SEEN - SE European Bird Migration Network
Autumn migration of Long-eared Owls counted visually at Pape 1985 - 2004
Guntis Graubics(1), Janis Baumanis(2), Karlis Millers(2)
1) Riga Zoo, Latvia
2) Institute of Biology Latvian University, Latvia

The Long-eared Owl (Asio otus) is both resident and migratory species which migration ecology is still poorly understood. Owls feed mainly on small rodents and obviously whole life cycle of these birds including migration strategy depends on prey population fluctuations. N European population may spend winter in the territory from Finland to NE Russia during years of abundant prey. Unfavourable feeding conditions cause a considerable part of northern population to migrate to wintering grounds further south to W Europe. Their migration route then goes along the SE coast of the Baltic Sea.
Autumn bird migration studies at Pape Ornithological Station started in 1967 with mass-scale trapping and ringing of migratory birds including owls. However, trapping results did not reflect the actual number of migrating owls nor made it possible to compare the migration intensity in different years as trapping results highly depend on weather conditions. In 1985 we introduced a completely new device enabling visual censuses of migrating birds during the night. A system of three spotlights (1000 W halogen bulbs) was in use. The spotlights were installed 4 m above the ground in open field about 200 m away from the seacoast. Spotlights were positioned in order to create a continued illuminated semicircle (ca 200 m in radius) perpendicular to the main direction of owl migration. Visual observations were continued during the whole migration season of the Long-eared Owl from September to late November. Observations were carried out each night. They lasted 30 minutes each and were repeated every 1.5 – 2 hours. The total number of migrating owls was estimated by extrapolation. During 20 years a total of 13 284 migrating Long-eared owls were recorded.
In the years of abundant migration first migrants appeared at Pape already in the first or second decade of September. However, in the years of minimal migration their movements started in the third decade of September. Migration usually ends in late November. Since juveniles start to migrate earlier in the season, the maximum migration date is later when the adult ratio in migration movements is higher. The estimated migration intensity in a season ranges from 443 (1998) to 4805 (1986) individuals. Our data show that the number of migrating owls fluctuates every 2-5 years. During 20 years of investigations some long-term decreasing tendency of migrating Long-eared Owls was recorded in Pape.
Created by Pronetix 2006