Complex timing of Marsh Harrier Circus aeruginosus migration due to pre- and post-migratory movements

We tracked three juvenile and 14 adult Marsh Harriers Circus aeruginosus
from southern Sweden via satellite to investigate migration strategies.
Four individuals were tracked for at least two years. All three juveniles
and four of the adults made west-oriented pre-migratory movements
well before the onset of autumn migration, and trans-Saharan
migrants visited post-migratory stopover areas in tropical Africa. By
these movements, the harriers presumably exploit short-term regional
variation in food abundance. Autumn and spring migration occurred in
a relatively narrow corridor, without distinct differences between sexes
in timing, speed, distance, and duration of migration, except that
females tended to migrate faster in spring than did males. Juveniles
migrated shorter distances than adults, and migration speeds were
lower. Spring migration was similar to autumn migration in terms of
speed and duration. Juveniles did not cross the Sahara Desert and three
birds, one female and two juveniles, wintered in Europe, which is in
accordance with a recent increase in the number of (juvenile) Marsh
Harriers wintering in northwestern Europe. All birds that crossed the
Sahara wintered in tropical West Africa. Harriers showed site fidelity to
breeding, wintering and stopover areas. The overall migration speed of
Marsh Harriers was similar to that of Ospreys Pandion haliaetus and
Honey Buzzards Pernis apivorus, two other trans-Saharan migrants.
Ospreys use fly-and-forage migration to promote resulting speed,
whereas Honey Buzzards are particularly apt to exploit thermal soaring.
How Marsh Harriers balance foraging versus travelling to accomplish
their rapid migration speeds remains to be resolved.