Biology

(from the Training Manual on Research and Monitoring Techniques for Birds of Prey in the Philippines)

theeagle_phpThe Philippine Eagle (Pithecophaga jefferyi) is a giant forest raptor endemic to the Philippines. It is considered one of the largest and most powerful eagles in the world. Unfortunately, it is also one of the world’s rarest and certainly among its most critical endangered vertebrate species. The eagle is known to be geographically restricted to the islands of Luzon, Samar, Leyte and Mindanao.

A. Breeding Biology

Philippine eagles are monogamous and they bond for life. But contrary to claims that they opt to remain unpaired in the death of their mate, evidences from natural pairing techniques and data from all other raptors indicate that they take in new mates as replacement. Females reach sexual maturity at around five years and males, at seven.

1. Courtship

Increased aerial displays, frequent stay near the nest and nest-building activity mark the start of the courtship period. In a study of a pair in 1999, courtship began as early as July. Aerial displays such as mutual soaring (paired soaring flight over the nesting territory), dive chase (diagonal drop by the female with the male trailing in pursuit), and mutual talon presentation (male extending talons to female’s back with the female flipping over to present its talons) were documented. The pair also performed cruising flights over the territory, and did frequent advertisement displays couple0d with vigorous calling. Delivery of nesting materials, although aimed at building nest, can be a form of display to signal readiness to breed. Repeated copulation on nest and nearby perches marks the peak of courtship. Besides ensuring successful fertilization, frequent copulation is also interpreted as another means to strengthen pair bond.

Although different in few details, the courtship behavior observed in this particular pair is generally similar to most diurnal raptors. Courtship displays are expected to aid in the establishment and defense of a nesting territory, attraction of a suitable mate, and the establishment of a strong pair bond, all which are necessary for successful breeding.

2. Timing of Breeding

Data from nesting pairs in Mindanao suggest that the nesting (egg-laying) season can start in September and may extend up to February of the following year. But in Luzon, it is between mid-December to mid-January. The factors responsible for seasonal timing of breeding are not known. However, rainfall patterns, such as the case in Luzon where the periods from September to November are peak typhoon season thus would not be advantageous for egg-laying, as well as the seasonal abundance of the prey have been suggested as possible environmental factors that trigger breeding. A complete breeding cycle, from courtship until the young eagle leaves the parents’ territory, lasts two years.

3. Egg-laying

Observation of captive females revealed that as egg laying draws near, the female appears to be sickly and would not take food for as long as 8 to 10 days. They have drooping wings, takes up a lot of water, continually do calls and builds nest. This condition is called “egg lethargy”. After this phase, the female lays one egg during the late after noon or at dusk.

4. Incubation, nestling and post-fledgling

For a complete breeding cycle, the females lay only a single egg. But if an egg failed to hatch or the chick died early during the first year, the eagles normally nest the following year. As soon as an egg is laid, the female would start incubating. Consequently, breeding behavior stops but sometimes it may still happen a few days after the egg is laid. It is believed that this is meant to ensure that a new egg gets laid just in case the egg under incubation failed.

Incubation lasts 58 to 68 days. Both the male and the female incubate the egg but the female has a greater share of the daytime, and apparently does all of nighttime, incubation. The female spent about two thirds of the incubation up to the early nestling period. After which, both hunt and feed the growing eaglet until independence. In one nest observed, the adults take turn brooding the young and covering it from the sun and the rain. But this ceased when the chick was left on its own in the nest when it was seven weeks old and thereafter. Once the egg is hatched, the eaglet will stay in the nest or about 5.5 months. The parents will take care of it for about 17 months until it leaves its parents territory in search of a vacant habitat.

Recent detailed observation gave revelations about play behavior in a juvenile Philippine Eagle. It was seen observing tree cavities and grasping the rim of knotholes using its tail as props and wing for balance while poking its head into the cavity. The young eagle also hangs itself upside down perhaps as an exercise in balance and was also seen doing mock attacks of inanimate objects on the ground or among tree crowns. All of these were done in the absence of the parents, which indicate that juveniles seem to learn hunting without parental intervention.

The table below summarizes the patterns of juvenile development as observed by Kennedy (1985).

kennedy 1985

5. Longevity

The Philippine Eagle is a long-lived species. A captive bird in Rome Zoo was received full grown in 1934 and died in 1976, making it at least 41 years old at death. A male eaglet at the Philippine Eagle Center arrived as a young bird in 1969 and it’s still alive and that makes it about 34 years old. It is still unknown how old eagles get in the wild. But based on the fact that wild birds face the many exigencies of the forest environment which is rather absent in the captive conditions, wild birds may live shorter than captive birds.

B. Feeding Ecology

The food habits of the Philippine Eagle are known from prey items brought into nests. Studies from 1978 to 1983 revealed that 15 species of vertebrate prey were used for feeding the young including flying lemurs, squirrels, snakes, civets, hornbill, bats and monkeys. But of these prey species, eagles seem to prefer flying lemurs and civets. For the past three decades, only three studies were done on Philippine Eagle breeding and food habits and contrary to persistent claims, no domestic animals were ever brought to nests. The table below shows the list of prey species identified during a study by Kennedy (1985). This table was modified from Kennedy.

kennedy 1985 feeding

The variety and size differences of prey suggest that the Philippine Eagle is an opportunistic hunter with preference for tree-dwelling species. Investigators also suspect that eagles are capable of shifting prey, choice of prey may also coincide with the breeding season of the particular prey.

Observations of their hunting behavior are scant. But more recent detailed observation provided starling revelations. The juvenile learns hunting behavior without parental intervention. Philippine Eagles hunt from perch, constantly observing knotholes or cavities in trees. Adults have been observed to poke their talons into tree cavities to apparently grab prey. One investigator believed that the relatively longer tarsus of the Philippine Eagles is an adaptation to taking prey from tree cavities. But this hypothesis needs further testing.

Meanwhile, the food habits of Philippine Eagles in Luzon have not yet been documented. Because of the difference in terms of the faunal composition of Luzon and Mindanao, them representing different faunal regions, the eagles there would definitely have a different diet regime. For example, flying lemurs, which are the preferred prey in Mindanao, is absent in Luzon. A food habit study in Luzon is long overdue.

C. Regional Breeding Density and Population Estimates

The current population status of the Philippine Eagle is not known. The species has been considered rare since it was discovered in 1896. Moreover, the eagle has always been difficult to census because of the significant logistic difficulties of working in dense, steep rainforest.

Previous attempts to survey or estimate the population status of the species have always been crude at best. Only scattered, individual reports occurred up through the 1960’s. Additionally, data from researchers in the 70’s to the early 80’s were difficult to interpret. And because of the small sample sizes and nature of approaches used, no confidence limits could be established for these estimates. However, based on systematic surveys in the last decade, breeding density estimates suggest there are about 200 pairs in Mindanao. Using the same estimates, about 300 pairs could be present in the other islands where it has been found.

The general indicators of population status continue to be alarming. Habitat and probably prey populations are continuing to disappear at a rapid rate. Thus, wild populations are losing places to live and are likely becoming food-stressed. Hunting and shooting of wild birds also persist. Eagles that were turned over to the Philippine Eagle Center in recent years either had gunshot wounds or were trapped illegally in the wild. Even birds that seemed healthy at the time of recovery or confiscation were found to have airgun pellets in their bodies after undergoing X-ray examinations.

Of the two primary characteristics of populations, i.e. reproductive rate and survival rate, the latter is the most important for populations of long-lived, slowly reproducing species such as the Philippine Eagle. Chance effects (such as weather fluctuations, epidemics, inbreeding, etc.) only make matters worse for small populations.

(from the Training Manual on Research and Monitoring Techniques for Birds of Prey in the Philippines)

D. Habitat Preferences

Except in Mindanao Island, no nest or nest site has ever been studied on other islands within the Philippine eagle’s range. In Mindanao, they are known to nest in a variety of habitats. Some nest on large trees in the lowlands and upper hill dipterocarp forests. Some may even nest at high elevations at transitions to montane or mossy forests. A few nests were in degraded forests near human habitations while others nest within forest interiors. Nest trees are found between 750 to 1590 meters in elevation and they are commonly along steep slopes and ravines, but not necessarily near river systems.

In nest site selection study done in 2001, six characteristics of nest trees seem to be selected for. These are namely height of nest tree, tree density, tree frequency and distance from nearest forest edge, forest trail and kaingin. They also select trees with denser canopies and large trunk spacing. The common Dipterocarp tree species used as nest tree include Shorea almon , S. contorta, S. polysperma, S. negrosensis , whereas the Non-dipterocarps were Balete Ficus sp ., Igem Dacrycarpus imbircatus, and Binuang Octomeles sumatrana. Other nest tree species that has been recorded are Parashorea plicata, Petersianthus quadrialata, and for a for a single record in Luzon, Agathis alba. Eagles don’t seem to prefer specific tree species. Because large trees remain relatively abundant in Mindanao, availability of nesting trees doesn’t seem to limit population there.

Trees towards northern slopes facing the mountain appear to be selected. This might be associated with cooler temperatures, less sunlight, and denser tree cover that increase protection on the nest. Nest trees in Mindanao predominantly have southern exposures (southwest and southeast) and crowns were open enough to facilitate flight to and from the nest.

The nest is normally located between 27 to 50 meters from the ground. They are built on either major branches or tree forks. These nests are large platforms of decaying twigs and sticks that piled atop each other because of the continued nest building and repeated use. Nests are also associated with large epiphytes. A nest could be anywhere between1.2 x 1.2 meters to 1.2 x 2.7 meters in size.