Populations of three Gyps vulture species in the Indian subcontinent have undergone recent rapid declines due to elevated mortality rates caused by diclofenac poisoning. Researchers have proposed that vultures adopt a previously-undescribed neck-drooping posture prior to death. Our study investigated two hypotheses: (1) neck drooping is temperature-dependent, (2) neck-drooping is indicative of poor health in Oriental White-backed Vultures (Gyps bengalensis) and is a prelude to death. Observations of neck-drooping were highly seasonal, with the majority of vultures observed neck-drooping in the hot season from April to October and no vultures observed neck-drooping during the cold months of December and January. Above a calculated threshold ambient temperature of 15.4°C (95% CI 9.9–22.5°C), there was a significant positive correlation between temperature and the proportion (expressed as an angular transformation) of vultures observed neck-drooping. Neck-drooping vultures were observed significantly more frequently with their back toward the sun with their head in their own shade than birds that were not neck-drooping, and vultures that were not neck-drooping were observed more frequently facing the sun than those neck-drooping. Together, these observations strongly suggest that neck-drooping posture has a role in thermoregulation. In contrast to the highly seasonal pattern of neck-drooping, mortality of vultures occurred in all months of the year. This finding indicates that neck-drooping has low specificity and sensitivity as an indicator of poor health and impending death in vultures.
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Vol. 41 • No. 1