Abstract from BTO Research Report No 335:

Burton, N.H.K., Marchant, J.H., Musgrove, A.J., Armitage, M.J.S., Phillips, J. & Holloway, S.J. (Dec 2003)

Low-Tide Distributions of Waterbirds on the Severn Estuary SPA:
Results of the 2002/03 WeBS Low Tide Counts and a Historical Analysis
ISBN 1-902576-99-3

Executive summary

1. This report is in two parts. The first summarises the results of the Wetland Bird Survey (WeBS) Low Tide Counts of the Severn Estuary undertaken in the winter of 2002/03. The second part analyses how the numbers and distributions of waterbirds on the estuary have recently changed, using data from past low tide surveys and from WeBS Core Counts.

2. The Severn Estuary is internationally important for waterbirds in the non-breeding seasons and as such is recognised as a Special Protection Area (SPA) and Ramsar site. Partly due to its size, however, recent surveys of the distributions of waterbirds on the estuary at low tide have been incomplete. Most notably, little was known of the populations of birds that might use intertidal areas in the centre of the estuary.

3. The 2002/03 low tide counts have provided the most complete recent survey of the low tide distributions of waterbirds of the Severn Estuary. Of a total of 195 count sections, data were received from 184. Coverage of the intertidal areas was almost complete, with just six sections not surveyed. The New Grounds at WWT Slimbridge were not covered by the survey, though the populations of birds here are well recorded by WeBS Core Counts.

4. A total of 58 waterbird species was recorded during the 2002/03 survey. Most numerous were Dunlin (a peak of 41,120), Lapwing (12,129), Curlew (3,610), Shelduck (3,493) and Wigeon (3,331). The highest bird densities were found between the Rhymney Estuary and the Gwent Levels Wetlands Reserve, on the mudflats adjacent to the New Grounds at Slimbridge, between Avonmouth and Oldbury, at Berrow and the Axe Estuary and at Bridgwater Bay. Few waders or wildfowl were found by surveys by boat on the mobile sediments in the centre of the estuary or on Flatholm or Steepholm, though large numbers of gulls were reported from these sites.

5. One species for which the site was designated a SPA – Whimbrel – was not recorded during the survey, as this species is most common on the Severn while on passage. The 2002/03 low tide counts have provided an accurate record of the distributions of the majority of the other 17 species for which the SPA is important. Bewick’s Swan and European White-fronted Goose, however, are almost entirely restricted to Slimbridge, and this site also holds important proportions of the SPA’s populations of other wildfowl.

6. The results of the 2002/03 survey were compared to those provided by the WeBS Low Tide Counts of 1998/99 and previous BTO surveys undertaken between 1987/88 and 1991/92 and put into context using data from WeBS Core Counts and other studies.

7. The numbers of many species had changed since the 1987/88-1991/92 surveys, though often in line with national trends. The increases in Wigeon, Gadwall, Teal and Shoveler numbers seen on parts of the Severn may be explained by the national trends for these species and likewise, declines in the numbers of European White-fronted Goose, Mallard, Ringed Plover, Grey Plover and Dunlin numbers also reflect recent national trends.

8. The most obvious local factors that have affected numbers and distributions have been the barraging and consequent loss of the intertidal mudflats of Cardiff Bay and the creation (as compensation for this) of the Gwent Levels Wetlands Reserve between Uskmouth and Goldcliff. The former has resulted in the displacement of considerable numbers of waders and wildfowl, notably Shelduck, Dunlin, Curlew and Redshank, though only for the latter species has there been a subsequent observed increase in numbers on neighbouring sections. The Gwent Levels Wetlands Reserve has been highly successful in attracting large numbers of wildfowl, including Wigeon, Gadwall, Teal, Mallard, Pintail, Shoveler and Tufted Duck.

9. Waterbirds might also have been affected by changes in their food resources associated with the recent improvements to waste water treatment on the estuary. Between Rhymney and Peterstone, for example, Pochard and Tufted Duck numbers have fallen sharply recently following the transferred of discharges to a new offshore pipe in 2001 and improved treatment in 2002. A decline in Shelduck numbers on the Axe Estuary has also been linked to improvements to waste water treatment there. However, it should be noted that in the majority of cases the changes to waste water treatment have been too recent for any changes in waterbird numbers to be apparent in the data presented. Changes are most likely to be detected in the future for those species, such as ducks and gulls, that feed directly on matter discharged from outfalls.

10. Other local factors which might have affected waterbird distributions and numbers within the estuary include the construction of the Second Severn Crossing, the loss of saltmarsh, drainage of adjacent farmland, sediment erosion and disturbance.

11. Three areas are considered worthy of further research. Firstly, it is important to know whether the creation of the Gwent Levels Wetlands Reserve leads (over perhaps the next five or 10 years) to an overall increase in the numbers of birds using the estuary or whether the reserve merely attracts birds away from other parts of the SPA. Secondly, it would also be extremely informative to undertake a wider survey of habitats suitable for waterbirds that border the SPA, e.g. riverine stretches of the Rhymney, Usk and Avon that may be used by Redshank and Teal and of farmland that may be used by Curlew and Lapwing. Lastly, further research is clearly needed to determine the impacts of the changes to waste water treatment currently being made around the estuary.

12. The following constraints of the study should be noted:

Firstly, it should be noted that in each winter only a maximum of four counts were made of each count section, one a month from November to February. Coverage also varied between the surveys – most notably there was little data from Bridgwater Bay or the Gloucestershire Severn during the 1998/99 survey.

Observation of the central areas of the estuary is also very difficult. In the earlier surveys all observations were from land, whereas in 2002/03 low tide counts of these central mud- and sand-flats were undertaken from a boat. In both cases, it is possible that the numbers of birds using these areas were underestimated.

Gulls were only recorded in the 2002/03 survey. However, even in that survey, coverage of these species was patchy.

The Severn is a highly dynamic estuary and thus the location and extent of many of the intertidal areas may have changed since the Ordnance Survey maps used for this project were created. The movements of sediments may potentially also cause marked differences in the distributions of invertebrates and thus waterbirds between years.

It should also be noted that the numbers of birds recorded on the Severn Estuary may vary annually due to weather conditions. In cold winters, the west coast of Britain may act as a refuge for many waterbirds that in milder winters would occur on the east coast or on the Continent. In cold winters, therefore, waterbirds may be more widely distributed across the estuary than they would in milder winters.

Lastly, in assessing the importance of different intertidal mudflats, it is also important to note that some species may use different areas during the night to those where they are recorded in the day.

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