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Harpy Eagles: Successful Hunters..!
José de Jesús Vargas González — in Harpy Eagle Conservation and Research    Share

Harpy Eagles: Successful Hunters...!

After a failure, there is always a learning lesson, and that was our lesson during the last trip in Darien. We had two weeks of heavy rain, strong winds, and high humidity. But this is Darien, the habitat of the Harpy Eagle, during the raining season! However, this is the more attractive season here in Panama because the forest shows different shades of colors and it is possible to observe a high biodiversity of birds, insects, and mammals. Despite the muddy roads, lots of mosquitoes and flooded streams, we spent spectacular days learning from our flagship species: the Harpy Eagle.

Small stream close to the Harpy Eagle nest

As I mentioned it took15 days, and our efforts to capture the juvenile Harpy Eagle from the nest located five hours from the Embera community of Llano Bonito were unsuccessful. We could not capture it because adults brought preys twice, and because by itself, this juvenile captured a Howler Monkey. Without mentioning the bad weather! The first event of feeding was done by the female, who brought the hind legs of a sub-adult Howler Monkey. "It was amazing to watch this female soared from the top of a mountain with the prey in her talons." The second event of feeding was done by the male, who provided a Three-Toed Sloth (possibly juvenile) to his juvenile. These observations let us infer that this young female Harpy Eagle has been well fed by their parents for the last 19 months of life, her current age.

During the time that we were waiting to capture the juvenile, we saw two very important and interesting events about feeding and hunting behavior of wild Harpy Eagles. The first event was the arrival of the adult male Harpy Eagle with Three-Toed Sloth that was still alive; and the second event was the foraging, observation, lurking and capture behavior of the juvenile Harpy Eagle.

The first observation occurred on June 12 at 09:00 hours, when this juvenile started to become interested in our trap, which was already set up. That morning, the juvenile vocalizations, and the subsequent response from another eagle, alerted us that one adult is arriving to the nest, probably with prey. Our experience observing the behavior of juveniles in their nests let us infer when an adult is approaching and sometimes it is possible to know if this is the female or the male.

Adult male Harpy Eagle leaving the nest tree

The juvenile flew to the platform that make up the branches of the Cuipo tree (where the nest was located), took a relaxed position (bent back, relaxed wings, lifted the head and shrank the neck) and began to vocalize. This behavior warned us, and we immediately began observing and listening more carefully. A few minutes later we heard a shrill whistle and strong vocals (similar to “crack, crack, crack…”).

Our prediction was accurate because almost immediately Rutilio (technician of the project) saw an adult male perched in a tree called Espavé (this tree is located few meters from the tree-nest). Due to the dense forest of the canopy, we could not see the prey that he brought in its claws. Few minutes later, he flew to the platform where the nest was and delivered the prey, went away, and perched in a nearby branch of the same tree. The juvenile immediately hijacked the prey, and opened more its wings. At this time, both individuals starting to vocalize, and took body positions somewhat similar to those described above.

The adult male did not stay a long time on the branch of the tree-nest (about 5 minutes), and then jumped into the void, and flew off into the dense vegetation of the area. Meanwhile, the juvenile was looking where the adult had moved, and continued vocalizing (whistles treble for around 15 minutes). These vocalizations performed in ranges of: 5 to 6 vocalizations - silent - 5 to 6 vocalizations, and so on. Then, the juvenile shook her claw and with the prey flew to a huge branch of the nest-tree. In this moment, we could observe that the sloth was moving, so it was alive. This behavior of bringing live prey by the adults to the juveniles was also observed in other nests.

Female juvenile Harpy Eagle camouflage in the canopy of the forest

On the branch we could see how the juvenile loosened and grabbed continually the sloth. It was a kind of practice, from there she flew to the platform again, and let the prey nearby, and began to kick the leg consecutively towards the prey, as well as she was hunting. From our position, it was not possible to see many details of how she held the prey. This behavior lasted about 20 minutes. At the end, the sloth was already dead. Then, she moved to a branch and began to clean and eat the prey. It was interesting to see how adults teach their juvenile to hunt, kill and clean food. This is perhaps a basic training necessary that all juvenile Harpy Eagles received from adults to be successful hunters. In natural environments the "survival of the fittest, the law of the jungle" determines whether an individual survives or perishes, whether for food, competition inter-or intra specific or natural events that may occur in their environment. Perhaps the basis of this training is founded on ensuring that the breeding effort results in the production of a juvenile that can adapt more easily to the environment. .

The second interesting observation we had the opportunity to witness was the hunting of a young Howler Monkey by the juvenile Harpy Eagle that we wanted to capture. This hunting event occurred in June 16 at 14:00. This day, the observations began at 07:00 am, when we left the camping area to set the trap (close to the nest-tree). This morning, we did not locate the juvenile at the nest tree. Because of that, we began to search in the vicinity. So the hours passed without success. At approximately 11:00 hours, we decided to continue the search following the suggestion of Indalecio (technician of the project). He mentioned that we should look in the direction where we heard vocalizations of a troop of Howler Monkeys. Because we were determined to exhaust all options to find this eaglet, we set out a new search, now to find the Howler Monkeys troop. This search was not very long because Howlers are monkeys that do not move much during the day. At 11:45 hours, we found the group of monkeys resting and feeding on top of a tree called Zorro. It was a relatively large troop of monkeys (roughly 20 individuals between males and adult females without small offspring that remain with their mothers).

Minutes passed, and we were thinking to stop searching in that region, when suddenly we heard a vocalization of a Harpy Eagle. It was the juvenile who was perched 20 meters high, well camouflaged in the dense cover provided by an Espavé tree. The juvenile was about 30 meters from the tree where the Howler Monkeys were.

When the juvenile Harpy Eagle vocalized, the monkeys began to make alarm calls. We observed three Howler Monkeys males approached, vocalized very aggressive and looked in the direction of the eaglet, while the other members of the group descended from the forest canopy. At that time we think that something interesting might happen. After 25 minutes of only observation by the juvenile, everything calmed down. Howler Monkeys stopped vocalizing, and took up resting positions (lying and leaning on the branches in the middle strata of the forest). The eaglet also took a relaxed position. She relaxes the right leg, and held only with one leg. Because nothing happen, we thought that this juvenile were only curious with this troop of monkeys. However, she kept watching the monkeys, shook his head from side to side, rotating and lifting the facial disc of feathers, and sometimes groomed, but without losing sight of the monkeys.

Adult of Howler Monkey making alarm calls

Another 15 minutes passed, when suddenly the juvenile Harpy Eagle changed her relaxed position, and began a rapid flight in direction of the Howler Monkeys. From our location in the understory, we could only watch how the juvenile was flying into the vegetation of the canopy and how the monkeys fled and emitted strong vocalizations.

We immediately moved to see what has happened. Indeed, this eaglet had been successful in her capture. Between her claws we saw a young Howler Monkey. She was on a thick branch at 15 meters high. With the binoculars, we watched only the head and arms of the monkey. We could determine that the monkey was dead at this moment (only five minutes passed since the juvenile flew, until we found it). This juvenile female Harpy Eagle had been successful in her capture, and with a “gallant” position she stood on her prey in turn she was observing the other monkeys move far away. Some adult males remained staring and vocalizing very strongly. Perhaps with the intention of intimidating, but this objective failed...!

The monkeys went away very quickly from the hunting site, and after that, the juvenile Harpy Eagle set out to clean up its prey. This behavior is part of what she learned from her parents, when they delivered live prey at the nest. Did not take long to clean up her prey, and after she moved to the nest tree. At the top of the Cuipo tree she perched and began feeding. At that time, we were ready to return to our camping site, because obviously she is going to ignore our trap.

Technicians observing the behavior of the juvenile Harpy Eagle

That evening, we talked about the successful prey capture by the juvenile, and that’s when Rutilio and Indalecio mentioned that they were watching the Howler Monkeys when the Harpy Eagle flew to hunt. They indicated that young monkeys were some distance from their mothers, and perhaps that moment of parental negligence, was expected by the juvenile Harpy Eagle to hunt, because she flew at this exactly moment. This is an expected behavior in opportunistic species, when the capture effort has to be effective to balance the energy investment in the hunting process. Obviously, a young Howler Monkey is much easier and less risky to hunt, that an adult who can defend and cause damage or even the death of the hunter, in this case to the juvenile Harpy Eagle.

These were two extremely attractive behaviors that we observed in this field trip: hunting instruction and successful hunt. Nevertheless, we left the area without success in the capture and radio tagging of this female juvenile Harpy Eagle. But, on the other hand, we left happy because we learned more about the behavior of this species. Behavior that results in relevant data for our research, as well as to educate local community members! That is why we think that Harpy Eagles are successful hunters..!

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