P. 2005. Differential migration of chiffchaffs Phylloscopus collybita and P. ibericus in Europe and Africa. Á/ J. Avian Biol. 36: 184 Á/190.Differential migration is a widespread, but poorly understood, phenomenon in birds. In this paper, we present the first detailed field study of differential migration in the Old World warbler (Sylviidae) family. We studied two chiffchaff Phylloscopus [collybita ] semispecies: the common chiffchaff P. [c. ] collybita and the Iberian chiffchaff P. [c. ] ibericus. Using data collected at several latitudes in Europe and Africa, we present convincing evidence for differential distance migration of sexes in chiffchaffs, with females moving further than males. Interestingly, while there was a pronounced gradient in the sex-ratios in Europe and North Africa (with an increasing proportion of females with declining latitude), no clear pattern was found south of the Sahara, where sex-ratios were more male-biased than predicted by a simple latitude model. This suggests that, amongst the chiffchaffs wintering in West Africa, a large proportion is composed by Iberian birds, and provides support to previous suggestions that Iberian chiffchaffs are long distance migrants. Results from detailed studies in Senegal also show that chiffchaffs display differential timing of spring migration, with males leaving the winter quarters considerably earlier than females. The results are discussed in the framework of the three main (non-mutually exclusive) hypotheses attempting to explain the latitudinal segregation of the sexes. Given the relative failure of standard comparative studies to discriminate between competing single-factor hypotheses to explain differential migration, it is argued that the chiffchaff species complex might be particularly suited to study this issue using a new approach suggested by Cristol et al. (1999): detailed (further) comparisons between closely related species (such as the common and the Iberian chiffchaffs) could help identifying the key factors to be incorporated into optimality models that can predict relative distance of migration of different sex or age classes.
To African-Palearctic migratory birds that spend the non-breeding season in sub-Saharan Africa, the Sahara desert is an ecological barrier that must be crossed using large energy reserves stored at stopover sites either side. Given the coincidence of spring migration with the height of the dry season in the west African Sahel, spring stopover conditions may be a limiting factor that regulates populations. By determining where, when and how reserves are accumulated prior to crossing the desert, we can identify possible population regulation mechanisms in Palearctic migrants and effectively target stopover sites for protection. We present data on the non-breeding season use of Djoudj National Park in the western Sahel by 38 Palearctic migrants based on constant effort mist-netting data. Twenty-four species showed evidence of using Djoudj either throughout the nonbreeding period or during spring migration. The remaining 14 species were largely absent from Djoudj and presumably accumulated energy reserves for spring migration south of the Sahel. Species preparing for spring migration at Djoudj began increasing their energy reserves in January or February. Energy reserves were generally accumulated at slow rates during long stopovers, and in all species examined, predicted take-off fuel loads were sufficient to cross the Sahara desert under certain conditions. Three species, Common Redstart Phoenicurus phoenicurus, Common Whitethroat Sylvia communis and Common Blackcap Sylvia atricapilla, achieved faster rates and had concurrently lower stopover durations. We predict that vulnerability to fluctuating environmental conditions in the Sahel region is greater in species that initiate fuelling later in the spring and in species with short stopover durations.
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 (
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