Biomonitoring heavy metals using the barn owl (Tyto alba guttata): Sources of variation especially relating to body condition

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Abstract
The feasibility of using the Barn Owl (Tyto alba guttata) to monitor environmental quality in the Netherlands was investigated, using Cd, Cu, Pb, Mn, and Fe as indicators for environmental contamination. Throughout 1992, bird-watchers, volunteers, and officials submitted 53 birds. The age and geographical distribution of these birds formed a representative sample of the population. The following interrelationships were investigated: cause of death, nutrient reserve, age, time of death, place of death, body measurements, sex, condition, and heavy metal concentration in kidney, liver, and tibia. Twenty-eight animals had died after collisions. Fifteen Barn Owls died of exhaustion. In total, twenty-four birds were exhausted, with coccidiosis or other parasitic gastrointestinal infections. The condition of the birds showed that as the birds' condition worsened, fat reserves were depleted before protein reserves. Significant linear relationships were found between decreasing protein reserves and decreasing dry weights of the liver, kidney, flight muscle and heart, but not of the tibia. An asymptotic, nonlinear relation was observed between dry organ weight and fat reserve. This suggested that fat reserves were only found when protein reserves exceeded 15% of the body mass at starvation. Concentrations of Cu and Fe in liver and kidney rose as protein reserves fell; the total content of Cu and Fe per organ, however, remained constant. The Mn concentration of these organs remained constant; Mn content increased with increasing organ sizes. Neither Cd nor Pb showed a clear relationship with parameters of body condition. The ratio between the organ content of Pb or Cd and the dry organ weight, however, revealed some birds from contaminated habitats. The findings suggested that concentrations of environmental contaminants should be measured on a dry weight basis. Furthermore, depending on the pharmacokinetic characteristics of a contaminant, the total content of that contaminant per organ can be more informative than the concentration. In this one year sample of Barn Owls, no indications were found of toxic levels of Cd, Cu, Pb, Mn, or Fe in the Netherlands. It is concluded that the Barn Owl is a suitable biomonitor. Furthermore, a network of volunteers can produce an informative sample of the Barn Owl population without interfering with the population.