The role of seas as a geographical barrier for migratory landbirds. An approach to the Bay of Biscay.

Abstract

The East-Atlantic flyway represents one of the main bird migration routes worldwide, comprising the Bay of Biscayas an inherent geographical barrier. So far, the significance of the Bay of Biscay for migrants and its potentialimpact on migratory routes have not received much attention in research. The general aim of this thesis is to unravelthe role of the Bay of Biscay as a geographic barrier both at a small and large scale shaping the nocturnalmigration of landbirds moving along the East-Atlantic flyway. All observation tools available in the study regiondeemed suitable for nocturnal studies were employed: operational wind profiler and weather radars, thermalimaging and moonwatching. Previous knowledge from visual observations and bird ringing available in literaturewas also taken into account. Furthermore, in a pioneer approach, the study assessed the potential use ofwind profiler data in an ornithological context. An objective qualitative and quantitative approach validated bythermal imaging was established to extract migration parameters.Migration traffic rates, flight directions and altitudes obtained by the two types of radars and thermal imagingfor various sites along the bay are provided and discussed in a meteorological and ecological context. Verticalhistorical wind profiler and current thermal imaging data indicated pronounced broad-front migration in earlyspring vs. more eastward (i.e. sea-avoidance) migration in autumn. Moonwatching provided additional informationon bird composition and confirmed predominance of passerines. Finally, horizontal weather radar datarevealed a north-south gradient along the French coast in spring, with higher intensity in the south-easternstudy area close to the main migration axis of the East-Atlantic flyway.