SummaryThe Balearic Shearwater Puffinus mauretanicus is considered one of the most threatened seabirds in the world, with the breeding population thought to be in the range of 2,000–3,200 breeding pairs, from which global population has been inferred as 10,000 to 15,000 birds. To test whether the actual population of Balearic Shearwaters is larger than presently thought, we analysed the data from four land-based census campaigns of Balearic Shearwater post-breeding migration through the Strait of Gibraltar (mid-May to mid-July 2007–2010). The raw results of the counts, covering from 37% to 67% of the daylight time throughout the migratory period, all revealed figures in excess of 12,000 birds, and went up to almost 18,000 in two years. Generalised Additive Models were used to estimate the numbers of birds passing during the time periods in which counts were not undertaken (count gaps), and their associated error. The addition of both counted and estimated birds reveals figures of between 23,780 and 26,535 Balearic Shearwaters migrating along the north coast of the Strait of Gibraltar in each of the four years of our study. The effects of several sources of bias suggest a slight potential underestimation in our results. These figures reveal the urgent need to reformulate the population viability analysis for the species, and then if necessary reconsider its conservation status.
Wind is an important selective agent in bird migration. In this study, we investigated the influence of wind on the air speed of pelagic seabirds in the Strait of Gibraltar by means of radar measurements. The birds were identified visually at the species level. A total of 354 radar tracks of migrating birds were analysed, with an average tracking time of 147 s per target. An interspecific comparison under negligible wind speeds showed a slight increase of air speed in the order from Cory's Shearwater, a typically gliding species (12.8 m s -1 ), Balearic Shearwater (gliding with flapping, 14.7 m s -1 ), gannets (flap-gliding, 15.0 m s -1 ), Great Skua (flapping, 16.0 m s -1 ) to auks (fast flapping, 17.4 m s -1 ). All of the studied species decreased their air speed with increasing wind increment (ground speed minus air speed), and this occurred in following winds slightly less than in opposing winds. Auks adjusted air speed to wind increment only in opposing winds, suggesting that auks are not able to reduce their high air speed in following winds due to extremely high wing loading. Zusammenfassung Fluggeschwindigkeiten ziehender Meeresvögel in der Strasse von Gibraltar in Relation zum WindWährend einer Frühlings-und einer Herbstzugperiode wurden auf dem Plan-Positions-Indikator eines auf der Südspitze von Tarifa positionierten S-Band-Schiffradars die Flugwege ziehender Meeresvögel aufgezeichnet. Gleichzeitig wurde deren Artzugehörigkeit durch Feldbeobachter bestimmt. Stündliche Windmessungen auf der Flughöhe der Vögel erlaubten es, den Einfluss des Windes auf die Eigengeschwindigkeit der pelagischen Zugvögel zu untersuchen. 354 Flugwege mit einer durchschnittlichen Aufzeichnungsdauer von 147 s wurden analysiert. Ein interspezifischer Vergleich unter vernachlässigbaren Windgeschwin-digkeiten zeigte eine leichte Zunahme der Eigengeschwindigkeit vom GelbschnabelSturmtaucher (einem typischen dynamischen Gleiter mit 12.8 m s -1 ) über den Balearen-Sturmvogel (Gleiter mit Flügelschlägen: 14.7 m s -1 ), bis zu Basstölpel (Schlag-und Gleitflug: 15.0 m s -1 ), Skua (Schlagflug: 16.0 m s -1 ) und Alken (rascher Schlagflug: 17.4 m s -1 ). All diese Arten senkten ihre Eigengeschwindigkeit wenn die Differenz zwischen ihrer Geschwindigkeit über Grund und ihrer Eigengeschwindigkeit zunahm. Der Effekt war besonders stark bei abnehmendem Gegenwind, etwas weniger stark bei zunehmendem Rückenwind. Alken (Tordalk und Papageitaucher) passten ihre Eigengeschwindigkeit nur bei abnehmendem Gegenwind an. Es scheint, dass sie aufgrund der extrem hohen Flächen-belastung ihrer Flügel nicht in der Lage sind, ihre Eigengeschwindigkeit unter die Geschwindigkeit bei Windstille zu senken. Communicated by A. Hedenström. M. Mateos-Rodríguez
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