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7. WETLAND AND COASTAL RESEARCH

7.1 Low Tide Counts

The Wetland Bird Survey (WeBS) began in October 1993 as a major collaborative scheme between the BTO, WWT, RSPB and JNCC. It represents a critical component in the strategic monitoring programme for birds in the UK. One of the main inputs of the BTO to WeBS is the operation of the Low Tide Counts scheme. Low tide counts provide information on the spatial distribution of feeding waterfowl within estuaries during the winter months. At present most estuaries are covered on a rolling programme. Counts are made monthly between November and February, two hours either side of low tide. The counts permit an assessment of the relative importance of different intertidal feeding areas for wintering waterfowl - for example they greatly contribute to the conservation of waterfowl through the network of Special Protection Areas (SPAs). The programme also provides information needed to assess the potential effects on waterfowl populations of a variety of human activities that change the extent or value of intertidal habitats. GIS applications of the low tide counts are currently being developed. Work has now started on an atlas of low tide counts, which will collate, present and analyse the results of the scheme to date.

7.2 Surveys of Non-estuarine Coastal Waterfowl

The major concentrations of non-breeding waterfowl within British estuaries are monitored through the WeBS Core Count scheme which is administered by the WeBS Secretariat based at WWT, Slimbridge, Gloucestershire. However, very important coastal populations of waterfowl occur outside estuaries and these have been assessed through two large-scale winter surveys. The first was the 1984-85 Winter Shorebird Count Survey. The most recent, co-ordinated by the BTO, was the 1997-98 Non-estuarine Coastal Waterfowl Survey (NEWS) which was the first Europe-wide survey of coastal waterfowl. A total of 15 countries participated. Very high levels of coverage were obtained in several countries. Over 3,000km of UK coastline was counted by 500 volunteers. The UK data have been analysed and input into a GIS. The Europe-wide results will be presented in a Wader Study Group Special Volume that should be ready for publication in September 2000. The results of NEWS will make a major contribution towards revising the wader population estimates for the UK and the East Atlantic Flyway.

7.3 Survey of Little Egret Roosts

Although it used to be a rare visitor to the UK, from 1989 onwards the Little Egret has shown a spectacular rise in numbers, with over 1,000 birds now recorded annually, principally within estuaries along the southern coasts of England and Wales. The species has also recently started to breed in the UK. It is widely considered that the most effective way to survey egret numbers is by counting them as they fly into their communal night-time roosts. The principal method of surveying UK estuaries is, however, the WeBS Core Count scheme. A survey is being carried out in which observers count egrets at their roosts on (or near to) the date of the equivalent WeBS Core Count at the site. The results will enable a more accurate interpretation of WeBS Core Counts of Little Egrets in the future. The survey should also enable a useful population estimate to be made at this stage of the colonisation of the Little Egret.

7.4 Research on Wetland Bird Populations and Habitat Change

The unit has recently undertaken a large range of research projects and impact-related work that makes use both of existing WeBS data and new field data collected to answer specific questions. Some recent examples of these projects are summarised below.

1.	Systems of regional indexing of waterfowl numbers have been implemented and alert systems for warning of possible serious population declines are being developed. Recent major declines in wintering waterfowl in Wales have been identified, the reasons for which are now being investigated in detail. As the number of snow and sleet days have decreased over the last 40 years, five species of common waders have shifted their distribution eastwards. A detailed analysis for Ringed Plover has shown that having controlled for long-term trends, the easting of the central tendency of over-winter distribution is inversely related to the number of days on which ground frost is recorded.

2.	The total number of waders on a site can be partly explained by water quality. The higher the value for Biochemical Oxygen Demand (BOD) the higher the densities of waders in general (waders were summed in terms of basal metabolic rate) suggesting that, in general, cleaner coastal waters will supress wader numbers. Redshank were found to prefer cleaner water, densities increasing with increased percentage of dissolved oxygen. High nitrate loads were found to lead to lower densities of Curlew.

3.	Two major improvements have been made to the Underhill Index. First, moving windows have been incorporated into the programme. This ensures that any missing counts are only estimated from counts made in adjacent years rather than from counts that could have been made many years previously when the populations of the birds could have been quite different. Second, the indices have been made more robust by each bootstrap sample being based on one set of sites randomly selected with replacement. For each bootstrap any missing counts are then imputed to generate each bootstrapped consistency interval. Due to computing limitations, the original Underhill programme imputed the missing values once for the complete data set and then randomly selected a sample of sites with replacement to generate each bootstrap sample.

4.	A WeBS system has been developed for assessing population change in waterfowl. This “Alerts” system uses a smoothing function to remove year-to-year variations in the WeBS indices to reveal an underlying trend. Two models (a revised version of the Underhill program and one that used General Additive Models) were assessed and found to give similar results. Population change over five, 10 and 25 year periods are calculated from these smoothed indices and alerts raised if the change exceeds 25 (a 25% alert) or 50 (a 50% alert). Whether this change is significant from zero can be calculated by employing a bootstrapping procedure. This system will be first used on the 1998/99 WeBS data.

5.	The concept of maximum numbers on estuaries (i.e. carrying capacity) has been examined, using Grey Plover as an example. This work is relevant to the effects of habitat loss on the numbers of waterfowl using an estuary. Population trends on individual sites were compared to the national population trend. It had been suggested that by 1988 almost half the estuaries were approaching carrying capacity, but even though the number of Grey Plovers wintering in Great Britain has doubled since 1988 there is little evidence that maximum numbers have been reached on most sites for which sufficient data are available. On six sites the population has decreased relative to the national trend and on two others numbers have remained relatively stable in recent years. The decreasing sites have all been subject to substantial developments.

6.	Long-term studies at Cardiff Bay have increased our understanding of the phenology of waterbird recovery from disturbance due to construction work. This project involves studying the effect of the permanent loss of wintering grounds on the movements and survival of Redshank Tringa totanus (Burton, in press).

7.	Collaborative work with ITE has developed models which allow estuarine waterfowl densities to be predicted country-wide from a suite of relatively inexpensively collected environmental variables. These models can be used to predict the likely impact of habitat change due to natural factors (e.g. Spurn Point), Global Climate Change or human developments on waterfowl populations. As part of the “Modelling Natural Resource Responses to Climatic Change” project (MONARCH) the BTO is collaborating with the Environmental Change Unit, University of Oxford. An adapted form of the models developed with ITE are being used to predict wader and wildfowl populations on a range of estuaries for different scenarios of sea-level rise and taking into account various options for hard sea defenses and managed retreat.

8.	A project has been started which follows on from the assessment of how disturbance-free refuges on the Wash should be distributed to optimise wader usage. This new project assesses the likely impact of habitat loss on the Moray Complex on wader roosts. Preliminary results indicate that waders tend to be as equally site-faithful on the Moray as on the Wash.

9.	As part of a project funded by CCW, the impact of the Sea Empress oil spill of February 1996 on wintering waterbirds at the Cleddau estuary was assessed. WeBS count data for the last 20 years and intensive through-the-tidal-cycle count data for the three winters following the spill were analysed to determine the effects of the oil spill and subsequent recovery of bird populations. There was evidence that Shelduck, Wigeon, Oystercatcher, Curlew and Redshank were negatively affected by the oil spill, although most species had at least partly recovered by the third winter.

10.	A review of the status of introduced non-native waterbird species in the area of the African-Eurasian Waterbird Agreement (over 100 countries) has been carried out for the DETR. This work highlighted the countries most affected by non-indigenous waterfowl species and those most at risk.

11.	Following the 1997 Breeding Sawbill Survey, funded by DETR, a paper has been submitted to Bird Study documenting the continued increases in density and range of goosanders, most notably in the Welsh population.

12.	The results of a three-year DETR-funded project to assess the population size, distribution, movements and survival of three species of piscivorous birds, the Cormorant, Red-breasted Merganser and Goosander has been published (Rehfisch et al., 1999).

Key References

Armitage, M.J.S., Rehfisch, M.M. & Burton, N.H.K. (1999). The Impact of the Sea Empress Oil Spill on the Abundance and Distribution of Waterbirds within Milford Haven. Year 3 Final Report, July 1999. A report by the BTO under contract to CCW and the Sea Empress Environmental Evaluation Committee.

Austin, G.E. & Rehfisch, M.M. (1998). Had Some British Estuaries Really Reached Carrying Capacity for Grey Plovers Pluvialis squatarola L. by the Mid-1980s?: A Re-analysis of Long-term Population Trends with the Benefit of Hindsight. A report by the British Trust for Ornithology to the WeBS partners.

Austin, G.E., Rehfisch, M.M. & Waters, R.J. (1995). Regional Trends in Wader Populations. A report by the British Trust for Ornithology to the WeBS partners.

Austin, G., Rehfisch, M.M., Holloway, S.J., Clark, N.A., Balmer, D.E., Yates, M.G., Clarke, R.T., Swetnam, R.D., Eastwood, J.A., Durell, S.E.A. le V. dit, West, J.R. & Goss-Custard, J.D. (1996). Estuary, Sediments and Shorebirds III. Predicting Waterfowl Densities on Intertidal Areas. BTO Research Report No. 160. (ETSU Project T/04/00207/REP). ETSU, Harwell.

Blair, M., Musgrove, A.J. & Rehfisch, M.M. (1999). Review of the Status of Introduced Non-native Waterbird Species in the Agreement Area of the African-Eurasian Waterbird Agreement. BTO Research Report.

Burton, N.H.K. (in press). Winter site-fidelity and survival of Redshank Tringa totanus at Cardiff, South Wales. Bird Study.

Clark, N.A. & Prís-Jones, R.P. (1994). Low tide distribution of wintering waders and shelduck on the Severn Estuary in relation to the proposed tidal barrage. Biological Journal of the Linnean Society, 51: 199-217.

Holloway, S.J., Rehfisch, M.M., Clark, N.A., Balmer, D.E., Austin, G., Yates, M.G., Clarke, R.T., Swetnam, R.D., Eastwood, J.A., Durell, S.E.A. le V. dit, Goss-Custard, J.D. & West, J.R. (1996). Estuary, Sediments and Shorebirds II. Shorebird Usage of Intertidal Areas. BTO Research Report No. 156. (ETSU Project T/04/00206/REP). ETSU, Harwell.

McCulloch, M.N. & Clark, N.A. (1991). Habitat Utilisation by Dunlin on British Estuaries. BTO Research Report No. 86. BTO, Thetford.

Moser, M.E. (1988). Limits to the numbers of grey plovers Pluvialis squatarola wintering on British estuaries: an analysis of long-term population trends. Journal of Applied Ecology, 25: 473-485.

Rehfisch, M.M. & Austin, G.E. (in press) . The effect of water quality on overwintering waterfowl populations. Marine Forum.

Rehfisch, M.M., Clark, N.A., Langston, R.H.W. & Greenwood, J.J.D. (1996). A guide to the provision of refuges for waders: an analysis of thirty years of ringing data from the Wash, England. Journal of Applied Ecology, 33: 673-687.

Rehfisch, M.M., Holloway, S.J., Yates, M.G., Clarke, R.T., Austin, G., Clark, N.A., Durell, S.E.A. le V. dit, Eastwood, J.A., Goss-Custard, J.D., Swetnam, R.D. & West, J.R. (1997). Predicting the effect of habitat change on waterfowl communities: a novel empirical approach. In: Predicting Habitat Loss (eds. J. Goss-Custard, R. Rufino & A. Luis), pp. 116-126. The Stationery Office, London.

Rehfisch, M.M., Wernham, C.V. & Marchant, J.H. (eds) (1999). Population, Distribution, Movements and Survival of Fish-eating Birds in Great Britain. DETR, London.

Underhill, L.G. & Prís-Jones, R.P. (1994). Index numbers for waterfowl populations. I. Review and methodology. Journal of Applied Ecology, 31: 463-480.

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