In the Amazon rainforest, the canopy is a haven for arboreal species and home to some of Earth’s most charismatic species including sloths, tamarins, and spider monkeys. Unfortunately for them, their presence makes the treetops prime hunting grounds for birds of prey including the harpy eagle (Harpia harpyja), one of the world’s largest eagles. The enormous birds, which are the largest by weight of any eagle, can scoop up even sizable prey such as red howler monkeys and even small deer. Their voracious appetites mean that regular feeding is a must, but in an environment where habitats are falling by the day, a reliable source of food isn’t guaranteed even to those at the top of the food chain.
New research, published in the journal Scientific Reports, found that harpy eagles in the Amazonian forests of Mato Grosso, Brazil, are unable to sufficiently feed their young as a result of deforestation. Habitat degradation in these areas causes a sharp decline in suitable prey species occupying the canopy, meaning food is sparse and harder to find for harpy eagle parents.
To reach their conclusions, the authors of the study monitored 16 active harpy eagle nests across a range of landscapes. The habitat quality varied at each site, with the surrounding forest loss ranging from 0 to 85 percent. They placed camera traps directed at the nests and snuck around beneath them picking up bone fragments to ascertain what the eagles were bringing back for their young. Based on these analyses, the birds totted up 306 prey animals scooped up by hungry harpies, most of which were arboreal species. The top three were two-toed sloths, brown capuchin monkeys, and gray wooly monkeys.
As forest loss increased, the researchers observed that feeding rates decreased and by the end of the study three eaglets had died of starvation in areas with forest loss ranging from 50 to 70 percent. This, combined with the fact that no nests were found in areas with more than 70 percent forest loss, has been interpreted to mean that there is a 50 percent forest cover threshold upon which the viability of harpy eagle nests hinges.
This is a worrying statistic, as a scaling-up estimate conducted by the researchers showed that, of their 428,800 kilometers2 (165,560.6 miles2) Amazonian “Arc of Deforestation” study region, 35 percent of habitats would be unable to support breeding harpy eagles.
While the eagle nests studied showed that parents tended to bring less food rather than seek out alternative prey, one nest that was excluded from the study served as a curious outlier – the birds here switched to a diet almost entirely made up of armadillos. But because the animals included in the study were often dropped belly-down, the eaglets were unable to tip them and therefore they went uneaten. Evidently, for these young birds to survive a sufficient supply of appropriate prey is vital.
“Apex predators are threatened globally, and their local extinctions are often driven by failures in sustaining prey acquisition under contexts of severe prey scarcity,” wrote the study authors. “Our results suggest that restoring harpy eagle population viability within highly fragmented forest landscapes critically depends on decisive forest conservation action.”