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Bewick's Swans: Their Feeding Ecology and Coexistence with Other Grazing Anatidae

Eileen C. Rees
Journal of Applied Ecology
Vol. 27, No. 3 (Dec., 1990), pp. 939-951
DOI: 10.2307/2404388
Stable URL: http://www.jstor.org/stable/2404388
Page Count: 13
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Bewick's Swans: Their Feeding Ecology and Coexistence with Other Grazing Anatidae
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Abstract

(1) The distribution of Bewick's swans (Cygnus columbianus bewickii Yarrell) wintering in Gloucestershire was monitored daily during the 1986-87 season. Habitat variables at the feeding sites were measured each month and analysed to assess factors affecting the movement of swans within the county. (2) The swans grazed on flooded pasture during the day for 73% of the study period, but the absolute depth of flooding did not have a significant effect upon their site selection. (3) Multivariate analyses indicated that the swans occurred most frequently and in greatest numbers at sites where the highest biomass of green grass was available. The percentage of protein in the vegetation was a relevant secondary factor. (4) When visiting a refuge frequented by other grazing species of waterfowl, the swans continued to select fields according to the quantity of grass available but site usage was more closely correlated with maximum sward length than with total grass biomass. The correlation between sward length and grass biomass on the refuge was not statistically significant. (5) There was a strong inverse correlation between the number of swan-days and the use of the fields by other grazing wildfowl. (6) The biomass of grass on the refuge was reduced by the feeding activities of other waterfowl. The swans therefore tended to use fields outside the refuge area and to feed in smaller groups as the food supply diminished.

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