Migration of four Sylvia warblers through northern Senegal

Abstract

We discuss the migratory strategies of the different species in relation to their pre-migratory fueldeposition and how far they can fly theoretically with the recorded fat contents. Most birds carry enough fat at take off to take them at least to Morocco, probably Iberia and, with tailwinds, possibly even further north. M ost insectivorous passerine birds that breed in northern Europe migrate to Africa during the winter. This means that many birds conduct single migratory journeys of 5,000km or more and they have to cross both the Mediterranean Sea and the Sahara (Moreau 1972). Since, with few exceptions, birds have no opportunity to feed en route when passing these vast geographical obstacles they have to rely on stored fat and possibly on favourable winds (

Spring passerine migrants stopping over in the Sahara are not fall-outs

“…Taking the Garden Warbler and the Common Whitethroat as examples, the mean body mass of individuals nist-netted during the entire project on bird migration across the Sahara with both a fat score and a muscle score of 0 was 12.8 g (±0.09 SD, n = 121) and 10.9 g (±0.23 SD, n = 17), respectively (Swiss Ornithological Institute, unpublished data). Previous studies calculated potential flight ranges using 14.8 g or 16.1 g for as lean body mass for the Garden Warbler (Pilastro and Spina 1997;Hilgerloh and Wiltschko 2000) and 12.2 g or 12.9 g for Common Whitethroat (Pilastro and Spina 1997;Ottosson et al 2001). The difference will lead to higher estimated flight ranges for birds with larger fuel loads such as the birds in this study.…”

Garden WarblerSylvia borinmigration in sub‐Saharan West Africa

“…However, captures were concentrated in just a handful of days, with 71% of captures in 1992 occurring in two days in April, whilst in 1993 one day accounted for 68% of captures (Fig 2). This pattern of captures suggests forced landfall rather than a regular passage as observed in other Sylvia warblers during the same period (Ottosson et al 2001). The mean mass of all spring birds at Djoudj was significantly higher than those at either Vom or Amurum (t-test, test mean = 21.8, t 63 = 7.16, P < 0.001), suggesting that the fuel for the Saharan crossing had been accumulated to the south and further supporting a forced interruption.…”

“…As spring Garden Warblers trapped at Amurum in the Guinea-savannah zone carried high fuel stores and few birds were caught at the more northerly sites, Ottosson et al (2005) suggested that, unlike many other Sylvia warblers (Ottosson et al 2001), Garden Warblers fuel in the Guinea-savannah zone on their northward migration and not closer to the desert edge. Djoudj is situated further north than both these sites, yet 62 birds were captured there during spring migration.…”

“…The migratory strategies of trans-Saharan migrant passerines have generally been well described in Europe, whilst strategies south of the Sahara have received relatively little attention (Ottosson et al 2001(Ottosson et al , 2005. Given the strong link between population fluctuations and climatic conditions in the Sahel region of West Africa (Newton 2004), knowledge both of the regions occupied during winter and of the sites used during migration is essential if we are to understand population-limiting processes.…”

Crossing the Sahara desert: migratory strategies of the Grasshopper Warbler Locustella naevia

“…We hypothesise that north-western European populations of Grasshopper Warbler prepare for the autumn crossing of the desert at sites in the Iberian Peninsula, as has been shown for the Reed Warbler Acrocephalus scirpaceus among other species (Bibby and Green 1981). For the return journey, we hypothesise that sites close to the southern edge of the Sahara desert are utilised, as is the case for the majority of Sylviidae examined to date (Ottosson et al 2001) but not all (Ottosson et al 2005). We expect fuelling rates at the southern edge of the Sahara to be lower than in Iberia due to the impacts of the west African dry season and we hypothesise that inter-annual variation in rainfall in sub-Saharan Africa will affect fuelling rates and fuel store magnitude.…”