© Marcel Burkhardt
Lead poisoning in Golden Eagles
Carcasses of wild animals often contain fragments of lead ammunition. The Swiss Ornithological Institute examines whether the organs of Golden Eagles and other raptor species that have been found dead show increased lead levels.
In the past years, there have been several instances of lead poisoning in Golden Eagles and Bearded Vultures in Switzerland and other countries. Poisoning is mainly caused by fragments of lead ammunition in wild game such as chamois, red deer and ibex. A comprehensive study of Golden Eagles that have been found dead investigated whether the lead-poisoned birds are rare and isolated cases or whether there is a level of contamination that could affect the development of the population. Another aim of the study was to clarify where the lead is from and how it is ingested by the examined birds.
The Swiss Ornithological Institute closely collaborates with gamekeepers and cantonal hunting administrations of various Alpine cantons, especially the Canton of Grisons. These agencies provide organs of Golden Eagles found dead. The analyses of liver, kidney, feather, blood and bone samples were conducted by the University of Zurich’s Institute of Forensic Medicine in collaboration with the Institute of Veterinary Pharmacology and Toxicology. In the first part of the project, 36 dead or dying Golden Eagles were examined. For comparison, dead Eurasian Eagle-Owls were analysed, as they rarely feed on carcasses and have little or no exposure to lead ammunition. In the second part of the project, the sample for lead analysis will be expanded to include other bird species such as Red Kite, Eurasian Sparrowhawk, Northern Goshawk and Northern Raven. The expanded sample will also include areas of the Central Plateau, where other hunting regulations apply.
At present, the Swiss Alps accommodate about 350 Golden Eagle breeding pairs, representing a vital but still vulnerable population. In particular, losses occur due to technical installations (e.g. casualties after collisions with transport cables and high-tension power lines), and broods are lost as a result of human disturbances. It is important, therefore, to keep an eye on causes of death that could weaken the population. Moreover, lead poisoning is also a danger to other raptor species such as the rare Bearded Vulture or Red Kite. The study results support the case for the use of lead-free ammunition.
Three Golden Eagles exhibited symptoms of acute lead poisoning (lead blood levels of 32–108 µg/dl). Lead concentration in liver and kidney were significantly higher in the dead Golden Eagles than in the Eurasian Eagle-Owls. Lead levels in the bones were as much as ten times higher in Golden Eagles than in Eurasian Eagle-Owls. Lead concentration in the bones of the Golden Eagles was therefore considerably higher than the values cited in the literature so far.
To find out where the lead comes from, lead isotopes were determined. The lead found in the bones of the Golden Eagles matched the lead from ammunition, but did not match lead found in the ground, in prey animals, or in the Eurasian Eagle-Owls. It can therefore be concluded that the source of the high lead levels in the Golden Eagles was, in all likelihood, ammunition.
The ingestion of lead by Golden Eagles is best explained by the ingestion of lead fragments in animal carcasses or gut piles left by hunters. The entrails of animals that have been shot often contain tiny fragments of lead from rifle bullets. A survey using camera traps during Ibex hunting season in the Canton of Grisons found that Golden Eagles systematically take advantage of gut piles: At six sites monitored by camera traps, the gut piles attracted five different Golden Eagles within just a few hours.
Hunting administrations of the cantons of Grison, St. Gallen, Glarus, Bern and Wallis
Jacqueline Kupper, Institute of Pharmacology and Toxicology, University of Zurich
Thomas Krämer, Institute of Forensic Medicine, Forensic Pharmacology and Toxicology, University of Zurich
Foundation for Bearded Vultures
The frequency distribution of lead concentration in feathers, blood, bone, kidney and liver of golden eagles Aquila chrysaetos: insights into the modes of uptake.
Excessive lead burden among golden eagles in the Swiss Alps.