WOJCIECHOWSKI, M; YOSEF, R; PINSHOW, B; Ben-Gurion University of the Negev, Israel; International Birding and Research Centre Eilat, Israel; Ben-Gurion University of the Negev, Israel: Body composition of small migrating birds at a stopover site in southern Israel.
To assess changes in body composition with changing body mass during migratory stopover, we scanned over 600 migrating birds of 21 species [body mass < 50 g] using dual-energy X-ray absorptiometry at a stopover site at Eilat, Israel. Analysis of four passerine and one wader species supports the idea that fat-free mass is replenished first, and only after it asymptotes does fat mass increase. The observed changes may reflect two functional phases of body mass gain. Non-fat tissues are rebuilt first, including those subsequently necessary to restore fat, which takes place second. Seasonal differences in body composition of fruit-eating Sylvia atricapilla and S. borin indicate that birds leave Eilat northwards in spring before storing fat to maximum capacity, but close to completion of fat-free mass regain. In contrast, S. atricapilla arrive at Eilat going south in autumn in the final stages of rebuilding their fat free mass, and during stopover they build-up fat stores before recommencing migration. The difference in strategies may be because fruits are reliably available to north-bound birds along their passage, but less to southbound birds during their Sinai and Sahara desert crossing. In invertebrate-eating birds (Calidris minuta, Acrocephalus scirpaceus, Phylloscopus collybita and Luscinia svecica), the process of rebuilding fuel reserves is completed before leaving Eilat, both during autumn and spring migration. We suggest that because invertebrate availability is dependent on substrate and weather, invertebrate eaters opportunistically complete fueling in both directions.
