Researchers have discovered a lot about the Philippine eagle. But its behavior in the wild remains largely unknown. Where they go, how much time they spend in the wild or how far they are willing to go hunting and leave their nest behind?
A recent research has succeeded in obtaining conclusive data about the bird’s behavior.
The study, “Space-time Home-range Estimates and Resource Selection for the Critically Endangered Philippine Eagle on Mindanao,” was made by Luke J. Sutton, Jayson C. Ibanez, Dennis I. Salvador, Rowell L. Taraya, Guiller S. Opiso, Tristan Luap P. Senarillos and Christoper J.W. McClure.
It was published on May 15, 2023, in IBIS International Journal of Avian Science.
For informed conservation action
Quantifying home-range size and habitat-resource selection are important elements in wildlife ecology and are useful for informing conservation action, the authors said in their paper.
“Many home-range estimators and resource-selection functions are currently in use. However, both methods are fraught with analytical issues inherent within autocorrelated movement data from irregular sampling and interpretation of resource selection model parameters to inform conservation management,” they explained.
The research used satellite remote sensing to provide updated estimates of the home-range size and first estimates of fine-scale resource selection for six adult Philippine eagles (Pithecophaga jefferyi), using a space–time autocorrelated kernel density estimate home-range estimator and non-parametric resource selection functions.
Findings: Distinct site fidelity
The study showed that all six adult eagles manifested distinct site fidelity, with continuous range residency between 2 km and 18 km, one month after tagging.
This means, the researchers said, the eagles kept coming back to the same areas over and over.
The scientists were able to determine that the eagles used habitat high in photosynthetic leaf and canopy structures, and avoided areas of old-growth biomass and denser areas of vegetation.
“This is possibly due to foraging forays into secondary forest and fragmented agricultural areas away from nesting sites,” the report said.
“For the first time, we determine two important fine-scale spatial processes for this critically endangered raptor that can help in directing conservation management,” the report added.
“Rather than employing traditional home-range estimators and resource-selection functions, we recommend that analysts consider space-time approaches and non-parametric resource-selection functions to animal movement data to explore fully space-time and resource selection,” the researchers said.
GPS tracking device
Global positioning satellite tracking devices were attached to each of the six Philippine eagles, allowing the team to monitor their exact location, McClure said in an interview via Zoom on January 12.
“It was the Philippine Eagle Foundation [PEF] that trapped the birds. The earliest was in 2013 and the latest was in 2021,” McClure, director of Conservation Science at the Peregrine Foundation, told the BusinessMirror.
McClure was in the Philippines last year as part of the study and visited some of the project sites of the PEF in Davao.
Feeding areas; fidelity to territories
He pointed out that the team learned about how the Philippine eagle hunts food.
“Basically, we knew the nesting habitats of these birds but we don’t know their foraging habitats,” he said.
“It [research] suggests that they use the lush areas for breeding, but will go elsewhere to hunt. They use the secondary forests or agricultural areas for hunting,” McClure said.
Likewise, the study was able to uncover the high fidelity of the eagles to the territories, underscoring the need to protect their habitat, and communicating to the communities not to harm or hunt the eagles.
Asked further to explain the research outcome, Ibanez said that as the paper predicted, it has something to do with prey animal availability and detection.
“If the forest has multiple canopy layers, it means there are more preys that can be accommodated. So there’s more food,” Ibanez, one of the authors of the paper and the director for Research and Conservation of PEF, told the BusinessMirror via Messenger on January 11.
On the other hand, Ibanez said that if the vegetation is not thick, it would be easier for the eagle to find potential prey and hunt.
“The eagle can easily maneuver and fly around the forest interiors,” he said.
Required forest area
The paper found out that the Philippine eagle requires at least 6,800 hectares (has), that is “27 times larger than the whole area of BGC in Taguig,” Ibanez explained, referring to Bonifacio Global City which is approximately 240 has.
“That’s how big a forest an eagle couple would require,” he pointed out.
However, 50 percent of the birds’ time is spent within a 1,200 has area, possibly a preferred or favorite hunting ground within its vast territory.
“This means the eagle hangs out and hunts for food in just 1,200 has of the 6,800 has home range,” Ibanez explained.
“It’s like a person who lives in a subdivision with a big area but he only goes to his house and the clubhouse,” he added.
This led the authors to conclude that the eagles are only using areas where preys are abundantly found and leave the rest of the territory unexplored.
Territorial raptor
As the Philippine eagle is territorial, it is also a solitary creature. Scientists believe that the iconic raptor, named as the country’s Philippine national bird, pairs for life.
Once it has found a partner, the pair typically builds a new nest, if there are no ancient nests in the area that they can use, on large dipterocarp trees like the native species, lauan.
Philippine eagles can live up to 40 years or more in captivity. But in the wild, scientists say they would probably live a much shorter life because of habitat destruction and hunting.
Scientists say its reproduction takes five to seven years to sexually mature, lay a single egg every two years and wait for their offspring to make it on their own before preparing for the next egg.
Both eagle parents incubate the egg for about 58 to 60 days, with the male eagle hunting most of the time during the first 40 days of the eaglet’s life, while the female stays to protect the young.
Critically endangered; major threats
The Philippine eagle, also called the monkey-eating eagle, suggests that it hunts and eats monkeys, such as the Philippine long-tailed macaque.
The eagle is one of the largest and most powerful among birds of prey with its 7-foot wingspan It stands 3 feet in height from the tip of its crown to its tail.
An endemic species, the Philippine eagle can only be found in four major islands in the Philippines—eastern Luzon, Samar, Leyte and Mindanao.
Habitat destruction and hunting caused its population to decline and join the list of critically endangered species of the International Union for the Conservation of Nature in 1994.
According to the PEF, at least one Philippine eagle is killed every year from hunter’s shooting.
Worse, as more forests are destroyed, the eagles are forced to hunt near human settlements. When this happens, humans protective of their livestock start hunting the birds.
Only around 392 eagle pairs are left in the wild according to latest estimate.
This figure, Ibanez said, can now be updated using a more exact model which they used in the latest study.
‘Protect not just the nest’
To strengthen the protection and conservation measures for the Philippine eagle, McClure pointed out that protecting just the nest site is not enough.
“We also need to protect them the moment they go out to hunt,” he said.
With the research findings, he said the PEF can now reassess the population estimate of the Philippine eagle, which he admitted will entail costs.
“We need to have more transmitters and all,” he noted.
According to McClure, the PEF is doing a good job protecting and conserving the Philippine eagle, highlighting how it taps the Indigenous People and communities as partners, adding that it needs more support and funding.