Two Radio-equipped Harpy Eagles Released on a Quiet Island for Science

For nearly a century, the monkeys and sloths of Barro Colorado Island lived without looking up at the sky. There were no large birds of prey to swoop down on them. Everything changed when scientists released a male and female harpy eagle, bred in captivity and equipped with radio transmitters. In just fourteen months, these two birds transformed a tranquil forest into a place where being alert again was a matter of life or death.

The experiment, now a classic in conservation biology, sought to answer two questions. Does a top predator raised by humans retain its hunting instinct when its cage is opened? And on the other hand, what happens to the prey when they haven’t seen that enemy for decades? The small Panamanian island offered a perfect natural laboratory to test this.

Barro Colorado Island was formed when part of the rainforest was flooded during the construction of the Panama Canal, and since the 1920s it has served as a scientific reserve managed by the Smithsonian Tropical Research Institute. For decades, harpy eagles had not been recorded in the area, and the primates lived without any real threat from the air. Monkeys and sloths moved slowly through the treetops and slept in plain sight. The rainforest seemed stable, but it was missing a link.

In 1999, things took a turn. A young male harpy eagle, raised in conservation programs, was released in June, and a female joined them in October. Both were subadults, fitted with radio transmitters, and came from projects coordinated with The Peregrine Fund and other organizations specializing in birds of prey. No one had taught them to hunt in the wild. Even so, they learned very quickly.

According to the study by biologist Janeene M. Touchton and her colleagues, the male was tracked for 89 days and the female for 205, between June 1999 and August 2000. During that period, the male captured 25 prey animals and the female 46, for a total of 71 animals. Most were medium-sized arboreal mammals, primarily sloths and various types of monkeys that, until then, had had little reason to fear a predator high in the trees.

The figures conceal several important details. Approximately half of the prey captured by both individuals were sloths, and the rest were social primates. The eagles were successful in between one-third and one-half of their hunting attempts, achieving a kill every three or four days. They particularly took advantage of cloudy days and the rainy season, when their prey’s activity changes and the vegetation offers them more cover.

All of this occurred without parents to show them how. The authors of the study conclude that captive-bred harpy eagles can quickly adapt to life in the wild when they find a habitat that offers sufficient food. This finding is crucial for reintroduction programs in Latin America. It indicates that breeding in specialized centers does not eliminate, at least in this species, the ability to hunt independently.

But the other half of the story lies with the prey. A subsequent study with howler monkeys on Barro Colorado Island found that in less than a year these primates went from ignoring the eagle’s calls to responding with increased vigilance, less time spent eating, and specific alarm vocalizations in the face of aerial danger. This reaction persisted even months after the raptors were removed from the island.

What does it mean for conservation?

Taken together, these studies show that reintroducing a large predator not only affects the number of animals that die, but also modifies behaviors and restores lost ecosystem interactions. Harpy eagles are considered a Vulnerable species by the International Union for Conservation of Nature (IUCN), and in countries like Panama, efforts are underway to recover their populations through forest protection and the release of captive-bred birds. The Barro Colorado case is a key reference point when discussing the reintroduction of predators to the wild.

The scientific study describing the behavior of these two captive-bred harpy eagles has been published in the journal Neotropical Ornithology.