Fénix, a female Chaco eagle, lived at San Rafael Zoo in Mendoza province, Argentina, until the zoo shut down in 2016. She was later transferred to the Buenos Aires Ecopark, an old zoo that had been converted into a research center, hospital and shelter for animals without a destination. There, she began a difficult training program led by Andrés Capdevielle, the coordinator of the ecopark’s Birds of Prey Conservation and Rescue Program. In 2019, after a long and complicated process, Fénix had regained her natural hunting skills and was ready to be released.
A few months later, while she was still in a monitoring period after her release, she fell into a tank of water and drowned.
This video shows the release of Fénix, a rescued Chaco eagle that ultimately did not survive. Video courtesy of Buenos Aires Ecopark/Caburé-í Foundation
The lives of Chaco eagles, or crowned eagles (Buteogallus coronatus), are often touched by tragedy. Drownings, electrocutions, shootings, poisonings, and habitat loss have all pushed this little-known and understudied bird toward the brink of extinction. The species is categorized as endangered on the IUCN Red List, but population data estimates indicate no one is entirely certain of its status. Current population estimates range from 250 to 999 mature individual Chaco eagles in the wild — a range broad enough to offer different prognoses about the future of the species.
“The problem is that there is almost nobody researching this eagle,” says Diego Gallego García, a biologist at the University of the Basque Country in Spain and a fellow at the National Scientific and Technical Research Council (CONICET) of Argentina. “We do not know what is happening in Brazil, Bolivia, Paraguay, nor in a good part of northern Argentina, which is where it is distributed.”
Gallego, originally from Bilbao, Spain, lives in Santa Rosa, Argentina, and is also part of the Center for the Study and Conservation of Birds of Prey in Argentina (CECARA). He’s an expert on the obstacles that Chaco eagles face throughout their lives. “Reaching the age of adulthood for a crowned [Chaco] eagle is an almost impossible mission,” Gallego says.
There’s been a persistent lack of data on the behavior and biology of the Chaco eagle since it was first described in 1817. Researchers were until recently unable to even settle on a name. Known in parts of central Argentina as coronada (Spanish for “crowned”), its name in English has varied from “crowned eagle” to “solitary eagle” to now “Chaco eagle.” This most recent change aimed to differentiate it from the crowned eagle of sub-Saharan Africa (Stephanoaetus coronatus), a species that has been studied much more extensively and that may even be genetically related to the Chaco eagle. Yet even this latest agreed-on name is something of a misnomer: the Chaco eagle’s distribution doesn’t exactly overlap with the Gran Chaco, the subtropical plains that form one of the best-known geographic regions of South America outside the Amazon.
Most of the records, sightings and studies on the nesting and behavior of the Chaco eagle have taken place within western La Pampa and southeastern Mendoza provinces in Argentina, far from the heart of the Gran Chaco. However, for Hernán Casañas, executive director of the organization Aves Argentinas, this distinction is relative: “The mountainous region, the edges of the Andes mountains, and the spine that goes around the prairies in the plains of La Pampa can also be considered geographically and environmentally part of the Chaco,” he says. “The provinces of La Pampa, San Luis, and Mendoza, where people have observed the largest number of Chaco eagles, are all included in that area, and so it would not be inappropriate to call it the Chaco.” Indeed, the vegetation in these regions are continuous with the vegetation found in the arid Chaco region.
Understanding the diet of the Chaco eagle
How much is really known about this mysterious raptor? Not a lot, and yet “much more than when we began in 2001,” says José Sarasola, an ecologist, director of CECARA, researcher at CONICET, and the architect of the campaign to save the Chaco eagle from extinction. “We started from zero, blindly, without even knowing what it ate,” adds Sarasola, a recipient of the Whitley Award — known as the “Green Oscars” — in 2019 for his work with the Chaco eagle.
The diet of the Chaco eagle is far from a minor issue. For decades, rural residents were convinced that the raptor preyed on lambs and goats, which gave rise to a sense of hostility that often led to people shooting the birds.
“It cannot be said that this was never true,” Sarasola says. “During the 19th century, in La Pampa and part of the province of Buenos Aires, livestock was mostly sheep, but it was also pretty extensive; the animals were loose. It is easy to think that the eagles may have taken advantage of the birth of a lamb, weighing no more than a kilogram, to take back to their nests, as older people still describe today.”
The answer to the question of what the eagles really eat today came about through the use of camera traps in 2012.
“We could see what prey the parents brought to the eaglets, and of the 600 that we identified, none came from domestic livestock,” Sarasola says. “Explaining these images allowed us to change people’s perception of the eagle and helped us in our conservation project because facing the [agricultural] sector is always complicated.”
The observations made it possible for researchers to decipher the true dietary habits of the Chaco eagle, detailed by Juan José Maceda in one of the few scientific studies published about the topic. The prey of choice, it turns out, is the screaming hairy armadillo (Chaetophractus vellerosus). The Chaco eagle also eats other animals, including smaller birds, skunks, weasels, turtles, and various types of snakes. Occasionally, it feeds on carrion.
Based on this demonstrable evidence, and through talks at schools and with farmers and landowners, it was possible to drastically reduce the shooting of Chaco eagles.
“Since 2016, we have no evidence of deaths due to gunshots in La Pampa,” Gallego says.
“The current problem is hunting for big game in the area,” Sarasola says, “because many armed people come from outside the area, and sometimes, to calibrate their vision or see what animal something is, they shoot a crowned [Chaco] eagle.”
CECARA, working with local authorities, is finalizing details to place posters along highways in the region notifying people of the presence of Chaco eagles and requesting they take the necessary precautions to avoid hitting them.
More deaths than births
Still, Chaco eagles are being killed at a rate faster than new eagles are being hatched.
“The reproductive rate is very slow, and the mortality rate during the juvenile stage is too high,” Sarasola says. “If we do not work seriously, we’re talking about a species condemned to extinction.”
The reduction and fragmentation of their habitats from forest loss is one factor that experts say needs to be addressed.
“If [an eagle] has fewer places to nest, it has fewer options to reproduce,” Sarasola says. La Pampa still hosts large swaths of Espinal dry forest with large caldén trees (Prosopis caldenia) that the eagles like to nest in. “But the species would also need to be present in southern Córdoba and Santa Fe, whose environments have both been highly modified,” Sarasola adds.
The situation for the eagles in Santa Fe province is particularly dire: its current distribution there is 30% smaller than historically — the same figure by which the Espinal has also shrunk in the past century.
Chaco eagles reach sexual maturity at 4 years old, and each pair lays one egg per reproductive cycle, which is once per year. But the success rate is less than three in five, according to Chaco eagle expert Maximiliano Galmes in his doctoral thesis in 2007 — the first in-depth study of the species. Compounding the problem is that only an estimated 30% of eagles live to adulthood. Most die as eaglets or juveniles, usually due to unexpected events during incubation or after leaving the nest. These include falling prey to other species (mostly pumas and wildcats), premature death of the parents, fires, or storms that can affect the nest or the egg.
A very difficult species to study
In early 2020, a summer storm broke a branch of the caldén tree that was home to Ñankul, a very young eagle. The nest fell to the ground, but Ñankul survived. A resident reported the incident, leading to an emergency rescue. Members of CECARA, led by Gallego, arrived at the location, replaced the missing branch with a wooden pallet, reconstructed the nest on the pallet, and returned the eaglet to the nest, hoping its parents would not abandon it. “And it worked,” Gallego says. “They returned and continued raising it.”
Another tale of rescue comes from Telteca Natural Reserve in Mendoza. Park rangers there noticed a dead eaglet in a nest; two days later, they found another eaglet lying on the ground. “We quickly decided to put a transmitter on the one that had fallen and lift it into the nest of the dead eaglet,” says Buenos Aires Ecopark’s Capdevielle. “There was no prior record of adoption in that species, but the couple that had lost their eaglet adopted the new eaglet, which survived and was able to fly.”
Stories like that of Ñankul — which inspired a short film that was short-listed for a prize at the Latin American Nature Awards in 2020 — show the fragility of the Chaco eagle. At the same time, they serve to expand our knowledge of the species while also highlighting the difficulties of conservation efforts. In many cases, the species depends on the goodwill of spontaneous collaborators, as Chaco eagles often require individual care and monitoring.
Even finding a nest isn’t easy. “Someone needs to tell you” where it is, Sarasola says. It’s even more difficult to monitor the nest occupation dynamics of Chaco eagle pairs. “Each one has more than one nest in areas of 4 or 5 kilometers [about 2 to 3 miles],” Sarasola says. “There are some that can appear to be abandoned for years, and all of a sudden, the old owners return, or young eagles take advantage of them. One season, we might monitor 14 occupied nests, and the next, only nine. At times, it becomes very frustrating.”
“We have between 25 and 30 identified reproductive territories,” Gallego says, “and although many of them were never used again, we visit them each year.”
Despite the challenges, the researchers have persisted. And in 2012, they caught a big break with the incorporation of satellite transmitters that allowed them to follow the birds over prolonged periods of time. Until then, the research was more “artisanal,” conducted through captures, banding and genetic studies. “The reality is that in the past, we never saw a banded individual again. Today it is different,” Sarasola says.
Gallego is in charge of processing the data generated by the GPS units on the 10 birds that have been tagged to date. The data from recent years show that the adult eagle couples are relatively stable and tend to be territorial about their nesting range. They move around in an area of about 50 to 100 square kilometers (19 to 39 square miles) during the breeding season, but can go much farther away during the rest of the year.
Ramps to prevent drownings
A better understanding of the fates of individual Chaco eagles helps in the search for solutions to reduce the mortality rate. For example, the cylindrical water tanks, known as “Australian tanks,” that abound in this parched region often attract all kinds of wildlife, including Chaco eagles — but can quickly turn into death traps when the animals fall in and can’t climb back out.
“In La Pampa alone, where there are 25,000 of those tanks, it is estimated that 250,000 birds [of various species] drown per year,” Sarasola says.
“Crowned eagles are large raptors, around 1.85 meters [6 feet] in wingspan and just over 3 kilograms [6.6 pounds] in weight,” Gallego says. “[When they’re] wet, their wings double their weight and stop them from being able to get out of the water, and they can become exhausted and drown.”
Having understood the problem, the researchers engaged with landowners to outfit the tanks with a wire-mesh ramp — an inexpensive and easy addition — that floundering birds can use to pull themselves out of the water.
“Initially, we thought of them as a possible way for the birds to get out if they fall in. However, what they do is use them as a ladder to lower themselves to drink water without fear of falling,” Sarasola says. Since then, follow-up research has shown that the ramps can reduce the number of drownings by 50%.
Preventing the birds from being electrocuted when they perch on electricity lines is another challenge. It requires convincing utility companies to make design changes to the pylons, which are very common in rural fields, so that a Chaco eagle spreading its wings isn’t in danger of touching two lines at the same time. In the case of an earthed pylon, just touching one line can be deadly. In Mendoza, local electricity company Edeste agreed to replace the metal lightning rods on its pylons with surge arresters that would protect the power lines without having to be earthed.
“It was a joint task by the provincial government, the Caburé-í Foundation, the [Buenos Aires] Ecopark, and the people from Edeste over 1,500 kilometers [930 miles] of electrical lines,” Capdevielle says. “We have managed to save about 5 million hectares [12 million acres] of forest from this threat.”
He adds they hope to repeat this work in other areas where the Chaco eagle occurs.
An undefined range
The next goal for the CECARA researchers is to determine more accurately the current distribution of the Chaco eagle. Most sightings to date, including of nests being monitored, are in La Pampa. The rest are spread out between the nearby provinces of Mendoza and San Luis. But with the satellite tags, it may now be possible to determine the exact locations of some of the breeding eagle pairs in the Chaco. At the very least, knowing this would help resolve the issue of the species’ name.
“If the pandemic allows it, in June we are going to western Santa Fe and eastern Santiago del Estero, in the sub-meridional lowlands of the Gran Chaco,” Gallego says. Funding for the project will come from a grant that Gallego received from the Rufford Foundation in the U.K. “The idea is to start working from scratch in that area, with talks in schools and with rural farmers and associations to spread knowledge of the species,” he says. Afterward, the researchers will search for nests, monitor them, band the eagles, and install transmitters on the newly hatched eaglets. They will also install rescue ramps wherever they’re needed.
The mission to save the Chaco eagle will not be easy, they acknowledge. Losing the species would be a very hard blow to the biodiversity in the areas where it occurs. “It is an apex predator and has a role as important as the jaguar or large carnivores, which, if they are removed from food chains, create crucial imbalances,” says Casañas from Aves Argentinas.
A monument in the main square of Santa Rosa, the capital of La Pampa, highlights the importance of the Chaco eagle, but the bird continues to be a great unknown — even for much of the scientific community. Yet even with so many factors weighing against the eagle’s survival, so far it has managed to resist extinction — thanks in no small part to the efforts of a dedicated band of researchers over the past 20 years.
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