A comparison of behaviour and habitat use by Bewick
The dynamics and behaviour of the Northwest European population of Bewick’s
Swans Cygnus columbianus bewickii has been studied over several decades, yet relatively
little is known about the more numerous Eastern population of the species.
Comparisons of the two populations could help explain why the Northwest
European population is in decline. Here we describe for the first time the swans’ use
of a wintering site (Shengjin Lake) in China, comparing these findings with studies
made of Bewick’s Swans wintering at Slimbridge, UK. First birds arrived in late
October at both sites, and swans remained at Shengjin Lake until late March, which
approximates to the time that Bewick’s Swans spend at wintering sites across Europe.
Monthly swan counts did not correlate significantly with water level variations at
Shengjin Lake (and by implication food accessibility). However, more extensive
monitoring, both of water depth and food abundance, is required to understand
whether fluctuating water levels affect site use by the birds. There was no significant
difference between the two sites in the proportion of birds recorded feeding
throughout the day, though this may reflect a high level of variance in the swans’
feeding activity. A bimodal pattern in feeding activity recorded for the swans in China
is similar to feeding patterns recorded for Bewick’s Swans at Slimbridge. Breeding
success recorded for Bewick’s Swans in the Eastern population was significantly
higher than for those in the Northwest European population every year from 2007
to 2010 inclusive. Further studies should investigate whether this is attributable to the carry-over effects of swans in the Eastern population feeding on aquatic vegetation
in traditional wetland habitats during winter, as opposed to the farmland used by
swans wintering in Europe.
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