East or west? Migration routes and wintering sites of Northern Gannets Morus bassanus from south-eastern Iceland

Abstract

Northern Gannets (Morus bassanus) are distributed in eastern and western North Atlantic breeding populations. The species’ colonies in Iceland lie between the European and North American colonies. To better understand their migratory patterns and to explore potential connections between the western and eastern populations, geo-location devices were used to track the migrations of Northern Gannets from Iceland. Findings support ringing records in demonstrating a primarily south-eastward movement following the breeding season, with no tracked birds wintering in western Atlantic waters. Fifteen successfully tracked adult birds wintered over a range of about 5000 km on continental shelf seas from NW Scotland to NW Africa with areas of concentration off Africa and in the Celtic Sea. Direct distance from the colony to the most distant point reached ranged from 1200 to 6100 km. Trips amounted to 16,100–33,500 km over the entire migration/winter period. While birds heading for NW Africa mostly showed a relatively straight migration direction, several round trips were recorded in (N)W Europe. Migration trips and over-winter colony absence lasted between 126 and 189 days. Birds departed from the colony from 9 to 24 September and returned from 19 January to 27 March. Timing and duration of migration and wintering periods varied substantially among individuals. Gannets staying in the waters of NW Africa experienced much higher sea surface temperatures than birds wintering further north, suggesting higher thermostatic costs for the latter.