Releasing Harpy Eagles at an Older Age May Help them Reach Independence Faster
Marta Curti— 30 November 2006 — in Harpy Eagle Conservation and Research    ShareWhen we first began releasing captive-bred Harpy Eagles in Panama, we did so when they were approximately six months old in order to mimic their natural fledging age. While the releases went smoothly, we noticed that it took the young birds between six months and up to two years before they were hunting on their own. This is consistent with the development patterns of wild-born Harpy Eagles as well. However, whereas adult Harpy Eagles are prepared to continue feeding their young for this amount of time, the effort was time consuming and difficult for our volunteers and biologists who were often tracking and feeding up to ten released birds or more at a time. In order to test the theory that releasing birds at an older age would significantly decrease the time it takes them to become independent, we began releasing them at around 18 months of age. To date, we have released four birds, two males and two females, at this advanced age.

All but one of these birds, a female named KC, began hunting almost immediately after their release. After four months of release, KC is still accepting supplemental food from us and has not shown any signs that she is ready to start hunting yet. In contrast, Male BC, or “Paco,” who was released in March of this year has never accepted supplemental food from us, despite our best efforts to track him and provide him with food.

The other two Harpy Eagles, a male named DK, or “Kilo,” and a female named KD, or “Delta,” were released together in August. Just like “Paco,” they were reluctant to accept food from us and only the female accepted one rat just a few days after her release. Since then, neither bird has shown any interest in the food we have tried to provide. Because of their behavior, we figured that they were hunting on their own, but as with all newly released birds, it was important for us to get proof that they were, in fact, able to survive without additional food from us. Though evidence of hunting is not always easy to find, there are several signs that we look for which indicate that a bird is successfully catching its own food, which include: seeing them with prey; observing them with a full crop; or finding prey remains near the bird. Both KD and DK made this particularly difficult, because they often fly off the minute we approach, making it very hard to see the birds, let alone check for signs that they are hunting.

However, a few weeks ago we had great luck in proving that both these birds have become successful hunters. Early one morning, volunteer Jason Sardell and I set out after female KD. We had hiked for only an hour or so when we began to pick up a relatively strong signal from her transmitter using our telemetry receiver. After taking a bearing with our compass, we headed off trail following her signal, — a series of “beeps” that grow louder as one gets closer to the bird. Less than half an hour later, as we were coming to a rise of a large, steep hill, her signal was very strong indicating that we were very close to her. We took a few more steps forward and heard a tremendous crash only a few feet from us. We looked and saw her flying up from the ground. She landed in a tangle of vines with her wings held out. After a few minutes, she flew off and clutched in her talons was a smallish dark mass – a prey item we could not identify, but prey nonetheless. After she disappeared into the forest, we investigated the area and found two old mutes and the remains of a dead sloth that had been picked clean and was probably several days old. All that was left was some fur and bones. We are pretty certain that we were seeing evidence of one of KD’s meals.

Feeling super happy and lucky, volunteers Miguel Angel Mayoral, Megan McSherry, and I went after DK two days later. We located him relatively easily after a short hour and a half walk. Again, we were following his signal and this time, as soon as we dropped down into a small river valley, we began to get a very strong signal. A few seconds later, a large troop of howler monkeys began vocalizing with their loud, thunderous calls, shaking tree branches and acting very defensive. Clearly, they were upset by DK’s presence in the area. We spent a while looking for him, though we were never able to get a glimpse of him. He was in an area very thick with trees and though he was nearby, he was impossible to see. According to his signal, he moved a little bit each time we got close. As we were tracking him, all three of us noticed a strong odor – something dead was nearby. We continued to follow his signal, and soon found the remains of a howler monkey that had probably been killed a day or two before. Alongside the kill, we found a super fresh mute –it was still wet. Though we did not see DK with the kill, it most likely was his.

Of course we will continue to monitor these birds, looking for evidence of future hunting successes. And though we will need to release more birds at 18 months of age in order to be able to say with certainty that it will consistently decrease the eagles’ time to independence, it appears that releasing birds at an older age really does make a difference. If this is so, we can reduce the amount of time, energy and money that must be spent on the release of each bird just by letting them go at an older age. This will make the release of a larger number of birds at a time much more manageable and therefore feasible. A good sign for future releases of Harpy Eagles in the Neotropics and, potentially, other large forest raptors worldwide.

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