the contiguous United States and northern Mexico. They are usually found near open water: coastlines, large rivers, reservoirs and productive wetlands.

Population by region (approximate):

Region	Approximate adult population
Squamish, British Columbia (winter)	1,000-2,000 wintering birds (November to February)
Contiguous United States	~316,000 total (2020 USFWS estimate)
Canada	Majority along the British Columbia coast, with large populations in Alberta, Saskatchewan, Manitoba and Ontario
Northern Mexico	Small, localised

Migration patterns depend on latitude and ice. Northern populations move south when lakes and rivers freeze, because open water is required for fishing. Alaskan and Canadian birds may winter in the northwestern states or along the Pacific coast. Southern populations are year-round residents and may even move north during summer for cooler conditions.

Migrating eagles follow ridgelines, coastlines and river valleys using thermal and ridge lift to minimise flapping. Concentrations of one to two thousand birds winter at Squamish, British Columbia, between November and February, attracted by spawning salmon.

Conservation History and Current Status

The bald eagle's conservation history is one of the most-cited recovery stories in modern wildlife management. In 1782, when the species was adopted as the national emblem, the United States had approximately 100,000 nesting eagles. By the mid-twentieth century the population had collapsed.

Causes of the decline:

• DDT. The pesticide dichloro-diphenyl-trichloroethane came into heavy agricultural use after 1945. DDT and its metabolite DDE accumulate in fish and are concentrated in the tissues of fish-eating predators. DDE disrupts calcium metabolism in female birds, which causes eggshells to thin. Bald eagle eggs broke under the weight of the incubating female, and reproductive success collapsed.
• Shooting. Bald eagles were shot as perceived competitors for game fish and waterfowl, and for bounty in some states until the mid-twentieth century.
• Habitat loss. Logging of tall shoreline trees destroyed nesting habitat. Damming changed river ecology and fish availability.
• Lead poisoning. Eagles that scavenge gut piles from hunters ingest lead fragments from rifle ammunition. Lead poisoning continues to kill bald eagles today and is one of the few remaining significant mortality causes.

By 1963 the breeding population in the contiguous 48 states had fallen to roughly 487 pairs. The species was on the verge of extinction in the lower 48. A series of interventions followed:

• 1940: Bald and Golden Eagle Protection Act prohibits killing or possession of the species in the United States.
• 1963: Low point of roughly 487 breeding pairs in the lower 48.
• 1972: DDT banned for most agricultural uses in the United States.
• 1973: Endangered Species Act signed into US law.
• 1978: Bald eagle listed as Endangered across most of the lower 48 and Threatened in a few states.
• 1995. The species was downgraded from Endangered to Threatened across the lower 48 states.
• 2007. The bald eagle was taken off the US Endangered Species Act list once recovery targets were met.
• 2020: US Fish and Wildlife Service announces a population of more than 316,000 individuals including roughly 71,000 breeding pairs in the lower 48.

The IUCN Red List classifies the bald eagle as Least Concern with an increasing population trend. The species is still protected under the Bald and Golden Eagle Protection Act, the Migratory Bird Treaty Act and comparable Canadian and Mexican laws. Possession of any part of a bald eagle, including feathers found on the ground, is illegal in the United States without specific federal permits, and the Act provides criminal penalties of fines and imprisonment.

Current threats include lead poisoning, collisions with wind turbines, electrocution on power lines, vehicle strikes, illegal shooting and localised habitat loss. None of these threats approach the historical scale of DDT, but several are being monitored as potential risks if unchecked.

Bald Eagles and Humans

The bald eagle has held central symbolic importance in many Indigenous North American cultures for thousands of years, longer, by orders of magnitude, than it has been a US national symbol. Eagle feathers are sacred in many nations and are used in ceremonies, regalia and spiritual practice.

The US National Eagle Repository distributes feathers and parts from naturally deceased birds to enrolled members of federally recognised tribes for ceremonial use, which is the legal channel that reconciles federal protection law with Indigenous religious rights.

Benjamin Franklin's 1784 letter to his daughter Sarah complained that the bald eagle was "a bird of bad moral character" because it scavenges and steals. Modern biology confirms Franklin's observations while rejecting his moral framing: bald eagles are opportunistic carnivores that scavenge heavily and kleptoparasitise smaller birds whenever opportunity allows.

These behaviours are energy-efficient and entirely appropriate for a large soaring raptor. Franklin preferred the wild turkey, calling it "a much more respectable bird," but the bald eagle had already been adopted on the Great Seal two years earlier.

Bald eagle tourism now generates meaningful revenue in places like Homer, Alaska, Squamish, British Columbia, and the upper Mississippi River during winter. Controlled viewing sites let visitors observe concentrations of eagles without disturbing birds. Livestreamed nest cameras at sites such as the Decorah, Iowa and Hanover, Pennsylvania nests have become a significant public-engagement tool for conservation.

Human-eagle conflict exists but is limited. Bald eagles occasionally take domestic fowl, small lambs and pet-sized animals, which provokes complaints in rural areas. Fatal attacks on humans are essentially unknown, though eagles will defend nests aggressively against people who climb too close.