SEEN - SE European Bird Migration Network
The Long-eared Owl (Asio otus) sex structure and autumn migration dynamics at Bukowo-Kopań station in 1999 - 2004
Robert Lasecki
Bird Migration Research Station, University of Gdańsk, Poland

This work presents an analysis of Long-eared Owls sex composition and migration dynamics at Bukowo–Kopań bird ringing station. Data were collected during autumn seasons 1999-2004. Over 2400 of Long-eared Owls Asio otus were caught (796 males and 1618 females). Data collection was done according to standard Operation Baltic methods – continuous mist–netting during whole migration season (special raptor mist-nets were used).
Passage of this species is characterized by three, distinct waves. Relations between migrants’ sex composition and migration time were analysed. There were differences in males’ and females’ passage in several migration waves (stat. signif. in autumns 2000, 2002 and 2004; in other years differences were statistically non-significant – probably because of too low number of birds of one sex category). Chi2 test applied to pooled data from 1999-2004 years showed highly significant differences (χ2 = 43,9; p<<0,0001). Females prevailed in all autumns (average 67%) but there were variations in several seasons. Males migrated earlier – from 1 to 6 days in different years and their percentage number was the highest in the last passage wave. In years, when the highest number of owls was caught, mean migration date of males was 7 days later than in years of low number of caught birds. Similar difference was found in females but it was 3 days. The observed pattern was probably a result of small rodent number fluctuation.
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