Uncertainty and Precaution in Estimating the Population Size of the Philippine Eagle

Other than the special case of the endangered California Condor (Gymnogyps californianus), no other large raptor has a distributional range as small as that of the Philippine Eagle (Pithecophaga jefferyi). Whereas the extent of occurrence (EOO) of the rather bigger Harpy Eagle (Harpia harpyja) is 15.5 million km2 and that of the rather heavier Steller's Sea-Eagle (Haliaeetus pelagicus) is (when breeding) 3 million km2, the Philippine Eagle is crowded into a comparatively restricted 550,000 km2 (BirdLife International 2024); indeed, the four islands that make up the bird's range—Luzon, Samar, Leyte, and Mindanao—have a combined area of only 215,000 km2. Given that large predatory species live at relatively low densities and necessarily possess large ranges (Colinvaux 1978), the situation of the Philippine Eagle is particularly unusual. Of three early estimates of the species' original (predeforestation, around 1910) population size, none ventured more than 6000 individuals (Collar et al. 1999, 2001).

Startlingly, however, of three estimates of the species' population size on Mindanao in the late 1960s and early 1970s—when, for a lack of sightings, numbers on the other three islands were considered negligible or nil—none was more than 100 (Collar et al. 1999, 2001). At that time such estimates depended on subjective impressions based on field experience. Even the first indication of the eagle's density involved a subjective impression, when the parents at a single nest during the nestling-rearing stage were judged to range over 100 km2 (Gonzales 1968). This value was then used by Bonnit et al. (1977), Kennedy (1977), and Krupa (1989) in their own estimations of population sizes, although Kennedy (1977) overrode it in favor of the sum of birds he had either seen (29) or been told about (74) in one-third of the estimated 29,000 km2 of habitat then remaining on Mindanao, thus extrapolating 309 individuals for the island (i.e. 1 pair/188 km2 assuming all birds to be adult; Collar 1997). Krupa (1989) returned to the 100 km2 estimated home range size, but used a reduced habitat cover estimate of 22,170 km2 and, accepting evidence of intense human persecution since the mid-1960s, made the novel allowance that 60% of habitat might be void of eagles (“empty forest”), thus offering a population range of 89–222 pairs for Mindanao.

By the 1990s the species had been confirmed as still present on Luzon, Samar, and Leyte, but its status on each island was highly uncertain. Adding to the uncertainty was the qualitative evidence that, even if it could sometimes be found in secondary forest, the eagle greatly preferred primary forest (Bonnit et al. 1977, Kennedy 1977); a telling piece of evidence in this regard was that the female at a nest in poor habitat deserted her egg after her mate provided too little food for her (Lewis 1986). In contrast, various observers, including Kennedy (1977), found higher densities and smaller home ranges in their studies (see Collar et al. 1999, 2001). Using all this information and allowing for the phenomenon of empty forest (40% and 100% occupancy), a detailed status review offered a table with four plausible range-wide densities (1 pair per 100, 50, 25 and 12.5 km2) set against official 1992 figures for closed-canopy forest habitat on the four islands (total 14,199 km2; Collar et al. 1999, 2001). The resulting population estimates ranged widely from 60 to 1120 pairs but Collar et al. (1999) conservatively settled on 113 pairs—1 pair per 50 km2 with 40% occupancy—for the application of the IUCN Red List criteria.

This attempt to embrace uncertainty and encourage research prompted at least one response. Bueser et al. (2003) measured eight inter-nest and four nearest-neighbor distances from data gathered on Mindanao in the years 1991–1998, yielding a mean of 12.7 km between nests and hence an estimated plot of 133 km2 per pair within which the mean amount of forest was 60–80 km2. Dividing these forest area values into the 14,000 km2 of the available forest “of the types . . . found in [these] circular plots,” (Bueser et al. 2003) based on 1997 government statistics, yielded 175–233 pairs on Mindanao, but dividing the entire area of the plots (range 96–170 km2) into the available forest produced a more conservative 82–146 pairs, so that overall the range was 82–233 pairs (Bueser et al. 2003), a value close to Krupa's (1989) 89–222. Bueser et al. (2003) admitted that allowance should be made for the empty forest factor (which Krupa's estimate did) but they did not suggest what the discount might be (using Krupa's 40% value reduces their estimate to 33–93 pairs, midpoint 63).

Subsequent authors ignored the lower value of 82 and augmented the upper one by attempting to approximate how many might be on the other islands—hence a global total of “probably fewer than 500 pairs” (Concepcion et al. 2006), “only . . . about 500 pairs” (Salvador and Ibañez 2006), “at most 500 [pairs]” (Luczon et al. 2014), “no more than 400 adult pairs” (Ibañez et al. 2016), and “an estimated 400 breeding pairs” (Senarillos et al. 2021). These estimates have ostensibly been validated by two recent analytical calculations. The first (Sutton et al. 2023) used species distribution models to calculate a range-wide area of suitable habitat (AOH) of 28,624 km2 (markedly larger than post-1970s estimates) and divided that by a mean home range size of 73 km2 derived from telemetry studies of six birds on Mindanao. This yielded a total population estimate of 392 breeding pairs (range: 318–447 pairs): 128 pairs on Luzon, 31 on Samar and Leyte combined, and 233 on Mindanao. A second study (Sutton et al. 2024) used the same telemetry data underpinning the density calculated in Sutton et al. (2023) but applied a different methodology and determined a marginally smaller average range size of 68 km2 (95% CI: 62–74 km2). Divided into the AOH published a year earlier, this results in a slightly higher estimate of 421 (387–462) pairs nationwide.