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

Författare

Roine Strandberg
Raymond Klaassen
Mikael Hake
Patrik Olofsson
Kasper Thorup
Thomas Alerstam

Summary, in English

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.

Avdelning/ar