Survival rates of wild and released White-rumped Vultures (Gyps bengalensis), and their implications for conservation of vultures in Nepal

Abstract

Beginning in the mid-1990s, populations of three species of Gyps vultures declined by more than 97% in South Asia in little more than a decade, caused by unintentional poisoning by the non-steroidal anti-inflammatory drug (NSAID) diclofenac. This led to a ban on the veterinary use of the drug, and establishment of conservation breeding programmes, throughout the region. Once much of Nepal had been confirmed as being free from diclofenac, beginning in 2017 White-rumped Vultures Gyps bengalensis were released from the captive breeding population. A total of 99 birds (n = 50 wild and n = 49 released) were fitted with GPS transmitters between 2017 and 2022 and monitored daily. Tag fixes suggesting death or ill-health were followed up and dead vultures were retrieved for post-mortem analysis. The estimated annual survival of wild adult vultures was 0.974 (95% confidence interval [CI] 0.910–0.997), while that of wild subadults was 0.880 (95% CI 0.721–0.966). Survival rates of released birds were lower than those of wild birds, being 0.644 (95% CI 0.490–0.778) for adults and 0.758 (95% CI 0.579–0.887) for subadults. Post-mortem analysis of dead vultures indicated several possible causes of death, including predation, infection and electrocution. There was no evidence that any birds died of NSAID poisoning. The high survival rates of wild birds, especially adults, and the lack of evidence for NSAID-caused mortality, suggest that vulture habitat in the Nepal Vulture Safe Zone is free from diclofenac and that other hazards are sufficiently infrequent to allow the vulture population to recover. The lower survival of released birds compared with their wild counterparts suggests a need to improve the conservation breeding programme and release protocol.