Abstract from BTO Research Report No 242:

Armitage, M.J.S., Holloway, S.J. & Rehfisch, M.M. (2006)

Monitoring the use made of Chara intermedia beds by waterfowl on Hickling Broad during the 1999/2000 winter.
ISBN: 1-904870-69-4

Executive Summary

1. Approximately 14 hectares of the extensive Chara intermedia beds, mainly adjacent to the central navigation channel at Hickling Broad, were cut in 1999. The aim of this study was to determine the impact of the cutting on the abundance and distribution of waterfowl on the Broad during winter 1999/2000.

2. Wetland Bird Survey (WeBS) counts of Coot, Mute Swan, Pochard and Tufted Duck during winter 1999/2000 showed similar patterns of numbers. The very high peak numbers of each species recorded in the first half of the winter decreased rapidly in the latter part of the winter. This effect may have been due to a combination of the rapid removal of the extensive Chara beds by the large numbers of waterfowl and bedding down of the vegetation intensified by the removal of a large amount of the plant material through cutting.

3. No difference could be detected between the height of the Chara beds in the cut and uncut sectors, thus making it less likely that the experimental set-up would detect differences in the waterfowl usage of the cut and uncut sectors of Hickling Broad. Significant positive relationships were detected between Chara height and densities of Coot, Mute Swan and Pochard.

4. All-day and all-night counts carried out during winter 1999/2000 showed no significant differences between the densities of feeding Coot or Tufted on the cut sectors and uncut sectors of the Broad. The densities of Mute Swan and Pochard were significantly higher on the cut areas than the uncut areas. However, it is important to note that most of the cut sectors were adjacent to the more disturbed navigation channel and that birds tended to favour the edge sectors, probably to avoid the disturbance that was greatest in the central part of the Broad. It may be that waterfowl do not favour either the cut or the uncut sectors in the central part of the Broad.

5. The fit of the generalised linear models used to analyse the data for each species was not ideal for the assumed Poisson distribution, probably because of over-dispersion of the data. It may be possible to improve the analyses by fitting alternative distribution models. These would provide a better basis for statistical inference.

6. The dabbling duck species identified as likely to feed on Chara intermedia were largely absent from the studied area of Hickling Broad during winter 1999/2000, probably due to the high water levels that kept the plants out of reach of these surface-feeding species.

7. The study has shown that waterfowl using Hickling Broad feed extensively on the Chara beds. In order to fully understand the effect of the cutting, further monitoring of the abundance and distribution of waterfowl is recommended. A suite of matched pairs of cut and uncut sectors would ideally be monitored over two winters. To make the analysis very robust the cutting treatment would be reversed during the second winter to make it possible to account for natural differences in the physical nature and geographical position of the sectors. Measurements of the depletion rates of Chara intermedia on the cut and uncut sectors would make it possible to estimate whether the decline in waterfowl occurred as a result of food depletion or environmental factors that are unrelated to food supply. It is also advised that foraging rates be measured for each sector type.

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