Keith L. Bildstein
3 November 2012
My first entry about Khao Dinsor, the new world-class raptor-migration watch-site in southern Thailand, failed to mention the vulture component of the flight… and with good reason.
Although several species of Asian vultures breed north of Khao Dinsor, none regularly migrates down the Malay Peninsula to southeastern-most continental Asia and the South Pacific Islands that lay beyond. Unlike Hawk Mountain Sanctuary in eastern Pennsylvania, where hundreds of vultures can be seen migrating each autumn, or the Strait of Gibraltar in southwestern Europe, where thousands of Griffon Vultures are seen each year, Khao Dinsor’s flight is emphatically “vulture-less.” The reason, or reasons, for the lack of scavenging birds of prey at the watchsite are not clear. That said several possibilities come to mind.
As a group, vultures are not nearly as migratory most other birds of prey. In fact, although many young vultures disperse from the breeding grounds, except for New World Turkey Vultures and European populations of Egyptian Vultures, most vultures do not regularly migrate as adults. The lack of migrating vultures at Khao Dinsor may simply reflect this fact. But that only begs an additional question: why don’t most species of vultures migrate in the first place?
Although some of my colleagues have argued that the lack of large-scale seasonal movements in vultures simply reflects their geography in that many vultures inhabit sub-tropical and tropical areas where carcasses are available year round. Nevertheless two species Himalayan Vultures and Cinereous Vultures breed in the Temperate Zone north of the Khao Dinsor and neither regularly migrate at the watchsite.
One important point to consider is that when vultures do migrate long distances, most move into wintering areas that already support populations of resident vultures. In North America, for example, northern populations of Black and Turkey Vultures travel south to winter in the southern United States or even farther south where resident populations of both species exist. And in fact, western North American populations of Turkey Vultures travel as far south as northern South America, where “wintertime” populations bulge by as much as five-fold over “summertime” populations.
Herein may be at least part of the explanation for why no vultures migrate past Khao Dinsor… there are no vulture populations south of Khao Dinsor. Southern continental South East Asia, and the South Pacific Islands that lie beyond, as just as “vulture-less” as Khao Dinsor. But again, this only raises an additional question: why are vultures missing from this part of the world?
Certainly, natural and human-dominated areas in this region support large populations of animals whose carcasses should be available to scavenging birds of prey including vultures. But are they? Human densities in the region tend to be high, and in many places people still live “off the land,” particularly as hunters and fishers.
Traveling through this region one sees little in the way of road kills. And my sense is that human inhabitants make certain that little goes to waste in the countryside. Several scavenging birds of prey do live in south of Khao Dinsor, but they species are so-called facultative, or part-time, scavengers, rather than obligate, full-time scavengers like vultures. They include the region’s widespread Brahminy Kites and Australia’s Wedge-tailed Eagles, species that hunt for live prey, as well as scavenging carcasses, as needed.
In the end humanity may be the answer to the question of why no vultures migrate past Khao Dinsor. There simply is no room in the landscape for obligate, full-time scavengers given human action. Additional study is certainly merited. But whatever the reason, the lack of vultures at the site should not deter you from missing this truly spectacular raptor-migration… the “Hawk Mountain of Asia” indeed.