Differential spring migration of adult and juvenile Levant Sparrowhawks (Accipiter brevipes) through Eilat, Israel

As many as 50 000 Levant Sparrowhawks (Accipiter brevipes) are counted during migration at the northern end of the Gulf of Aqaba each spring. We present data from 1819 migrants that were captured and ringed at Eilat, Israel: 459 from 1984-88, 21 from 1989-95, and 1345 captured from 1996-2000. Of these, 396 (22%) were adult females, 631 (35%) were adult males, 359 (20%) were juvenile females, and 433 (24%) were juvenile males. We compare migration timing and body sizes in juvenile (i.e., first-time spring migrants) and adult migrants, and in males and females. Wing chord length and body mass in males and females changed significantly with date of arrival. Further, a significant correlation was found for both sexes between wing chord length and body mass in spring. Within age classes, both wing chord and body mass declined significantly with date of ringing. Body mass was also significantly related to size obtained from PCA analyses (PCI), both in males and females. We computed also standardized residuals of body mass on PCI. Date of passage was also significantly correlated to the standardized residuals, both in males and females. This suggested, testing for allometry vs. isometry, that birds in better than expected 'condition' migrated earlier. Moreover, results from analysis of variance revealed that body mass and age were significantly related to the date of passage. The median date of passage for adults preceded that of juveniles by 2.5 days. We believe juveniles on their first spring passage migrate slower than adults and that they are more likely to be later and in poorer body condition.