The background to the decline in vulture numbers.
Tens of millions of vultures used to be present across the Indian sub-continent. The vast flocks present were due to the very large numbers of livestock reared across South Asia. Government statistics indicate that livestock numbers in India exceeded 400 million since the 1980s and reached more than 500 million in 2005.
In India and Nepal cows have a sacred status for Hindus and are not eaten. As a consequence, livestock carcasses became available for vultures in Asia and became the principal food source for the resident species. Vultures were so abundant that the Parsi religion in India and Buddhist communities on the Tibetan plateau utilised these birds for sky burials in order to cleanly and efficiently dispose of human bodies.
Vulture declines in India were first quantified at Keoladeo National Park, Rajasthan, by Dr Vibhu Prakash, Principal Scientist of the Bombay Natural History Society (BNHS). Between 1985-1986 and 1996-1997 the population of oriental white-backed vulture declined by an estimated 97% at Keoladeo, and in 2003 this colony was extinct. These declines were coupled with high mortality of all age classes widely recorded. Following the initial survey, in 2000 BNHS teams undertook over 11,000 km of road-based surveys, repeating 6,000 km of road-transects previously surveyed for raptors in the early 1990s, and confirmed that declines of >92% had occurred in all regions across northern India (Prakash et al 2003). Repeat surveys by the BNHS, covering the same route and methodology, were undertaken in 2001, 2003 and 2007 in order to monitor trends in numbers.
The survey in 2007 indicated that numbers of oriental white-backed vultures had declined by a staggering 99.9% over the preceding 15 years (Prakash et al 2007). Long-billed and slender- billed vultures decreased by 97% over the same period. Surveys across Nepal and Pakistan indicate vultures have declined at similar rates across the whole of south Asia, and within Pakistan both resident species (white-backed and long-billed) are on the edge of extinction.
The continuing rates of population decline were also of great concern, with white-backed vultures in India declining at an average rate of 48% a year for the period from 2001 to 2007 (to 11,000 birds in India). Long-billed (45,000) and slender-billed (1,000) vultures were estimated to be declining at around 22% a year. Populations of red-headed vultures and Egyptian vultures are also declining, at 41% and 35% a year in India (Cuthbert et al 2006).
Solving the mystery
Understanding the problems facing vultures
Research biologists from the Bombay Natural History Society (BNHS), Bird Conservation Nepal (BCN) and the Ornithological Society of Pakistan (OSP) were joined by international partners from the RSPB (UK), Zoological Society of London (UK) and The Peregrine Fund (USA). Because of the rapidity of the decline, simple population modelling established that they had to be caused by a major reduction in adult survival, as reduced breeding success could not account for declines of nearly 50% a year.
Through collecting carcasses of dead and dying vultures researchers quickly established that dead birds were often characterised by the presence of extensive visceral gout, and of 284 post-mortems carried out in Pakistan, India and Nepal gout was found in 84% of birds (Oaks et al 2004; Shultz et al 2004). Visceral gout is caused by a build up of uric acid, which at very high levels crystallises in the body coating all internal organs in a white ‘paste’. Uric acid is the white substance found in the guano of all birds, and the characteristic presence of visceral gout in vultures suggested the cause of death was likely to be related to kidney failure. Some birds appeared sick and lethargic for a protracted period before death, with a characteristic drooping head.
For several years, researchers battled to understand what might be the cause of the deaths. Dead birds were tested for pesticides, herbicides, toxic heavy metals and other environmental pollutants. While trace levels of some of these compounds were detected, in the majority they were at insufficient levels to cause physiological damage and there was no link between these compounds and the gout found in most dead birds. Because of the geographic range and speed of the declines one initial strong hypothesis was that a novel infectious disease agent was responsible for mortalities.
The diclofenac breakthrough was made in 2003 by researchers working for the Ornithological Society of Pakistan and The Peregrine Fund, led by Professor Lindsay Oaks from Washington State University, USA. Lindsay recognised that the class of painkiller known as Non-Steroidal Anti- Inflammatory Drugs (NSAIDs) had been linked to kidney failures and cases of visceral gout when some of these drugs were given to birds. Visiting pharmaceutical shops in Pakistan, the team found that a new NSAID, diclofenac, had recently come on sale and was commonly available. Investigating the carcasses revealed that every bird that had visceral gout also had traces of diclofenac, whereas those carcasses with no gout had no diclofenac. The team then gave diclofenac to vultures, either by injecting birds or by feeding flesh from buffalo and goats injected with diclofenac, and birds that received a high dose of diclofenac died within days of treatment, with extensive visceral gout. In 2004 the results of this work were published in the journal Nature(Oaks et al 2004). Extensive research established the same correlation between gout and diclofenac in birds from India and Nepal (Shultz et al 2004), modelled the amount of diclofenac required in the environment to cause the observed decline rates (Green et al 2004), measured the prevalence of diclofenac in cattle carcasses available for vultures (10% of carcasses, Taggart et al 2007), and modelled the prevalence of diclofenac and determined that diclofenac alone was responsible for the vulture population crash (Green et al 2007).
Other hypotheses put forward for the vulture declines include; reduced food availability, increased numbers of dogs, and habitat destruction, but none has significant evidence.