Thallium loading in owls and other birds of prey in Denmark

Due to previous findings of increased concentrations of the rodenticid, thallium, in free-living Danish carnivores, thallium analysis has been carried out on material from 77 owls and birds of prey. Thallium was found in 2/3 of the birds and in 10% the concentration was so high that the load of thallium may have caused the death of the birds. The load of thallium in birds of prey was lower than concentrations found in carnivores, probably because poisoning caused by direct intake of thallium baits placed for rodent control does not seem to occur among birds of prey. From the investigation of this limited number of birds, it is not possible to tell if thallium did have any influence on the maintenance of a healthy population of birds of prey, but the large number of thallium loaded birds also justify the restrictions in the sue of thallium which were given from the 1st of July, 1975.