Narrow sea crossings present major obstacles to migrating Griffon Vultures Gyps fulvus

Abstract

The flight behaviour of Griffon Vultures Gyps fulvus was studied at a major migration bottleneck, the Strait of Gibraltar in southernmost Spain, during the autumns of 2004 to 2007. The 14‐km‐wide sea channel significantly impeded the southern migration of the species into Africa, with many birds attempting repeated passage for weeks before crossing, and others not crossing at all and overwintering in Southern Spain. Water‐crossing attempts were restricted to times between 11:00 and 14:00 h on days with light or variable winds, or on days with strong winds from the north or west. No crossing attempts were made on days with strong winds from the south or east. Vultures attempted to cross the Strait in large flocks and never attempted to do so alone. Although 29% of the birds soared during crossing attempts, at least until they flew beyond visible range of approximately 4 km, most engaged in considerable flapping flight when attempting to cross. Overall, birds flying over water flapped more than 10 times as frequently as those flying over land prior to crossing attempts. Vultures did not flap continuously, but intermittently in brief bouts of flapping interspersed with periods of gliding or soaring flight. The number of flaps per bout over water was significantly greater than the number of flaps per bout over land. Vultures flying over water that flapped at rates of 20 flaps or more per minute typically aborted attempted crossings and returned to Spain in intermittent flapping and gliding flight. There are numerous reports of Vultures falling into the Strait and drowning while attempting to cross, as well as reports of returning Vultures collapsing on the beach having reached Spain in spring (Barrios Partida 2006). Our observations indicate that passage of Griffon Vultures at the Strait of Gibraltar is limited by the species’ over‐water flapping‐flight abilities, including its inability to flap continuously for even short periods of time. We suggest that even relatively short sea crossings represent significant obstacles to migrating Vultures and discuss the implications of this limitation on the distribution and abundance of the species.

Regional and seasonal flight speeds of soaring migrants and the role of weather conditions at hourly and daily scales

“…Only above sea we consistently underestimated the hourly and daily performance for both species. The general absence of thermals over large water bodies in the temperate zones prohibits stationary circle‐soaring there and requires soaring migrants to cross water bodies using flapping flight (Kerlinger , Bildstein , Bildstein et al ). We actually would expect that a transition to flapping flight lead to higher ground speeds than predicted from our model, given the model was parameterized on predominantly continental soaring migration data.…”

Maximum number of Griffon Vultures ever recorded on active migration in a single day at the Strait of Gibraltar

“…More importantly, big flocks of Griffon Vultures sometimes make several abortive attempts at crossing the sea, starting high over the European shore and often gaining several kilometres over the sea before coming back to the original coast. This typical behaviour has been described as 'hesitating' by the main authors that studied Griffon Vulture migration in the Strait (Bernis 1983, Griesinger 1994, Bildstein 2009). This behavioural feature makes monitoring difficult and counting imprecise from the European shore because birds that hesitate or abort attempted sea crossings can return to the Spanish shore and be recounted once they reintegrate into another flock about to depart.…”

Continental-style avian extinctions on an oceanic island

“…There are no indications that Trigonoceps sp. had limited flight ability, but their low-aspect-ratio wings make vultures ill-suited for crossing water bodies (Kerlinger, 1985;Bildstein et al, 2009), and imply a limited range as well.…”