The Science of Harpy Eagles
WHAT ARE HARPY EAGLES?
The harpy eagle (Harpia harpyjia) is one of the largest and most powerful raptors in the world. Named for the fearsome harpies of Greek mythology, harpy eagles are top predators of the rainforests they call home.
A harpy eagle from the Brookfield Zoo, Illinois. Image by user Tomfriedel.
Harpy eagles are large, heavy birds, with a height of up to 3.5 feet (1 meter) and wingspans reaching nearly 7 feet (2.1 meters). Like many raptors, female harpy eagles are larger and heavier than males; females weigh an average of 15-20 pounds (7-9 kg), while males weigh an average of 11-18 pounds (5-8 kg). Most impressive are the harpy eagle’s talons, which can measure up to 5 inches (12.5 cm)!
Harpy eagles are largely black and gray in coloration, with dark wing and back feathers and lighter faces and bellies. The light undersides of a harpy eagle’s wings are barred (striped) with black. Harpy eagles have long gray tail feathers that are similarly striped. Their legs and feet are yellow, and their talons are black. Harpy eagles also have a “crest” of long, dark feathers around the head; these feathers usually lie flat, but can be pulled upright, making the bird appear even larger.
A harpy eagle with raised crest feathers. Image by Brian Gratwicke.
Harpy eagles are built more for agility than for long-distance soaring. Despite their impressive size, a harpy eagle’s wings are actually relatively short compared to their body. This, along with their long tail feathers, helps them to steer and navigate short bursts of flight between tree perches and through dense rainforest vegetation.
Much like owls, harpy eagles have a facial disk, which helps direct sound to the ears located on the sides of their head. (The harpy eagle’s crest may also help direct sound!) This is believed to be an adaptation for hunting in the rainforest, where harpy eagles depend on their hearing to locate and capture their prey.
WHERE DO HARPY EAGLES LIVE?
Harpy eagles have historically been found from southern Mexico, throughout Central America, and through South America to northern Argentina. However, the harpy eagle’s modern range may be much smaller, as the eagles’ population shrinks and stresses to its habitat continue to grow.
Harpy eagles are almost exclusively found in lowland (low-elevation) rainforests, and within the rainforests are most frequently found in the canopy (the uppermost area of the forest). Though they prefer unaltered rainforest, harpy eagles can tolerate disturbances as long as the majority of their habitat remains covered by trees. Harpy eagles are not found in areas with less than 50% forest cover.
Like many rainforest species, due to lack of information and inaccessible habitat, the harpy eagle’s exact population is hard to determine. The IUCN estimates a total global population of over 100,000 to as many as 225,000 adult birds, with between 7,600-15,000 adults found in Central America and a much larger population of 110,000 to perhaps over 280,000 in South America. (Population sizes are often given as estimates of adult, or sexually mature, individuals; a population count including young individuals, or juveniles, would be larger.) Other population estimates are more conservative, with an upper limit of only 50,000 individuals worldwide.
Panama, a harpy eagle at the Belize Zoo. Harpy eagles are now rare or endangered in much of Central America, including Belize; as his name references, Panama was brought to Belize from Panama City as part of the Belize Harpy Eagle Reintroduction Program, or BHERP, discussed below. Video by Inspire EdVentures.
In all cases, it is agreed that the harpy eagle population is decreasing throughout its range, though in some areas faster than others. Harpy eagles are believed to be rare, endangered, or even extinct over much of its former range, including large portions of Central America. The bird’s Central American population has a stronghold in Panama; the country may contain between 1,600 and 2,400 mature birds, though this could be an overestimate.
Harpy eagles are more common in South America, though even there their population is shrinking. They are most prevalent in the Amazon Rainforest of Brazil and surrounding countries, but are decreasing in other areas, such as Brazil’s southeastern Atlantic Forest.
WHAT DO HARPY EAGLES EAT?
Harpy eagles are top predators of the rainforest, with over 60 recorded species. These powerful birds hunt mainly in the rainforest canopy; they move quickly and agilely between the branches of rainforest trees, and use their powerful legs and massive talons to capture and dispatch their prey, which can weigh over half as much as the bird itself.
The talons of a harpy eagle, Panama, at the Belize Zoo. Image by Inspire EdVentures.
Harpy eagles mainly target arboreal (tree-living) species, especially arboreal mammals. These include sloths, monkeys, opossums, coatimundi, and anteaters, and occasional ground-dwelling mammals such as agoutis, armadillos, porcupines, foxes, peccaries, and even young deer. Harpy eagles also feed on large reptiles, such as iguanas and snakes, and other birds like parrots and the great curassow.
Despite the number of their prey species, harpy eagles are highly specialized hunters. In South America, especially in the Amazon Rainforest, sloths make up the bulk of a harpy eagle’s diet; in fact, a study of harpy eagle prey species in the Amazon Rainforest found that sloths provide up to 78% of a harpy eagle’s food. Monkey species- typically more social than sloths, and perhaps harder for eagles to hunt- make up another 11% of the eagle’s diet. (In areas where sloths are not found, such as northern Central America, the harpy eagle’s diet is more diverse, with monkeys believed to make up a greater portion of captured food.) Overall, as much as 98% of prey species recorded in this study spend at least part of their lives in trees.
A harpy eagle with its prey, a brown capuchin monkey. Image from “Tropical deforestation induces thresholds of reproductive viability and habitat suitability in Earth’s largest eagles.” doi.org/10.1038/s41598-021-92372-z
The harpy eagle’s status as a top predator makes it an essential part of the rainforest ecosystem. Like other predators, harpy eagles are responsible for keeping the ecosystem in balance by controlling prey populations. For these reasons, harpy eagles are often considered “keystone” or “umbrella” species, meaning a species whose presence is critical to maintaining the health of its ecosystem and whose conservation benefits the ecosystem as a whole.
Though harpy eagles are commonly believed to prey on livestock or domesticated animals, there is no evidence that they actually target livestock species. Harpy eagles greatly prefer their natural prey, and will pursue this prey as long as there is adequate rainforest cover to support a healthy population. However, as we will discuss later, decreasing rainforest habitat and fear of harpy eagles attacking livestock frequently leads to conflict with humans, which is often deadly for the eagles.
HOW DO HARPY EAGLES BEHAVE?
Harpy eagles are diurnal, meaning that they are most active during the day. They spend most of their time in the rainforest canopy, often perching on branches in search of their prey. Like many top predators, harpy eagles are territorial; their established territories can range from as small as 10 square km (3.8 square miles), recorded in Panama, to as large as 79 square km (30.5 square miles), recorded in Venezuela.
Harpy eagles pair for life and nest in some of the tallest trees in the rainforest, including the kapok or silk cotton tree (Ceiba pentandra), which can grow up to 230 feet (70 m), and the Brazil nut tree (Berthorelia excelsa), which can grow up to 165 feet (50 m). Nests are built of large sticks and branches, typically at heights above 130 feet (40 m), and may be as large as 7 feet (2 meters) across. Both the male and female eagle participate in building the nest; some pairs return to the same nest for every clutch, while others have several nests that they move between.
A kapok tree (Ceiba pentandra), one of several tree species used by harpy eagles as nesting sites. Image by Dick Culbert.
Female harpy eagles usually lay eggs during the rainy season, between April and January. The female typically lays two eggs, but only one will hatch; once the first egg hatches, the female will stop caring for the second egg. The eggs are incubated for 55 days, largely by the female, while the male parent hunts for himself, his mate, and eventually the hatched young.
Young harpy eagles remain in the nest for up to five or six months, fledge, or begin to fly. However, even after reaching adult size, the young eagle isn’t ready to set out on its own; instead, juveniles remain with their parents for up to two years, during which time the female will not lay any more eggs. When the juvenile is able to hunt and survive on its own, it leaves its parents’ territory, and the parents may begin the process again. Harpy eagles reach sexual maturity at four to five years of age, and may live for over 30 years.
ARE HARPY EAGLES ENDANGERED?
Harpy eagles are listed as Vulnerable by the IUCN Red List, with an overall decreasing population throughout their range. They are considered vulnerable in Ecuador and Venezuela, endangered in Bolivia, Paraguay, and Argentina, and critically endangered in Belize and Panama, and are believed extinct in El Salvador.
The main threats to the harpy eagle are habitat degradation and hunting. Over the next 60 years- approximately three harpy eagle generations- the IUCN Red List predicts that between 27% and 38% of rainforest cover may be lost within the harpy eagle’s current range, largely due to human-driven deforestation. During the same amount of time, the overall population of harpy eagles could decline by between 27% to as much as 57%, with the populations outside the Amazon Rainforest- especially those in Central America- as the most vulnerable to local extinction.
Causes of habitat loss in the harpy eagle’s range include agriculture, mining, dam-building, and road-building. Tree species that harpy eagles rely on for their nest sites, especially those in the Amazon Rainforest, are frequently removed for timber harvesting. Though harpy eagles can live near humans if enough rainforest remains intact, they depend heavily on mature rainforest to provide their prey and nesting sites.
Deforestation for agricultural use of the Amazon Rainforest in Colombia. Image by Matt Zimmerman.
In fact, the harpy eagle’s specialization makes them even more sensitive to habitat disturbance. Frequently, in cases of habitat loss and fragmentation, it is the ecosystem’s top predators that are first to feel the effects and first to disappear. Harpy eagles’ specific habitat and requirements and reliance on arboreal prey leaves them vulnerable if the ecosystem they rely on is altered or destroyed. Additionally, the harpy eagle’s slow reproductive rate, with only one progeny produced every two or three years at most, makes it difficult for their population to recover from injury even if the overall habitat remains undamaged.
Equally threatening to harpy eagles is human persecution. Though they are occasional victims of hunting or poaching, harpy eagles, like many other large raptors, are most frequently killed by humans who mistakenly believe the eagles are a threat to themselves or their livestock. Harpy eagles are not known to prey on livestock or domesticated animals; however, as their habitat decreases and contact with humans increases, more deadly altercations will likely occur.
HOW CAN WE HELP HARPY EAGLES?
Conserving rainforests will help harpy eagles by preserving their habitat and prey. Additionally, habitat corridors- areas linking expanses of suitable habitat- can be preserved to prevent fragmentation and ensure harpy eagle and other rainforest species populations remain connected. To protect harpy eagles, their prey species, nesting trees, and the overall rainforest ecosystem must all be protected as well.
Harpy eagle reintroduction programs have also been implemented in Central America, where the harpy eagle population is perhaps the most vulnerable. First started by The Peregrine Fund in Boise, Idaho in 1989, and later relocated to Panama City, Panama in 2001, a breeding program aimed to combat the eagle’s decline by releasing captive-bred harpy eagles into the rainforests of Panama and, later, Belize, in partnership with the Belize Zoo and Tropical Education Center through the Belize Harpy Eagle Restoration Program (BHERP). Overall, 49 harpy eagles- 39 captive-bred eagles and 10 wild eagles rehabilitated after illness or injury- were released in Panama and Belize.
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However, simply releasing captive-bred eagles into the rainforest was not enough to reinforce the population, as harpy eagles were- and are- still persecuted by humans in these areas. Of the 18 released harpy eagles who died of known causes, eight, or 44%, were killed by gunshot wounds, by far the leading cause of death among eagles in the study. Many of the rehabilitated wild eagles first came into the program’s care due to gunshot wounds as well.
In response, The Peregrine Fund and the Belize Zoo began an education campaign to dispel harmful myths about the harpy eagle and foster understanding of the eagle’s history and importance to the environment. Today, the Belize Zoo has two harpy eagle ambassadors, Panama and Daqueen, both former members of The Peregrine Fund reintroduction program who could not be reintroduced to the wild. Panama and Daqueen continue to help educate zoo visitors about the plight of Central America’s harpy eagles and the importance of their conservation.
Jamal Andrewin-Bohn (right), Belize Zoo Conservation Program Manager, explains how harpy eagles Panama and Daqueen came to the Belize Zoo. Panama and Daqueen were both part of the Belize Harpy Eagle Restoration Program (BHERP), which operated in Belize from 2003 to 2009. Video by Inspire EdVentures.
Interested in learning more about Belize’s harpy eagles? Learn more about our Belize Zoo virtual tours at Belize Zoo LIVE!
REFERENCES & FURTHER READING
Harpy Eagles at The Peregrine Fund
The Belize Harpy Eagle Restoration Program
Aguiar-Silva, F. Helena, Tânia M. Sanaiotti, Benjamim B. Luz; Food Habits of the Harpy Eagle, a Top Predator from the Amazonian Rainforest Canopy. Journal of Raptor Research 1 March 2014; 48 (1): 24–35. doi: https://doi.org/10.3356/JRR-13-00017.1
BirdLife International. 2021. Harpia harpyja. The IUCN Red List of Threatened Species 2021: e.T22695998A197957213. https://dx.doi.org/10.2305/IUCN.UK.2021-3.RLTS.T22695998A197957213.en.
Shaner, K. 2011. "Harpia harpyja" (On-line), Animal Diversity Web. https://animaldiversity.org/accounts/Harpia_harpyja/
Watson, Richard T., Christopher J.W. McClure, F. Hernán Vargas, J. Peter Jenny; Trial Restoration of the Harpy Eagle, a Large, Long-lived, Tropical Forest Raptor, in Panama and Belize. Journal of Raptor Research 1 March 2016; 50 (1): 3–22. doi: https://doi.org/10.3356/rapt-50-01-3-22.1