![]() Interestingly, some studies suggest that birds also travel under suboptimal conditions, such as in headwinds or crosswinds, because waiting for optimal conditions would result in a major delay (Bruderer 1999, Karlsson et al. However, as the study region is characterized by a highly variable meteorological environment dominated by westerlies, the anticipated high numbers of autumn migrants would be faced with headwinds which might prevent them from flying over water. So basically, tailwinds (north- to north-easterly in autumn south- to south-westerly in spring) both in autumn and spring would be expected to promote migration over the open sea of the Bay of Biscay. Prevailing conditions over a given barrier could then eventually determine if migrants decide to cross or to make a detour. Adverse weather, such as headwinds or precipitation, halt or delay the crossing of barriers, as it could entail excessive energy costs or increase mortality risk (Richardson 1978, Liechti & Bruderer 1998). For instance, in Iberia some migrants select more western (Atlantic) flyways in autumn, but more eastern (Mediterranean) flyways in spring (Bairlein 2001).Īlso prevailing seasonal meteorological conditions could have an impact on migratory routes, for example, when crossing geographical barriers (Thorup et al. It is well known that variations in migration strategies found in short- and long-distance migrants cause changes in the spatial and seasonal flow of migration (Bruderer & Liechti 1999) resulting in different routes in spring and autumn (Klaassen et al. By contrast, a coastal operational radar located in the southeastern Bay of Biscay revealed high nocturnal activity in spring and low activity in autumn (Weisshaupt et al. ( 2014) reported much weaker passage in spring compared to autumn. In another study, which was based on passerines ringed at several coastal stopover sites along the Bay of Biscay, Arizaga et al. They concluded that migration was proceeding on a broad front. In their study on diurnal autumn migration through the Pyrenees, Lack & Lack ( 1953) observed great numbers of birds coming in from the Bay of Biscay as well as travelling along the coast and then continuing inland. Few studies have been published concerning the role of the Bay of Biscay in bird migration. The East-Atlantic flyway represents one of the principal migration routes in Europe, comprising the Bay of Biscay as a geographical barrier for migratory landbirds. ![]() In both seasons, there was no significant response to wind conditions. The majority of the targets were passerines.Ĭonclusions: The patterns observed suggest spring migration occurs on a broader front, potentially involving sea crossing further to the west, while autumn migration concentrates more eastwards over land. ![]() Tailwind had no significant effect on migration intensity, but migration ceased during strong cross- or headwinds despite clear skies. ![]() Autumn migration was weak at the western coast, but strong at the eastern coast and inland. Results: Migration was strong at both coastal sites in early spring. Methods: A thermal-imaging camera was used at two coastal study sites (Punta Galea, Cape Higuer) in spring and three study sites (coast: Punta Galea, Cape Higuer inland: Iregua Valley) in autumn for four hours from sunset over 90 nights in 20. Capsule: Bird migration was recorded by an infrared device at three sites in the southeastern Bay of Biscay, indicating seasonal east–west differences in migration flow.Īims: The main aims of this study were to quantify and describe nocturnal migration dynamics in proximity of a sea barrier, and to assess seasonal and geographical drivers of migration patterns. ![]()
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