Blood lead levels in an endangered vulture decline following changes in hunting activity

Lead ammunition stands out as one of the most pervasive pollutants affecting wildlife. Its impact on bird populations has spurred efforts for the phase-out of leaded gunshots in several countries, although with varying scopes and applications. Ongoing and future policy changes require data to assess the effectiveness of adopted measures, particularly in the current context of biodiversity loss. Here, we assessed the long-term changes in blood lead (Pb) levels of Egyptian vultures from the Canary Islands, Spain, which have been severely affected by Pb poisoning over the past two decades. During this period, the reduction in hunting pressure and changes in legislation regarding firearms usage for small game hunting likely contributed to a decrease in environmental Pb availability. As anticipated, our results show a reduction in Pb levels, especially after the ban on wild rabbit hunting with shotguns since 2010. This effect was stronger in the preadult fraction of the vulture population. However, we still observed elevated blood Pb levels above the background and clinical thresholds in 5.6% and 1.5% of individuals, respectively. Our results highlight the positive impact of reducing the availability of Pb from ammunition sources on individual health. Nonetheless, the continued use of Pb gunshot remains an important source of poisoning, even lethal, mainly affecting adult individuals. This poses particular concerns for long-lived birds, compounding by potential chronic effects associated with Pb bioaccumulation. Our findings align with recent studies indicating insufficient reductions in Pb levels among European birds of prey, attributed to limited policy changes and their uneven implementation. We anticipated further reductions in Pb levels among Egyptian vultures with expanded restrictions on hunting practices, including a blanket ban on Pb shot usage across all small game species.

Methods
344 blood samples (1 mL) of Canarian Egyptian vultures were collected between 1999 and 2022. Samples were stored in tubes containing lithium-heparin and immediately frozen at -20ºC for blood Pb analyses. Samples were analysed by atomic absorption spectroscopy with a graphite furnace (GF-AAS) after diluting whole blood samples in Triton at 0,1% with standard addition calibration. The limits of quantification of Pb obtained in the different years ranged between 0,1 and 0,6 µg/dL. For blood samples collected in 2021 and 2022, analyses were performed by ICP-MS after the digestion in a microwave oven with nitric acid and hydrogen peroxide. The limit of detection obtained with this technique was 0,1 µg/dL.