Andean eagles have managed to adapt to fragmenting habitats — for now

By Maxwell Radwin

  • A new study looked at black-and-chestnut eagles’ (Spizaetus isidori) ability to survive in fragmented forests in the Andean regions of Colombia and Argentina.
  • Researchers found that the eagles were able to fly between fragmented forests on different mountain ranges and survive better than terrestrial predators
  • However, juvenile eagles had higher mortality rates than their adult counterparts, suggesting that conservation efforts should be focused on ensuring young eagles survive into adulthood.

Even the greatest, most mobile predators aren’t immune to the shrinking of South America’s forests. The endangered black-and-chestnut eagle (Spizaetus isidori), one of the largest raptors on the continent, has been forced to change its behavior to survive in increasingly fragmented habitats and growing threats from humans.

new study in Global Ecology and Conservation found that this aerial predator is successfully adapting to shrinking tropical and subtropical forests in the Andean mountains, but may still face serious threat of extinction if forest loss continues.

“In order to maintain viable populations of this top predator and its key ecological functions,” the study said, “it is urgent to mitigate human persecution.”

Between 2015 and 2020, researchers tagged eight eagles — which have a 180-centimeter (70-inch) wingspan and can weigh as much as 3.5 kilograms (7.7 pounds) — in Colombia and Argentina, in hopes of better understanding how the species was hunting, nesting and traveling in an ever-diminishing forest ecosystem.

Were they staying in one patch of fragmented forest or traveling between different patches? Were they able to move between mountain ranges? If they were making the trip, how often were they surviving?

A Black-and-chestnut eagle and its offspring in a nest. (Photo courtesy of Américo Vilte)

“These are eagles,” Manuel Grande, report co-author and professor at the National University of La Pampa in Argentina, told Mongabay, “so we know they can fly long distances. But on the other hand, it’s a forest species. So we weren’t sure if they were going to be able to move between the fragmented forests.”

Andean forests are one of the most degraded ecosystems in the Americas. Natural vegetation loss ranges between 28% and 60%, with the worst of it occurring in the lowland foothill forests, the study said. In Colombia and Argentina, where the study was carried out, there are less than 50,000 square kilometers (19,300 square miles) of forest remaining.

Deforestation and forest fragmentation are mostly the result of agricultural expansion, particularly in the lowlands. Logging has also contributed to forest loss and degradation. People involved in these activities also build houses and other infrastructure in some areas, which has led to the construction of roads.

Only the most inaccessible parts of the forest remain, the study said, often on the steep slopes of mountain ranges. The other forest fragments are surrounded by people, who have been known to defend their domesticated animals and crops from wildlife, the black-and-chestnut eagle included.

A juvenile Black-and-chestnut eagle. (Photo courtesy of Rodrigo Aráoz)

In Colombia, it’s common for farmers to shoot the eagles when they try to hunt domesticated poultry. However, the eagles only go after the poultry when the fragmented forests can’t provide the wild prey they’re accustomed to.

“One of the biggest challenges the species has to deal with when it comes to habitat loss is feeding itself,” said Santiago Zuluaga, the report study’s lead author. “The problem may be even more concerning than the loss of habitat.”

Because the eagle is a top predator in Colombia, Ecuador, Bolivia, Peru, Venezuela and Argentina, losing it could disrupt Andean forest ecosystems regionally, as there will be less population control of its prey, such as monkeys and rodents.

The black-and-chestnut eagle is considered endangered on the IUCN Red List, with an estimated population of fewer than 1,000 mature individuals. Some efforts have been made to help conserve birds of prey in these regions, including international funding to better track eagles and their nests. However, for the most part, there haven’t been organized projects to help them.

A juvenile Black-and-chestnut eagle. (Photo courtesy of Gonzalo Ignazi)

The study found that juvenile black-and chestnut eagles tend to fare worse than the adults. Of the six juvenile eagles tagged for the study, four ultimately died. This was mostly due to the lack of experience compared to their adult counterparts when it comes to hunting prey and avoiding the threats posed by humans, the study said.

This could be a problem for conserving the species because, like many predators, the eagle has a relatively delayed maturity and spends more of its life in the nesting and juvenile stages than species lower down on the food chain.

Once the eagle reaches maturity, however, it tends to navigate fragmented forests without many problems, moving between different mountain ranges at higher altitudes and on steeper slopes where there’s less of a human presence. The key, the study said, is to take action to protect the species so that it can reach maturity and be able to protect itself.

“Right now, there isn’t much else for the eagle to do,” Zuluaga said, “but adapt or die out.”

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