DEPARTURE DECISIONS OF A MIGRATORY PASSERINE, THE COMMON REED-WARBLER ACROCEPHALUS SCIRPACEUS, IN RELATION TO FUEL LOAD AND GEOGRAPHICAL BARRIER PROXIMITY

Research on the stopover ecology of migrant birds has examined the role of multiple factors, both intrinsic and extrinsic, on departure decisions. However, a single factor may influence stopover decisions differentially, depending on the relative geographic location of a stopover site within the flyway and the expectation of ecological challenges ahead. For instance, actual fuel load may have a decisive influence on departure decisions before a crossing of major geographical barriers, such as seas or deserts, but may be less important when crossing vast continental areas offering plenty of opportunities to refuel. The aim here was to test whether the relative influence of fuel load on departure decisions of the Common Reed-warbler Acrocephalus scirpaceus, a long-distance migrant songbird, varies in relation to proximity to a geographical barrier: the sea expanse between Iberia and Africa. We expected that departure from southern Iberian stopover sites would be highly influenced by fuel load, since migrants would not fly to Africa until they had sufficient fuel to cross the sea. We used Cormack-Jolly-Seber (CJS) models to test for the effect of fuel load on emigration (departure) probability from a number of stopover sites situated in northern and southern Iberia during the autumn migration period. Reed-warblers stopping over for longer than one day were more likely to depart if they were in northern Iberia than in southern sites. Moreover, large fuel loads promoted emigration from stopover sites in northern, but not in southern, Iberia. Therefore, we found no evidence supporting the hypothesis that Reed-warblers wait until they acquire large fuel loads before leaving their stopover sites near the edge of the barrier. This study shows that the same parameters may not act in the same way along migration route, in particular in relation to geographical barrier proximity.