Philippine Eagle Biology and Ecology

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

arny eagleThe 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).

Age when the head is held up 7-10 days
Stood momentarily 25-27 days
First wing flapping 32 days
First pin feathers appear in scapulars 37 days
Walking 45 days
First eating by itself 54 days
First time off nest 118-151 days
Fledged (departed nest tree) 130-164 days
Started to wander from nest area 246-288 days
First observed kill 304 days
Last seen in parent's home range 640 days

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.


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