SEEN - SE European Bird Migration Network
Measuring body composition of small migrating birds with dual-energy X-ray absorptiometry at a migratory stopover in Eilat, Israel
Michał S. Wojciechowski(1), (2), Reuven Yosef(3), C. Korine(1), B. Pinshow(1), (4)
1) Mitrani Dept. Desert Ecology, The Jacob Blaustein Inst. Desert Research, Israel
2) Present address: Dept. Animal Physiology, N. Copernicus University, Toruń, Poland
3) Int. Birding & Research Center in Eilat, Israel
4) Dept. Life Sciences, Ben-Gurion University of the Negev, Israel

Dual-energy X-ray absorptiometry (DEXA) is a method of measuring body composition in animals which gives a tissue signature separated into bone, fat and non-fat masses.  We used DEXA to measure body composition of 600 individual birds of 21 species.  Analysis of four passerine and one wader species supports the concept that when birds increase body mass (mb) when feeding at staging sites during migration, fat-free mass is replenished first, and only after it asymptotes does fat mass increase.  The clear correlations of body composition with mb allow a "reverse" analysis of body composition of small migrants based on mb data. Frequency distributions of body masses of birds ringed in spring and autumn indicate that most Sylvia atricapilla and S. borin, both mostly frugivorous during migration, arrive at Eilat in the spring with their fat mass at its lowest but beginning to rebuild non-fat mass.  By contrast, in autumn south-bound S. atricapilla arrive at Eilat in the final stages of rebuilding their fat free mass.  In invertebrate-eating birds (Calidris minuta, Acrocephalus scirpaceus, Phylloscopus collybita and Luscinia svecica), we did not observe seasonal changes in mb and body composition of birds caught in Eilat. 
We suggest that because availability of fruit and invertebrates differ, birds that have different diets use different refueling strategies during migration.  Frugivores probably need larger fuel stores before flight because the distances between patches rich in fruit are large, whereas invertebrate eaters may feed on the way during both spring and autumn because insects, although temporarily unpredictable, are present in both seasons.  In addition, above data may provide a useful tool for ringers. To date most stations evaluate fat and muscle using a subjective score.  Presented relationship between body composition and mb allows to estimate with high confidence the amount of fat and non-fat tissue in a bird based on body mass alone and provides an objective method of estimating body composition of small migrants.
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