Breeding performance of Tengmalm's owl in a heavy metal pollution gradient

1. Breeding performance and liver accumulation of metals in Tengmalm's owl and liver accumulation in its prey species (small mammals) were studied in a heavy metal pollution gradient from a smelter in northern Sweden. Owls breeding in nest boxes were studied in 1981-82 and 1984-85 and represented the situation during two different vole population cycles. 2. In general, breeding performance decreased with decreasing distance to the smelter. This applied to breeding population density (% nest boxes with greater than or equal to 1 egg), clutch size (except in 1985), breeding success, and to embryo/nestling survival. 3. Heavy metal contamination of the owls' prey by arsenic, cadmium, lead, and mercury increased towards the smelter. Despite this, Tengmalm's owl nestlings did not show a corresponding increase in metal accumulation in their liver tissue, except for arsenic. 4. According to snap-trapping data, the supply of voles (owls' staple food) in late spring was less near the smelter than in the control area in 2 out of 3 years. 5. Although the study was performed in very pronounced heavy metal polluted environments, direct metal toxicity effects upon the breeding performance of the owls seemed unlikely. Instead, a reduced food supply is likely to have caused a reduction in the breeding performance of the owls. Heavy metal and sulphur dioxide pollution could have been important factors limiting the small mammal populations in the heavily contaminated study area.