Meteorological drivers of autumn migration onset in breeding and non-breeding Arctic swans

Migratory habit was developed in birds in response to unfavorable winter conditions in their high-latitudinal breeding grounds. Autumn weather conditions should determine the decision on departure in Arctic birds. Large-bodied Bewick's Swan Cygnus bewickii of the West Pacific population must pass mountain ranges and open sea between their Arctic breeding areas (Chaun delta in this study) and temperate winter quarters (Japan). In this paper we use data from individually telemetry tagged Bewick's Swans (both families with young and adult individuals presumably non-breeders or failed in breeding) to analyze the prevailing meteorological factors at the point of their departure from their breeding/moulting grounds. We compare these conditions to those at their departure from major autumn stop-over sites in the Kolyma Lowland for the longest migration boot without stops to Sakhalin Island. We hypothesize different meteorolocal factors for a short longitudinal journey of the first migration boot and second the longest migration boot. We also predict a difference between families with young and adult swans in migration strategy. Families departed from their natal site in the Chaun delta significantly later than non-breeding adults; however, both groups left synchronously from stopover sites in the Kolyma Lowland. In the Chaun Delta, swan families were dependent on tailwinds and rainfall for the timing of their departures, with these factors working in conjunction with decreasing temperatures. In contrast, non-breeding swans did not take wind conditions into account but still avoided migration during rainy weather.