Abstract
Garden Warblers Sylvia borin were studied during autumn stopover in Crete before crossing the barrier of the Mediterranean Sea and the Sahara Desert. Birds followed with transmitters show extensive stopover periods, which were longer in first-year birds, 16 days, compared with adult birds, 14 days. The distribution of body masses from birds trapped in fig trees were used to estimate the departure body mass and the results found indicate that both age categories on average depart with a fuel load close to 100% of lean body mass. The movement of transmitter birds shows differences between first-year and adult birds. Adult birds move further away from the release site and many also left the study area. Several were found settled outside the study area, up to 17 km away, indicating that they regularly make longer stopover movements. It is suggested that this might be a result of that they return to a place where they stayed during an earlier migration. It was shown that stopover site fidelity exists and nine garden warblers were recaptured in the area during a following autumn. The results found highlights the importance of stopover areas close to the Sahara Desert.
Highlights
About 2 billion songbirds breeding in the Western Palearctic cross the Saharan Desert every autumn to reach their wintering grounds (Hahn, Bauer & Liechti, 2009)
Mean body mass of birds trapped using tape lures on southern Crete was lower for first-year birds compared to adult birds
Adult birds trapped by means of luring show an increasing body mass against date (Spearman correlation; RS = 0.15, p = 0.001, n = 450) while no such correlation was found in first year birds (RS = 0.03, p = 0.437, n = 658)
Summary
About 2 billion songbirds breeding in the Western Palearctic cross the Saharan Desert every autumn to reach their wintering grounds (Hahn, Bauer & Liechti, 2009). Gradual variations in the extent of the desert has occurred over time, most recently from a humid period to a period of desertification in North Africa seem to have started about 6000 years ago (Holmes, 2008; Kropelin et al, 2008). These conditions have probably resulted in fluctuations in the difficulty for birds to cross the barrier. Most of the long distance migrants are not adapted to refuel in oases or in the surrounding vegetation (Jenni-Eiermann et al, 2011) and extensive fuel loads are stored in advance as can been seen from the very high body masses of birds close to the barrier (e.g., Finlayson, 1981; Fransson et al, 2006; Fransson et al, 2008)
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