It’s Philippine Eagle Week and it is an opportune time to know about our national bird which, incidentally is also the largest and most endangered eagle in the world. The Philippine Eagle Pithecophaga jefferyi is endemic or unique to our country and can only be found in Sierra Madre in Luzon, Samar, Leyte and Mindanao.
It was discovered in 1896 in Samar by John Whitehead, an English explorer and naturalist. I read Haribon-BirdLife’s Threatened Birds of the Philippines (1999), which provides a comprehensive discussion about the bird—its habitat and distribution, feeding and breeding habits, threats and conservation measures. Reading through the section on the Philippine eagle’s recorded sightings, I felt disheartened because most of all the records were incidents related to it being hunted, found dead or dying or being caught. But what is interesting is that in 1907, there were sightings of Philippine Eagle in nearby Marikina Watershed particularly in Boso-Boso, Montalban and in San Mateo.
Philippine Eagle is an inhabitant of the primary forest but also occurs in second growth forest and, according to Kennedy, has partially adapted to human upslope encroachment by hunting over cleared land. Unpublished research on prey density and diversity indicates a positive correlation with primary forest, clear evidence for the vital importance of such habitat to the long-term survival of the species. Hunting for prey is usually done in the morning with a typical hunt beginning at a perch high on a forested hillside, the bird dropping with flapping its wings in a short glide to another perch where it pauses and scans, then glides down to another perch and so on for an hour or so until near the floor of the valley, then glides off in horizontal flight, finds a thermal updraught and circles up to regain its previous elevation. The variety and size of prey items recorded (from a 10 g bat to a 14 kg deer) at a nest studied by Kennedy in 1985 suggest that Philippine eagle is an opportunistic feeder meaning that it preys on whatever is available and vulnerable.
The generic name of Philippine Eagle, Pithecophaga, which led to its original English name “monkey-eating,” was the result of the natives of Samar reporting that it “preys chiefly on green monkeys.” In Whitehead’s view (1899), monkeys are the only animals sufficiently abundant in the forest to support the large bird. However, it was found out later by Gonzales (1968) that monkeys formed only a small proportion of the diet of a breeding pair. Moreover, it may well be that diets change with area, elevation, forest type, rainfall distribution and/or stages in the breeding cycle: during the incubation and brooding periods one member of a pair is required for nest duty, so the hunting of monkeys may then be greatly reduced however locally abundant they are.
Philippine Eagles studied at nests proved to be in monogamous pairs and it is an assumption that runs through all work on the population and biology of the species that monogamy is the exclusive condition under which it reproduces. One or two eggs may be laid although one appears the more frequent and in cases of two, generally only one eaglet fledges. The male does most of the hunting from incubation up to the first third of the nestling period and the female doing two-thirds of the incubation and almost all of the chick-feeding duties in this period. Thereafter, the two adults share hunting and provisioning for the eaglet.
Incubation is approximately 60 days while nestling or the period where the parents care for the young is about 105 days. In a nest that was observed, for the first seven weeks of the chick’s life, its parents shielded it from the sun and rain but after that, left it entirely to care for itself. Eaglets remain in the nest for almost five and a half months after hatching and were entirely dependent for several months more. In one instance, the offspring remained within its parents’ home range for almost 17 months after leaving the nest. Thus, it is estimated that a successful nesting cycle requires two years. Life span is estimated at 40 years.