Avian migration strategies may include intra-specific variations, also known as differential migration. These migratory adaptations between conspecifics are particularly important given their implications for population dynamics. We aimed to examine the differential migration pattern as a function of age and sex, and whether this pattern also determines body morphology, in a passerine whose migration strategy includes movements to both pre- and sub-Saharan wintering areas. Thus, 1,100 common chiffchaffs Phylloscopus collybita were sampled during 2009–2012 at eight wintering locations within the Palearctic and Afrotropical regions. Biometric and body condition variables were studied for their relationship to geographical, climatic and productivity indexes. A higher proportion of adults and females were observed at lower latitudes. Common chiffchaffs were larger and in better body condition in sub-Saharan areas, coinciding with greater plant productivity and less annual rainfall. The selection of the southernmost areas by adults may be related to fidelity to more favourable sites. Southern and coastal locations offer better conditions for chiffchaffs, despite the challenges involved in crossing the Sahara Desert. Moreover, the greater skills of adults in finding abundant food resources may also drive greater relative abundance south of the Sahara. Greater body mass and fat deposits in birds wintering in sub-Saharan areas reflect higher resource availability and may imply that the birds are in better physical condition.
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Vol. 62 • No. 2