Spring passage of Willow Warbler Phylloscopus trochilus across the western Mediterranean:: comparing islands with the mainland

Spring migration of Willow Warbler Phylloscopus trochilus over the western Mediterranean was studied using observations collected at a ringing station on the mainland of Spain (Aiguamolls de l'Emporda), and at an island station (Cabrera, one of the Balearics). During the years 1993-2001, birds were caught and measured between 16 April and 15 May. The peak of captures and mean arrival date was seven days earlier on the continental station than on the island. At both stations long-winged individuals passed before short-winged birds, and birds at the continental station had longer wings than at the island. The number of recaptures was low at both locations suggesting that most birds departed at the day of arrival. The mean stopover period for recaptured individuals at the continental and island station was 2.2 and 2.6 days, respectively. Body mass remained more or less constant during this stay. Our observations suggest that subpopulations of Willow Warblers, differing both in morphology and timing of migration, have different migration routes over the western Mediterranean; one migrates over the Iberian Peninsula and another crosses over the sea.