Barn owl release in lowland southern England - a twenty-one year study

摘要

Twenty-one years of carefully documented barn owl (Tyto alba) study and release in England by the South Midlands Barn Owl Conservation Group (SMBOCG) have necessitated a re-examination of some of the premises on which such release schemes are judged. Fifteen years after the first releases, the number of independently breeding owls (i.e. breeding 'wild' or 'as though wild') in a 1200 kin 2 study area is still increasing sharply and now far exceeds the number released in any one year, with the rate of increase outstripping the cumulative number of release events. Ringing returns indicate that fledged young dispersed further than adults, and that released adults which immediately deserted their release sites were less likely to survive their first 30 days than those staying in the vicinity of their release, where they could take advantage of supplementary feeding. Artificial feeding at release sites led to pellets containing lower liveweight equivalent of wild-caught prey but higher numbers of fledged young. Otherwise, and importantly, no significant difference in survival, mortality, dispersal, foraging success or breeding success was found between wild/independent owls and various categories of released bird. Availability of field vole habitat was not as important for barn owl breeding success as was found in previous studies, with owls able to replace this species in their diet with the less habitat-specific wood mouse. Evidence is provided that starvation was not as important a cause of mortality as has been proved elsewhere. Although barn owl nests often occurred in extreme close proximity, breeding density had no effect on either foraging or breeding success. The possible genetic consequences of barn owl release in Britain are briefly discussed. Very few barn owl release schemes have attempted to prove that their work is worthwhile, so long-term, well-documented studies such as that presented here are vital in establishing some of the principles involved in releasing barn owls into the wild.