The Effects of Weather on the Wintering of Bewick's Swans Cygnus columbianus bewickii at Slimbridge, England

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Abstract
Bewick''s swans at a newly established wintering site were identified individually from 1963-1964, both within a season and from one season to the next, through bill markings and leg rings readable in the field. The daily registers of the swans present from 1968-1969 to 1975-1976 were analyzed for correlations with weather conditions. For both arrivals and departures wind direction was vital, tail winds being preferred, followed by calm, then side winds, with head winds generally being avoided. Speeds of the prevaling winds (south to west) were not significantly correlated with arrivals and departures. Arrivals were associated with colder weather, departures with warmer weather. Rainfall was less during periods of major movement. The state of the moon was only of marginal importance. The proportions of the prevailing winds correlated each winter with numbers of swan units, proportions of swans missing a winter, temporary absences and numbers of swan unit-days. The effects of weather on site usage should be taken into account when evaluating refuges and when estimating population sizes.

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