Long-Term Viability of the Re-Introduced Population of the White-Tailed Eagle Haliaeetus albicilla in Scotland

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The white-tailed eagle became extinct in Scotland early in the twentieth century after persecution by humans. A population has been re-established in western Scotland by the release, mainly as juveniles, of 82 wild-bred birds imported from Norway between 1975 and 1985. Some released immatures wandered widely, but birds settled on fixed home ranges within 100 km of the release site on Rum, usually at 3-4 years old. In 1992 there were c. 19 individuals, including eight pairs, occupying fixed home ranges. There was also an unknown number of immature birds. Annual survival rates were estimated at 73% for young birds prior to settlement and 94% thereafter from the observed numbers of released and established birds. However, estimates were imprecise because of the small size of the population and the lack of reliable individual marks. By 1992 a total of 29 young had been fledged in the wild from five home ranges. None of these young have yet bred. The mean breeding success of released birds increased with age up to 0.465 fledged young/pair/year at about 6 years old. There was significant variation in breeding success among individuals and/or home ranges. 13% of bird-years of occupation of fixed home ranges involved either unmated birds or supernumeraries in the home ranges of pairs, creating trios. A simulation model indicated that the observed proportion of unmated birds was similar to that expected from differences in the numbers of males and females arising by chance. It was estimated from the available demographic data that the population would be more likely to grow slowly than decrease if stochastic effects due to small population size did not operate. However, the 95% confidence interval for the estimated population multiplication rate encompassed decline at 11% per year and growth at 11% per year. Simulation models indicated that, with realistic assumptions about variation among individual birds or ranges and demographic parameters set at their maximum-likelihood values, there was a probability of about 0.6 that the present population would become extinct within 100 years. Only conditions which would permit population growth at 2-3% per year in the absence of stochastic, small-population effects would reduce the probability of extinction within 100 years to less than 0.1. Simulation of supplementary releases of imported juveniles indicated a substantial reduction in extinction risk from the release of a further 60 juveniles. Variation in the rate of release, within a range considered feasible (10 birds per year -1 or 20 birds year -1 ), had little effect on this result.