Migrating Blackcaps Sylvia atricapilla were mist netted at the desert edge in northern Israel and in Elat (southern Israel) during spring and autumn migrations between 1970 and 1991, Birds in spring in northern Israel were representative of birds that had completed the crossing of the Sahara, while those in Elat still had to cross the 150 km of the Negev Desert, which separates Elat and northern Israel. In autumn, birds captured in northern Israel were representative of those about to cross the Sahara Desert, while those in Elat had already started to cross the desert. The data allowed analysis of seasonal and location differences in the physiological state of Blackcaps before and after crossing the Sahara. Data analysed included body mass, visible fat score and calculated fat content. Autumn migrants were in better physiological condition than spring migrants at both locations, probably as a consequence of their migration route through fertile areas in autumn compared with the crossing of the Sahara in spring. Body mass was less variable after the Sahara crossing in spring than before the crossing in autumn. In spring, 71% and 67% of the birds were fat depleted (fat scores 0 and 1) at Elat and in northern Israel, respectively, while in autumn 34% and 42% were fat depleted. Blackcaps at Elat were 1.6 g lighter than those in northern Israel in autumn and 1.9 g lighter in spring. Potential flight ranges were estimated on the basis of meteorological conditions and flight altitude of passerines above the Negev in Israel (northern Sahara edge) during migration and on a simulation model that considered both energy and water as potential limiting factors for flight duration and distance. The simulation model predicted that half of the Blackcaps that stopped over in Flat and the majority of those that stopped over in northern Israel could not make a nonstop flight over the Sahara Desert in autumn without the assistance of at least an 8 m per s tailwind. Such a wind would still not be sufficient for 34% of the birds in Elat and 42% in northern Israel, and clearly they had insufficient fat reserves to cross the Sahara in a single flight. Although the fattest Blackcaps had accumulated sufficient fat to enable them to traverse the Sahara in a single flight, they probably faced dehydration by at least 12% of their initial body mass when they reached the southern Sahara edge. These birds should use intermittent migration with stopovers at sites with drinking and feeding potential. Their decision to stop over during the day in the desert at sites with shade but without food and water would be beneficial if the meteorological conditions during daytime migration imposed greater risks of dehydration than at night. Spring migrants could not reach their breeding areas in Europe without feeding, but those examined in Flat could cross the remainder of the desert in a single flight.
|Number of pages||11|
|State||Published - Apr 1998|
ASJC Scopus subject areas
- Ecology, Evolution, Behavior and Systematics
- Animal Science and Zoology