We analysed fat reserves of 28 species of trans-Saharan migrants after their crossing of the Sahara and most of the central Mediterranean on their way towards the breeding grounds in spring. A considerable variability was observed, with both fat and lean species, indicating species-specific strategies adopted to cross the Sahara and the Mediterranean. Lean species showed mean body mass values well below those reported during the breeding season, suggesting a pronounced physical stress caused by the migratory flight. To investigate the factors influencing the levels of residual fat reserves, we analysed correlates of mean fat load of each species at this stage of the journey. A stepwise multiple regression was run, with the fat reserve level as the dependent variable and the mean latitude of the wintering and breeding quarters, the northernmost latitude of the preferred habitat south of the Sahara, and wing morphology as independent variables. Residual fat load was standardised for each species' flying efficiency, using the residual of a regression of fat load on fat consumption per unit of distance covered, calculated using Pennycuick's (1989) model. After standardisation, residual fat load was significantly correlated only with the northernmost latitude of the optimal habitat south of the Sahara. The explained variance did not change when admitting for the species adapted to open habitats to make a stopover in northern Africa. Morphology (wing shape and tail/wing ratio) was significantly related to the distance covered but not to the residual fat levels. Our results suggest that the crossing of the Sahara and Mediterranean is constrained by the distribution of preferred habitat south of the Sahara. The ongoing desertification process in sub-Saharan Africa might therefore have a negative impact not only on the species wintering in the sub-Saharan area, but also on those species wintering in equatorial Africa.
Ecological and morphological correlates of residual fat reserves in passerine migrants at their spring arrival in southern Europe
PILASTRO, ANDREA AUGUSTO;
1997
Abstract
We analysed fat reserves of 28 species of trans-Saharan migrants after their crossing of the Sahara and most of the central Mediterranean on their way towards the breeding grounds in spring. A considerable variability was observed, with both fat and lean species, indicating species-specific strategies adopted to cross the Sahara and the Mediterranean. Lean species showed mean body mass values well below those reported during the breeding season, suggesting a pronounced physical stress caused by the migratory flight. To investigate the factors influencing the levels of residual fat reserves, we analysed correlates of mean fat load of each species at this stage of the journey. A stepwise multiple regression was run, with the fat reserve level as the dependent variable and the mean latitude of the wintering and breeding quarters, the northernmost latitude of the preferred habitat south of the Sahara, and wing morphology as independent variables. Residual fat load was standardised for each species' flying efficiency, using the residual of a regression of fat load on fat consumption per unit of distance covered, calculated using Pennycuick's (1989) model. After standardisation, residual fat load was significantly correlated only with the northernmost latitude of the optimal habitat south of the Sahara. The explained variance did not change when admitting for the species adapted to open habitats to make a stopover in northern Africa. Morphology (wing shape and tail/wing ratio) was significantly related to the distance covered but not to the residual fat levels. Our results suggest that the crossing of the Sahara and Mediterranean is constrained by the distribution of preferred habitat south of the Sahara. The ongoing desertification process in sub-Saharan Africa might therefore have a negative impact not only on the species wintering in the sub-Saharan area, but also on those species wintering in equatorial Africa.
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