Age Structure of the Long-Eared Owl (Asio otus) Migration at Bukowo-Kopań Station (Southern Baltic Coast) in Autumns 1996-2003

Over 2000 Long-eared Owls were caught (217 adult and 1827 immatures) at Bukowo-Kopań station (54°21'N, 16°17'E / 54°28'N, 16°25'E), in the central part of the Polish Baltic coast, during autumn seasons 1996-2003. Migration of this species had wave-like pattern with three distinguished passage waves. There were differences between young and adult birds passage in subsequent passage waves (statistically significant only in autumns 2000 and 2003, in other seasons they were non-significant. The value of X²-test calculated for all seasons together was high and differences were significant (X² = 10.96, p < 0.01). Generally, very high variation must be stressed. Young birds dominated during autumn migration (on average they constituted 89%) but this number varied distinctly in different years. The immatures were the most numerous in the second and third wave (they were especially numerous in the third wave in years with the highest number of caught birds). Adult owls migrated a bit earlier - average passage data was earlier by 1 to 16 days in various seasons. All data point at very high variability of the Long-eared Owl migration and necessity of deeper studies on this phenomenon.