Coastal Migration and Wind Drift Compensation in Nocturnal Passerine Migrants

Abstract

The orientation of nocturnal passerine migrants in relation to winds and coastlines was studied by visual observations (ceilometer observations). Numbers and track directions of migrating birds passing at low altitude (<200 m) through a vertical light beam were recorded simultaneously at sites along two coastlines at opposite sides of the Falsterbo peninsula in southwestern Sweden in the autumn of 1989. The directions of the coastlines differed by 340 at the two observation sites, which were only 350 m apart. Observations were made during the first part of the night a few hours after sunset. Wind direction and speed were measured at 11 m altitude and the headings of individual birds were calculated. The observed mean track direction of the migrants differed significantly between the two sites, and coincided with the orientation of the coastline at each site. At site 1 partial compensation for wind drift was observed, while at site 2 wind compensation was complete. The results show that nocturnal passerine migrants flying at low altitude follow coastlines and that they may use the coastline to compensate for wind drift. It remains to be shown how widespread coastal migration and the use of topographical landmarks are for wind drift compensation among nocturnal migrants.