JAKARTA — Most of the habitat of the Javan hawk-eagle is protected, but threats of forest degradation and isolation loom over the surviving population of Indonesia’s national bird, a new study shows.
The latest population estimate for the Javan hawk-eagle (Nisaetus bartelsi) is 511 breeding pairs, an increase from 325 pairs in 2009, according to the research.
It added that 70% of the species’ habitat lies within protected areas across the islands of Java and Bali, while the remainder is in farmland.
“Improved habitat distribution data are needed to better estimate the current population size and to facilitate development of new strategies and action plans,” says the study published Nov. 21 in the Journal of Raptor Research.
The researchers conducted field surveys between 2008 and 2019 by observing the nests of eagle couples and revisiting them during the breeding season, and also collected information through interviews with local communities, key informants from local nongovernmental organizations, government officials, and site managers.
To analyze the data, they included improved study methods, such as higher-resolution satellite imagery, which helped in identifying the important habitats.
The scientists credited these improved methods with helping them come up with a population estimate that was higher than previously calculated. They also found that while the species prefers big forest areas for breeding, they can adapt to smaller patches when necessary and available, especially in areas where much of the land is used by humans for farming.
But the researchers also noted a slight decrease in suitable habitat for the hawk-eagles during the study period due to significant degradation of primary forest, and also a strong indication of habitat isolation associated with the small size of forest patches.
“The Javan hawk-eagle is very dependent on primary forests because of the availability of emergent trees which are its preference for making nests,” study lead author Syartinilia, a professor in landscape management at the Bogor Institute of Agriculture (IPB), told Mongabay in an email.
She said further forest degradation would cut the number of emergent trees, which grow higher than the rest of the forest canopy, and reduce the quality of food that’s available for the hawk-eagles. “So, in the long term, forest degradation will threaten the sustainability of the Javan hawk-eagle,” she said.
Syartinilia also said that poor connectivity between the small habitat pockets, each of which has very limited carrying capacity to support the species, would lower the bird’s ability to defend against threats from the forest edge compared to larger patches.
“To ensure the preservation of the Javan hawk-eagle’s habitat in this small patch, the main solution is to connect it with other patches, especially large ones, to facilitate flow/movement from small patches to large patches,” she said.
Syartinilia suggested that isolation could be reduced through a habitat connectivity plan by recommending protecting land cover that still contains emergent trees even though they’re not in protected forest areas. She added it could also include incorporating home gardens or other vegetated land for agricultural purposes and other existing tree corridors.
In addition to habitat degradation and loss, the researchers noted that the illegal wildlife trade, facilitated by online sales, was another key threat to the Javan hawk-eagle.
The species is a prominent predator that can reach a length of about 60 centimeters (24 inches). Locally referred to as Garuda, after a mythical Buddhist-Hindu bird that serves as the national emblem of Indonesia, these raptors are key bioindicator species as they offer clues about the health of the ecosystems in which they live.
Other factors that threaten the species include natural events such as volcanic eruptions and human activity such as logging, both of which contribute to changes in the forested areas of Java, the most populated island in Indonesia.
In 2013, Indonesia drew up a 10-year plan for Javan hawk-eagle conservation, and in 2015 declared the bird a priority species for conservation. The plan was meant to grow the bird’s population by 10% from a 2019 baseline, but this hasn’t happened yet, according to the recent study.
The researchers have called for more studies to continue long-term surveys, including to verify the presence of individual nesting hawk-eagles, monitor their habitats, and track their movements in real time.
“This research needs to be carried out because information on home ranges that correspond to the quality of habitat is important for estimating populations that are closer to existing conditions,” Syartinilia said.
Syartinilia, Mulyani, Y. A., Suyitno, R. A., Condro, A. A., Tsuyuki, S., & van Balen, S. (2023). Population estimates of the endangered Javan hawk-eagle based on habitat distribution modeling and patch occupancy surveys. Journal of Raptor Research, 57(4), 1-14. doi:10.3356/JRR-22-16
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This article by Basten Gokkon was first published by Mongabay.com on 12 December 2023. Lead Image: A juvenile Javan hawk-eagle in Bromo Tengger Semeru National Park. Image courtesy of Heru Cahyono.