Continued population recovery and high annual survival rates of White-rumped Vultures in Nepal

Abstract

Populations of Gyps vultures in South Asia underwent catastrophic declines caused by unintentional poisoning by the veterinary drug diclofenac. Since the drug was banned in 2006–2010, declines have halted across the region, including in Nepal. The present study updates population trends of G. bengalensis in Nepal based on recent road transect surveys. The population has continued to increase at a rate of 12.7% per annum since the population low point in 2012. Assuming an age of first breeding (α) of 4 years, we composed a simple age-structured Leslie population matrix model, which was parameterized using estimates of annual survival of adult and immature GPS-tagged birds (0.947 and 0.955, respectively), and juveniles (0.65) of the related G. fulvus, and productivity figures from annual nest monitoring across Nepal (0.69 fledglings per nest per year). The intrinsic rate of population increase during the population recovery (λ (95% confidence limits)) estimated from the matrix model was 1.087 (1.033, 1.127) (i.e. 8.7% increase per annum). Values of λ were 1.099 and 1.075 for α = 3 and 5 years, respectively. The estimate of λ derived from the Demographic Invariants Method, utilizing estimates of annual adult survival and α (4 years), was 1.097, that is 9.7% increase per annum (λ = 1.116 and 1.084 for α = 3 and 5 years, respectively). The matrix-derived estimate of λ was more sensitive to changes in adult survival than any other demographic rate. Given the non-significant differences in the estimates of λ, there is no conclusive evidence that the recovery has been augmented by immigration from India. There was no evidence that variation in population trends across Nepal was associated with proximity to supplementary food. Numbers of vultures are still low relative to predecline levels, so actions to improve regulations and restrict the availability of vulture-toxic NSAIDs remain high conservation priorities.