Seasonal variation in body fat and weight of migratory Sylvia Warblers in central Jordan By Fares Khoury Abstract: KHOURY , F. (2004): engl. Seasonal variation in body fat and weight of migratory Sylvia Warblers in central Jordan. Vogelwarte 42: –. The physical state of Sylvia Warblers stopping over in a large plantation on the edge of the eastern desert of Jordan was studied during spring and autumn migration. Sylvia species caught in autumn had generally higher fat loads than in spring, which can be attributed to common aspects of migration habits. However, variations in strate- gies of different species were indicated by different distributions of fat scores and physical changes during stop- ping over. In spring, Blackcaps stopping over replenished their fat reserves, possibly as preparation for crossing further desert areas in the Middle East. Several Lesser Whitethroats stopped over in spring without replenishing fat reserves. Thus their choice of a suitable site with low predation risk was probably to recover water balance and/or muscle tissue after crossing the Sahara. It is moreover assumed that Lesser Whitethroats are able to gradually cross open deserts of the Middle East where they may feed en route in spring. Many Garden Warblers stopped over without replenishing fat reserves but with a significant increase in body mass, which indicated recovery of water balance and/or muscle tissue. Garden Warblers were not recorded in open desert areas, thus they are believed to avoid further deserts and migrate gradually northwards along more fertile areas of the Middle East, where they feed en route. In autumn, most birds had high fat loads, but leaner Blackcaps and Lesser Whitethroats were frequent, some of which were found to stop over and replenish fat reserves, apparently in prepa- ration for crossing the Sahara. Variations in migration patterns among closely related long distance migrants are especially evident after crossing the Sahara in spring, and might reflect different migration strategies. Key words: Sylvia, physical state, bird migration, Middle East, Jordan. Address: Department of Biological Sciences, Hashemite University, P. O. Box 150459, Zarqa 13115, Jordan. E-mail: avijordan2000@yahoo.com 1. Introduction Migratory birds which travel to and from winter quarters in Africa have to cross vast areas of sea and/or deserts of the Middle East and the Sahara of North Africa. These ecological barriers require special physiological adaptations, including deposition of sufficient fat reserves (BAIRLEIN 1992, YOM-TOV & BEN-SHAHAR 1995) and mechanisms related to water balance (BIEBACH 1990, IZHAKI & MAITAV 1998, KLAASSEN et al. 1999). Migrant passerines with high fat reserves are assumed to cross the Sahara without refuelling, if they can rely on adequate tail winds (BIEBACH 1992, IZHAKI & MAITAV 1998). Most birds landing in the desert just wait for the next evening to continue their migration. However, weak/lean birds may stop over for longer periods to replenish their fat reserves if they land in or near suitable habitats, e.g. oasis (BAIRLEIN 1985, BIEBACH et al. 1986, LAVEE & SAFRIEL 1989). BIEBACH (1990) suggested four possible strategies of desert crossing, ranging from gradual, nocturnal migratory movements to non-stop flight over the entire Sahara. The different possibilities are not mutually exclusive and combinations are possible, even within one species (BIEBACH et al. 2000). Several Eurasian populations of various migrant species cross the Middle East, including Jordan, every autumn and spring on their way to and from Africa. Yet bird migration has not been studied in detail in Jordan and most other parts of the Middle East. A large proportion of the Mid- dle East, including Jordan, is arid desert with few areas containing habitats with lush vegetation and abundant water and food resources all year round. Around 85% of Jordan's area receives annually an average rainfall of less than 200 mm (Meteorological Department, Marka-Amman). In autumn, food resources become scarce after the long summer characterised by drought and high tempera- tures. From the perspective of an autumn migrant, the greater part of the Middle East is thus con- sidered as a further ecological barrier, where feeding is nearly impossible. In spring, conditions are Die Vogelwarte 42, 2004: –