Spring migration of adult and immature buzzards (Buteo buteo) through Elat, Israel: Timing and body size

More than 300000 Common Buzzards (Buteo buteo), particularly steppe buzzards (B, b, vulpinus), are counted at the northern end of the Gulf of Aqaba (a.k.a. Gulf of Elat) each spring (Shirihai et al. 2000). In 1996-2000 we captured, banded, and measured 1420 of these northbound migrants at a trapping station north of the city of Elat, Israel. We used information collected from these birds, together with information from 1472 individuals that had been trapped in 1984-88 (Gorney and Yom-Tov 1994) to examine migration timing and body sizes in juvenile (i.e., first-time spring migrants) versus adult migrants at the site. Almost all migrants trapped (>98%) were considered to be steppe blizzards by plumage; 65% were juveniles. The median date of passage for adults (9 April) preceded that of juveniles (26 April) by more than two weeks. Within both age classes, both wing chord and body mass declined significantly with date of capture. Gorney and Yom-Tov (1994) demonstrated that once they had taken overall size into account, juvenile migrants weighed less than did adult migrants. Of the birds trapped, 6.2% had oiled or tarred feathers or toes. A significantly higher proportion of juveniles than adults were oiled or tarred. Our results, together with those of Gorney and Yom-Tov (1994), lead us to conclude that juveniles on their first spring passage are less efficient migrants than arc adults, and that they are more likely to succumb to both natural and human-related risks en route.