Fat, hydration condition, and moult of Steppe Buzzards Buteo buteo vulpinus on spring migration

Abstract

Physical condition of migrating Steppe Buzzards Buteo buteo vulpinus was determined during spring migration at Elat, southern Israel, 1984–1988. An index based on measurements of body mass, wing‐length and culmen was used to estimate fat content, based on fat extraction from nine buzzards. In addition, hydration condition was measured in 1988 and moult was checked in 1985–1988. More immatures (1001) than adults (459) were trapped, and a greater percentage of immatures (19.8%) than adults (8.4%) was retrapped during the same season. Adults had significantly greater fat reserves (4.5% of body mass) than immatures (3.8%). Most adults were in some stage of interrupted moult: however, active primary moult was noted only in immatures (10%). These findings suggest that different age groups use different migration strategies. Most of the buzzards trapped in this study could not have completed their entire migration using only stored fat: thus, hunting was probably necessary to replenish their energy reserves. No indication of water stress was found in the spring of 1988.

The magnitude and timing of migration by soaring raptors, pelicans and storks over Israel

“…Comparison of autumn with spring migration data shows that for most species the spring migratory wave is longer than the autumn one (Tables 5 and 6). This is due to the fact that in spring adults migrate first and within a few days, whereas young birds migrate later and for a longer period of time, as was shown by Gorney and Yom-Tov (1994) for the Steppe Buzzard, by E. Gorney (unpubl. MSc thesis, Tel Aviv University) for the Marsh Harrier and the Levant Sparrowhawk and by this study for the White Stork.…”

Potential prey make excellent ornithologists: adaptive, flexible responses towards avian predation threat by Arabian Babblers Turdoides squamiceps living at a migratory hotspot

“…Depending on the proportion of the predator's diet consisting of birds of similar size and type as the Babblers, we scored threat level as low, intermediate or high. Any raptor species of high threat level that does not hunt on migration but instead relies on stored energy deposits (Gorney & Yom‐Tov 1994, Yosef 1996) was given a lower, intermediate level of threat. For predators of high threat level, we classified the actual threat of each individual observation on the basis of our general experience of migratory and hunting behaviour in the area: high soaring or gliding (often in groups) is harmless, whereas individuals hovering or flying low and searching the area are threatening.…”

“…This migration also involves hundreds of thousands of birds of prey, adding to the already present resident raptor species. Some of the migrating raptors rely on energy stores to complete their migrations, whereas other species may hunt during stopover or during migration (Gorney & Yom‐Tov 1994, Yosef 1996). Thus, the level of predation threat to Babblers differs between species of raptor.…”

Can conditions experienced during migration limit the population levels of birds?

“…In many species, adults trapped at stopover sites put on weight more rapidly, achieved greater body reserves, and stayed for shorter periods than first-year birds; and within age-groups, males put on weight more rapidly, achieved greater body reserves, and stayed for shorter periods than females (for age differences see Veiga 1986;Serie and Sharp 1989;Ellegren 1991;Carpenter et al 1993;Holmgren et al 1993;Gorney and Yom-Tov 1994;Morris et al 1996;Woodrey and Moore 1997;Lindstro¨m et al 1990;Yong et al 1998;Woodrey 2000;Heise and Moore 2003; for sex differences, see Morris et al 1994;Otahal 1995). These differences may have arisen because young birds were at a competitive disadvantage in the presence of older ones, and females in the presence of males.…”