Migracje ga̧siorka na obrzezach pustyń: Badania w Eilat, Izrael

Migratory Red-backed Shrikes were mist-netted during the spring and autumn migrations (n = 1031 individuals) from 1984 to 2001 in Eilat (Israel). In a similar pattern of trapping, more than four times as many shrikes were caught during autumn than in spring. Males migrated significantly earlier than females in spring but not during the autumn migration, which suggests that in males there is a stronger drive to reach their breeding territories early. In both seasons and between both sexes we did not find any significant relations between the body measurements of individuals and the time of passage. The spring migration was much shorter time than the autumn migration. This was expressed by the minimum stopover duration, as well as by the time when Red-backed Shrike occurred in Eilat. There are significant differences between wing chord length, body mass and fat scores between seasons. In autumn males had longer wings, and both sexes were heavier and in better condition than in spring. The data suggest that the differences are an adaptation to their having to cross the Sahara Desert. The birds spend a statistically significantly shorter period of time at the Eilat stopover site in spring than in autumn (median 1 ± 1.5 days vs 5 ± 6.5 days). In autumn, recaptured birds were 6.3% heavier than during the first capture. The change in body mass was significantly correlated to the duration of the stopover period. The results suggest that migration over desert is energetically costly and that Eilat is a very important stopover site for migrating Red-backed Shrikes.