Garden Warbler Sylvia borin migration in sub-Saharan West Africa: phenology and body mass changes
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The Garden Warbler is a classic subject for the study of Palaearctic-African bird migration strategies. Most studies have considered the situation close to the breeding areas, while the African and especially the sub-Saharan part of the species' migration have received comparatively little attention. Here we use autumn and spring ringing data from Nigeria and The Gambia to study the movements and energetics of the species in West Africa during the non-breeding season. The first Garden Warblers arrive south of the desert around the beginning of September, roughly at the same time as the median date for their passage through the Baltic Sea region and c. 3 weeks before their median passage date through southern Italy. In the Nigerian Sahel savannahs, where, owing to the rainy season and its associated increase in food availability, many more Garden Warblers stop over in autumn than in the dry spring, the median date of passage is 1 October. The body mass on arrival south of the desert is normally only a few grams more than the lean body mass (LBM; 15 g) - with a mean of 16.6 g (sd = +/- 1.8 g) in The Gambia and 17.4 g (sd = +/- 1.8 g) in the Nigerian Sahel. After resting and refuelling in the Sahel, Sudan and Guinea-type savannahs the Garden Warblers depart during November-December for wintering areas further south. Before leaving, they again increase their body mass, with an average fuel load of c. 20%, and often more than 50% relative to LBM. Some of the birds passing through Nigeria probably spend midwinter around the Congo Basin. During spring they return northwards to the Guinea savannah zone in April and fuel-up there for the trans-Sahara passage. At this time they normally increase their body reserves to around 50% of the LBM, but c. 10% of the birds gain 100%, thus doubling their mass. The passage there peaks around 20 April and continues well into May. That the main take-off northwards is directly from the Guinea savannahs is indicated by the very low numbers trapped in the Sahel during spring.