Response to weather and light conditions of migrating Whooper Swans Cygnus cygnus and flying height profiles, observed with the Argos satellite system

• Published in: Ibis
• Main Authors: PENNYCUICK, C.J., BRADBURY, T.A.M., EINARSSON, ÓLAFUR, OWEN, MYRFYN
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1999
• Subjects: Cygnus cygnus; Iceland
• Online Access: http://dx.doi.org/10.1111/j.1474-919x.1999.tb04412.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fj.1474-919X.1999.tb04412.x; https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1474-919X.1999.tb04412.x

Ten satellite tracks of Whooper Swans migrating between Iceland and Britain or Ireland were analysed in relation to detailed weather and astronomical data. Surface pressure, visibility, cloud cover, precipitation intensity and type, sun altitude and moon altitude were estimated separately for each of 414 location points, of which 217 were over land or offshore islands, and 197 were over the open sea. Height profiles for four northbound and four southbound flights included two swans that flew continuously for most or all of the sea crossing, one of which reached 1856 m asl, the maximum height recorded. The others flew low, and landed often on the water, sometimes for prolonged periods. Elapsed times for the sea crossing varied from 12.7 to 101 hours. Statistical analysis showed that the swans tended to move onward during the sea crossings, provided that the altitude above the horizon of either the sun or the moon was higher than ‐4d̀, and also that the visibility was greater than 2 km; otherwise, they tended to stop on the water. This effect was seen only when the swans were crossing the open sea, not when they were flying over land or islands. It was interpreted as suggesting that they need a visible horizon to navigate when out of sight of land. If this inference is correct, it would eliminate the possibility that the swans use inertial navigation, but strengthen the case for celestial navigation.