Migrating whooper swans Cygnus cygnus: Satellite tracks and flight performance calculations

The ARGOS satellite system was used to follow seven migratory flights of six Whooper Swans, comprising five autumn flights from Iceland to Scotland in 1994 and 1995, and two spring flights in the opposite direction in 1995. We monitored the wind and weather in detail at migration times. The transmitters were type PTT-100 from Microwave Telemetry, equipped with air pressure and temperature sensors for altitude telemetry, and were attached by a harness made from neoprene tape. None of the swans hew any higher than necessary for terrain clearance when crossing land. Two swans migrating on 27 October 1995 flew at heights between 500 and 1700 m ASL when crossing the ocean, with evidence of small climbs in lee waves downwind of Iceland. Otherwise, the swans flew very low, with frequent, and sometimes prolonged stops on the water. The swans hew by day, or at night when there was moonlight or clear sky without moon, but landed in conditions of low cloud and poor visibility, and also in darkness when there was heavy cloud cover and no moon. The southbound swans flew straight tracks across the ocean, three making landfall in the Outer Hebrides, one on the north coast of Scotland, and one in Donegal Bay after a direct crossing from northeastern Iceland. The two northbound swans were both forced away from the direct track by contrary gales, but eventually made their landfalls near their respective staging areas on the south coast of Iceland. One was four days at sea, and the other over 30 hours. Flight performance calculations indicated that an amount of fat equivalent to 25% of the lean mass should suffice for the crossing with adequate reserves. A medium-sized female should have sufficient muscle power to fly with 25% of her lean mass as added fat, whereas this would be marginal for a large male. Females, being smaller, would be able to arrive in Iceland in spring with more reserve fat. Little or no excess power would be available for climbing, even at sea level. It is not credible that these swans could climb by muscle power alone to the great heights previously reported, although this might be possible if they can indeed soar in lee waves.