Molt during spring migration: A comparison of four species of raptors

We quantified the extent of molt in Steppe Buzzards (Buteo buteo vulpinus), Marsh Harriers (Circus aeruginosus), Levant Sparrowhawks (Accipiter brevipes), and Eurasian Sparrowhawks (A. nisus) caught during spring migration 1985-1988 in Elat, southern Israel. Ten percent of yearling Steppe Buzzards (58 of 550) and four percent of yearling Marsh Harriers (3 of 77) were actively molting their remiges while on migration. These findings are contrary to suggestions that birds do not molt their flight feathers during migration when they should avoid extra energy expenditure and maintain flight performance. Active molt of primaries and secondaries, however, was not found among adult buzzards and harriers, or in any individual of the two Accipiter species. Molt strategies may be related to differences in flight mode during migration between species that primarily soar (buzzards and harriers) and species that flap more (sparrowhawks). Body condition was not related to the state of molt. This, and the fact that only yearling birds were in active molt of remiges, suggest that a delay in arrival on the breeding grounds may be a significant cost of molt during spring migration.