Citation
Marchant, J.H., Calbrade, N.A. & Austin, G.E. 2009. Methods for comparing low-tide trends for Wetland Bird Survey count sectors with wider regions: a pilot study for three wader species on the Stour and Orwell Estuaries SPA. BTO Research Report 540: British Trust for Ornithology, Thetford
Overview
This report documents analysis of Wetland Bird Survey data for the Stour and Orwell SPA, identifying changes in counts over a 10-year period for Knot, Dunlin and Black-tailed Godwit. Through this, it shows the value of the SPA for these species.
In more detail
- Managers of wetland sites can assess the status of a particular count sector on a site counted for the Wetland Bird Survey (WeBS) by comparing the numbers of birds found there, and their population trends, with wider areas. Where trends at a sector differ from those expected from the data for wider areas, and habitat change has also occurred, it may be possible to infer ways to improve the conservation management of the site.
- WeBS runs two major count schemes covering all estuaries in the UK. The WeBS Core Count Scheme was developed specifically for surveillance of numbers of waterbirds at the site level, and to derive population estimates, and has been in use for over 50 years. The underlying methodology of the Core Count Scheme aims to deal with overcounting resulting from multiple recording of the same individuals. The second scheme, the Low Tide Count Scheme was developed to monitor relative usage of the intertidal habitat within an estuary. Because waterbirds may make use of different parts of an estuary over the same low tide period, avoidance of overcounting is not a primary concern addressed by the methodology of the Low Tide Count Scheme and WeBS does not routinely use low tide count data to derive site totals for waterbirds.
- Consequently population trend generated from the monthly core counts (CCs) may be expected to differ from the trend generated by summing low tide counts (LTCs) from relevant count sectors. Conclusions about the relative favourability of a count sector may depend, therefore on the methodology used.
- Previous ornithological reports for the Stour and Orwell SPA have derived estimates of site totals from summed LTCs. Because there has been concern that summing LTCs may have resulted in an unknown and probably changing degree of overcounting, we here test a method of comparing trends obtained from LTCs on individual sectors with trends derived from CCs for wider areas. Although WeBS routinely compares CCs from individual count sectors with population derived from summed CCs, previously no analyses have been done that compare LTCs from individual count sectors with CC totals. In principle, however, there is no obvious argument against putting trends in usage of intertidal areas within estuaries determined by LTCs in the context of the site population derived from CCs.
- The study compares counts for Knot, Dunlin and Black-tailed Godwit on count sectors of the Stour and Orwell SPA over a 10-year period. These are placed in the context of the data for lower, mid and upper estuary consolidations (for which summed LTCs provide the only available background data), the whole estuary (Stour or Orwell), the Stour and Orwell together, a wider region including the Deben Estuary and Hamford Water, and the Environment Agency’s East Anglian region based on CC data.
- The analysis suggests that conditions across the Stour & Orwell SPA as a whole are favourable to Knot. Indeed Knot have increased on the site and more rapidly so than across East Anglia as a whole. There have been some local declines within the SPA, with decreasing numbers recorded at Bathside Bay and Seafield Bay on the Stour and from sectors close to the Orwell Bridge indicating redistribution within the site but these birds have been absorbed elsewhere. All things being equal one would expect increases across all sectors given the increased numbers on individuals on the SPA and so these local declines may indicate pressures due to habitat change or increased disturbance in these particular areas. However, same redistribution could arise it other areas have become increasingly attractive to this species. This cannot be determined from analysis of trends alone.
- Although Dunlin numbers are showing a strong decline throughout East Anglia and indeed the UK, believed to be a response to global climate change (with increasing numbers wintering on the Waddensea) this analysis has identified that the steep decline of Dunlin across the SPA exceeds that expected from the wider trend. This indicates that this SPA is becoming increasingly less favourable for this species.
- The decline in Black-tailed Godwit on the SPA is in stark contrast with almost every other site within the species UK winter range. There has also been a redistribution within the SPA.
- With only three species analysed, it is difficult to draw conclusions with regard to potential pressures that may be driving changes in bird numbers. It is the identification of similar trends in species with similar ecological requirements that focuses attention on potential pressures. Despite this, it is notable that all three species have shown declines in the upper reaches of the Orwell in the vicinity of the Orwell Bridge.
- The method of using Core Counts, rather than summed LTCs, has successfully identified sectors where trends have differed from those in wider areas. The method can help to determine the effects on key estuarine bird species of management activities, such as dredging and sediment placement, by identifying sectors where population changes have not been in line with those expected. We recommend the use of this method across all species in similar future studies.
Staff author(s)
Graham Austin