Global Change Biology (2005) 11, 31–38, doi: 10.1111/j.1365-2486.2004.00876.x

Shifting nonbreeding distributions of migratory fauna in relation to climatic change

GRAHAM E. AUSTIN andMARK M. REHFISCH British Trust for Ornithology, The Nunnery, Thetford, Norfolk IP24 2PU, UK

Abstract The distributions of eight out of nine common of (Charadrii) overwintering on UK estuaries have changed in association with recent climate change. These represent a high proportion of various populations from breeding grounds as far apart as Greenland to the west to high-arctic Russia to the east. During warmer winters, smaller proportions of seven species wintered in south-west Britain. The distributions of the smaller species show the greatest temperature dependence. The opposite was found for the largest species and no relationship was found for a particularly site-faithful species. In north-west , the winter isotherms have a broadly north to south alignment, with the east being colder than the west. The average minimum winter temperatures across the UK having increased by about 1.5 1C since the mid-1980s, the temperatures on the east coast during recent winters have been similar to those of the west coast during the mid-1980s. On average, estuaries on the east and south coasts of Britain have muddier sediments than those on the west coast and thus support a higher biomass of the prey of waders. We suggest that, with global climatic change, the advantage gained by waders wintering in the milder west to avoid cold weather-induced mortality is diminished. Consequently, more choose to winter in the east and thus benefit from better foraging opportunities. The implications of these results are considered in terms of a site-based approach to wildlife protection used in Europe and elsewhere. Keywords: body weight, conservation, global change, re-distribution, shorebirds, site designation, waders, winter iskotherms

Received 27 February 2004; received in revised form and accepted 8 July 2004

The internationally important numbers of waders Introduction and wildfowl wintering in the United Kingdom Meta-analyses have shown a clear effect of climatic (Rehfisch et al., 2003a, b) are attracted there by change on nature (Parmesan & Yohe, 2003; Root et al., productive wetlands and relatively mild winters. These 2003). With climatic change, phenological changes in birds arrive during the late Northern Hemisphere plants (Myneni et al., 1997), birds (Crick & Sparks, 1999; summer from breeding grounds as far apart as Green- Thomas et al., 2001) and amphibians (Beebee, 1995), land (e.g. Red Knot canutus and Ringed Plover range shifts of plants (Easterling et al., 2000), butterflies Charadrius hiaticula) and Iceland (e.g. Eurasian Oyster- (Parmesan et al., 1999), fish (Finney et al., 2000) and birds catcher Haematopus ostralegus, Ringed Plover, Sander- (Thomas & Lennon, 1999) and changes in the demo- ling Calidris alba and Common Redshank totanus) graphy of mammals (Coulson et al., 2001) and birds to the west and high-arctic Russia to the east (e.g. Grey (Sillett et al., 2000) have been documented. This paper Plover Pluvialis squatarola) and from wide-ranging presents the first analysis to date to demonstrate changes locations within northern Europe, particularly Fennos- in the winter distribution of migratory fauna with milder candia (e.g. Eurasian Oystercatcher, Ringed Plover, winters, probably to take advantage of better feeding Dunlin Calidris alpina, Eurasian Numenius conditions and to be nearer breeding grounds. arquata and Common Redshank), but a few species breed within the UK itself (e.g. Eurasian Oystercatcher, Correspondence: Graham E. Austin, Eurasian Curlew and Common Redshank) (Wernham e-mail: [email protected] et al., 2002). Since 1969/1970, most populations r 2005 Blackwell Publishing Ltd 31 32 G. E. AUSTIN & M. M. REHFISCH overwintering on UK estuaries have remained fairly relatively high invertebrate densities and in turn, much stable or have increased. However, a general pattern is higher densities of all the species to be considered here evident across many species: numbers on the south- with the exception of Eurasian Oystercatcher (Austin west coast of Britain (Wales and the English counties of et al., 1996; Rehfisch et al., 1997). The majority of Red Avon, Devon, Cornwall and Dorset) have either tended Knot, Sanderling, Grey Plover, Dunlin, Bar-tailed God- to increase less than numbers on the east coast, wit (Limosa lapponica) and Eurasian Oystercatcher and particularly in eastern England (Essex, Suffolk, Norfolk Eurasian Curlew that breed elsewhere but winter in the and Lincolnshire), or have decreased (Austin et al., UK make landfall in the east. Consequently, wintering 2000). These contrasting trends have only been evident on east coast estuaries rather than those further west since the mid-1980s. While the extent and distribution incurs lower migration costs. Also, birds in their first of intertidal habitat has not changed markedly during year of life, which in many of the species considered this period, there have been clear climatic trends, migrate asynchronously to older birds, will have no including those towards milder winters throughout knowledge of estuaries further to the west upon first Britain (Hulme & Jenkins, 1998). In order to predict arrival. UK breeding populations of Eurasian Curlew, future changes and assess the need for habitat Eurasian Oystercatcher, Ringed Plover and Common conservation, it is important to determine whether the Redshank either remain close to their breeding grounds shifts in distributions have resulted from detrimental or shift towards the south-west and west to winter changes in south-west Britain, or from favourable (Wernham et al., 2002). changes in the east of Britain. Hypothesis Choice of wintering area with respect to climate We hypothesize that, while in the past it would have Severe winter weather will have both a direct effect on been relatively more advantageous for individuals of the birds themselves and an effect on their invertebrate certain species to settle further west, relatively lower prey. In most species of wader, individuals remain site prey abundance perhaps coupled with the extra cost faithful with the onset of harsh weather (Myers et al., associated with a longer migration route may have 1979; Davidson & Clark, 1985; Townshend, 1985), which, caused overwintering in south-west Britain to become a coupled with their already relatively high daily energy suboptimal strategy as winters have become more expenditure (Wiersma & Piersma, 1994) and decreased benign. intake rates under such conditions (Goss-Custard et al., Here we consider whether observed trends in wader 1977; Pienkowski, 1981; Zwarts & Wanink, 1993), can distribution with winter weather patterns in the UK are result in substantial mortality when the weather is consistent with this hypothesis. Furthermore, we particularly severe over a prolonged period. In Britain, discuss the implications of our results for conservation such major mortality events occur more often on the management given current scenarios for global climatic relatively colder east coast (Clark, 1982; Davidson & change. Clark, 1985). In a local context, significantly reduced annual survival rates because of severe winter weather Methods have been reported for Common Redshank (Swann & Etheridge, 1989; Insley et al., 1997) and Eurasian Wader counts Oystercatcher (Durell et al., 2001). Despite the tendency towards site faithfulness (Warnock & Takekawa, 1996; Wader numbers have been monitored on all but a few Rehfisch et al., 1996, 2003c), some individuals will move UK estuaries since the winter of 1969/1970 by monthly in response to particularly severe weather (Clark, 1982; co-ordinated counts as part of the Wetland Survey Davidson & Clark, 1985). Species such as Red Knot, (WeBS) (Musgrove et al., 2001). We considered all which are close to their upper limit of metabolic widely distributed, common estuarine species: Ringed expenditure in north-west Europe (Wiersma & Piersma, Plover, Red Knot, Sanderling, Bar-tailed and 1994), are especially mobile (Davidson & Wilson, 1992). Common Redshank (species that have shown marked declines in numbers in south-west Britain); Grey Plover, Dunlin and Eurasian Curlew (species for which the Choice of wintering area with respect to food availability overall population increase has been more pronounced and migration routes in the east); and Eurasian Oystercatcher (a species for East coast estuaries are generally muddier than those which numbers have remained stable in south-west on the west coast because of differences in estuary Britain) (Austin et al., 2000). We use data from the morphology and tidal influence and thus support winters of 1974/1975 (since when monthly coverage of

r 2005 Blackwell Publishing Ltd, Global Change Biology, 11, 31–38, SHIFTING NONBREEDING WADER DISTRIBUTIONS 33 all British estuaries can be considered to be complete) to UK wader populations can vary between years and 1997/1998 (the latest winter for which suitable data differences in distribution could result from the buffer were available) inclusive. effect whereby less attractive sites only support high numbers of waders in years when UK populations are high (Brown, 1969; Fretwell & Lucas, 1970), although Climatic variation given that the increases to the east have been balanced by decreases rather than stability in the west this effect All available data for daily temperature, wind and could at best only provide a partial explanation. rainfall were obtained from meteorological recording However, a precautionary approach has been taken stations within 5 km of 96 UK estuaries, for all years and the models therefore addressed this possibility between 1974 and 1998 inclusive, through the British by including the UK national index ðNATINDEX¼ Atmospheric Data Centre. From these, monthly Total Total Þ as an explanatory vari- averages were calculated for minimum daily tempera- UK UKforJanuary1997 able. The principal circumstance under which the UK ture, daily rainfall and mean wind speed. Regional National index might significantly improve a species means were calculated using the monthly averages model would be where movement into or out of the from relevant estuaries. Weather conditions for south- UK, either because of annual fluctuation or a long-term west Britain were compared with those for the rest of trend, has been disproportional between regions. the UK using linear regression analysis. Regional trends The principal explanatory variables of interest were in climatic conditions were identified from smoothed MINTEMP, RAIN and WIND, the monthly averages of daily trend lines obtained by fitting generalized additive minimum temperature, rainfall and windspeed, respec- models (Hastie & Tibshirani, 1990). tively. Models were fitted to the data using an iterative approach. Consequently, the final model for each species would include those parameters from the Analyses equation The existence of possible relationships between the logit ðTotalSOUTH-WEST TotalUKÞ¼mþai þ b proportion of the UK population of each species ðMINTEMPÞþgðRAINÞþdðWINDÞ ð1Þ overwintering on estuaries in south-west Britain and þ eðNATINDEXÞ the climatic conditions in the UK was investigated by a generalized linear model (McCullagh & Nelder, 1989) that were significant in explaining the variation in the using the SAS software package. The proportion, proportion of the UK population overwintering in calculated as the total south-west population (TotalSW) south-west Britain. A negative parameter estimate for divided by the total UK population (TotalUK), was minimum temperature would indicate that during modelled by a logistic regression. The models were warmer winters a smaller proportion of the species binomial and specified a logit link function, and wintered in south-west Britain (consistent with our overdispersion was addressed by the application of a hypothesis). Similarly, positive parameter estimates for scale factor estimated from the square root of Pearson’s rain or wind would indicate that a higher proportion of w2 statistic divided by its degrees of freedom. Popula- the population was found in south-west Britain during tion values were obtained having used the Underhill wetter or windier winters, respectively. Because the Indexing Program (Underhill & Pryˆs-Jones, 1994) to susceptibility of species to weather conditions may estimate missing counts. relate to their relative energy budgets, the parameter Wader numbers on UK estuaries vary markedly estimates of the weather variables (see Eqn. (1)) may be through the winter, normally reaching peaks during expected to correlate with species body weights. midwinter, although details differ between species and sites. Distributions may also shift within winters for Results reasons other than severe weather movements, of which movements between favoured moulting, over- Climatic trends and geographic weather patterns wintering and premigration sites are most apparent. Thus, the relative importance of different sites may Means of the climate variables for south-west Britain change through the course of a winter and one may were strongly correlated with those for the rest of the therefore expect a monthly pattern in distribution to UK, both for the winter as a whole and for individual emerge regardless of any pattern associated with months. The south-west was warmer and wetter, but weather or between-year trend. The modelling ad- less windy, than the rest of the UK during November to dressed this seasonal variation by considering month as March, 1969/1970 to 1997/1998 (Fig. 1 shows January a class variable, represented by the estimable factor a. as an example). r 2005 Blackwell Publishing Ltd, Global Change Biology, 11, 31–38, 34 G. E. AUSTIN & M. M. REHFISCH

6.0 5 Mean minimum temperature (°C ) 4 5.0 C ) ° 3

4.0 2

1

3.0 Mean minimum temperature ( SW Britain 0 2.0 −1

1.0 0.30 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Sanderling Rest of the UK 0.25

Fig. 1 Average minimum temperature in south-west (SW) 0.20 Britain relative to those elsewhere in the UK. The pattern of 0.15 the relationship for January, given here, is typical of all months. Unity is indicated by a dashed line. 0.10

0.05

The fitted nonlinear trend lines (those for eastern 0.00 England are given as the appropriate examples but are 0.24 paralleled by those of other regions) indicate that Ringed Plover temperatures increased in the mid-1980s following a 0.22 decade of relatively low temperature and then re- 0.20

mained relatively high (Fig. 2a). There was no long- Proportion of UK population

wintering in south-west Britain 0.18 term trend in mean rainfall. Average windspeeds 0.16 tended to decrease during the 1980s but were higher 0.14 since then. 0.12 0.10 Population trends in waders 0.08 Trends in the proportion of the UK populations over- 74 78 82 86 90 94 98 wintering in south-west Britain have varied according Winter to species, although for most there has been a Fig. 2 (a) Average minimum temperature in eastern England discernable decline between the winters of 1969/1970 and Proportions of the UK populations of (b) Sanderling and (c) and 1996/1997. For Ringed Plover, Sanderling, Dunlin, Ringed Plover overwintering in south-west Britain. The Bar-tailed Godwit and Common Redshank, this decline smoothed lines of fit were obtained using generalized additive has been particularly marked since the mid-1980s models. (Sanderling and Ringed Plover are given as examples, Figs 2b and c).

P 5 0.0006), Dunlin (F1, 114 5 9.22, P 5 0.0030) and Eur- asian Curlew (F 5 4.0, P 5 0.0455) in south-west Relationship between wader population distribution and 1, 113 Britain were related to MINTEMP in the UK. Nonsigni- winter weather ficant trends with MINTEMP were found for Eurasian

Models comprising only those variables that explained Oystercatcher (F1, 113 5 3.12, P 5 0.0799) and Bar-tailed a significant proportion of the variance were obtained Godwit (F1, 117 5 2.71, P 5 0.1022). Except for Eurasian for all species. MONTH was retained in the models for all Curlew, the proportions of all species in south-west except Bar-tailed Godwit and Sanderling. NATINDEX was Britain were lower when the winter temperature was retained for all except Ringed Plover, Sanderling and higher. More Ringed Plover and Grey Plover over-

Dunlin. Once these effects had been accounted for, the wintered in south-west Britain in windier winters (F1, 71 proportions of Ringed Plover (F1, 71 516.98, P 5 0.0001), 5 5.26, P 5 0.0248) and wetter winters (F1, 112 5 6.01, Grey Plover (F1, 112 5 5.81, P 5 0.0159), Red Knot P 5 0.0157), respectively. The parameter estimate for the (F1, 113 5 4.80, P 5 0.0304), Sanderling (F1, 118 5 12.36, relationship between minimum winter temperature

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0.05 Greater proportions of the Eurasian Oystercatcher, G J Bar-tailed Godwit, Sanderling, Dunlin, Red Knot, Grey 0.00 Plover and Ringed Plover populations have wintered in south-west Britain in colder winters as expected given MINTEMP A − 0.05 E D F that the area is consistently warmer than the average for the rest of the UK and that the daily energy B requirement of waders is higher and more difficult to − 0.10 meet during colder winters (Wiersma & Piersma, 1994). C Furthermore, the influence of cold weather in the − 0.15 H models is strongest in the smaller species. With

Parameter estimate of increasingly mild winters, the advantages of wintering − 0.20 to the south-west of Britain to avoid cold weather- 0 200 400 600 800 Average weight (g) induced mortality are diminished and consequently the observed distributional shifts towards areas that are Fig. 3 Parameter estimates for MINTEMP (b in Eqn. (1)) plotted beneficial to the birds as they offer better feeding against average body weight (Brough, 1983). A, Eurasian conditions. Also, east coast estuaries are closer on the Oystercatcher; B, Red Knot; C, Sanderling; D, Grey Plover; E, migration routes to the breeding grounds than are those Dunlin; F, Bar-tailed Godwit; G, Eurasian Curlew; H, Ringed on the west coast, the majority of birds from popula- Plover; J, Common Redshank. A negative parameter estimate for tions breeding in Iceland, Greenland, Scandanavia and minimum temperature would indicate that during warmer winters a smaller proportion of the species wintered in south- Arctic Russia arriving via staging sites along the North west Britain. Sea coastline (Wernham et al., 2002). In contrast to previous studies of breeding fauna in which climate (MINTEMP) and the proportion of birds wintering in warming predicts redistribution to the north, winter south-west Britain (b in Eqn. (1)) was significantly isotherms are aligned north to south in Britain and the related to size (Spearman’s rank correlation Rs 5 0.70, birds have redistributed themselves in the expected P 5 0.0358, n 5 9), becoming less negative the larger the easterly direction. species (Fig. 3). Thus, the winter distributions of Since systematic recording began, the densities of all smaller species are more strongly influenced by winter of the species for which the weather variables sig- temperature than are those of larger species. nificantly explain distributions have been much higher on east and south coast estuaries than on west coast estuaries (Austin et al., 1996). However, during the Discussion period considered, this study has shown that there has been a relative west to east shift in both density and Are overwintering wader distributions related to winter abundance. This is consistent with large numbers of severity? these species having only moved west when cold Our analysis indicates that the proportion of the weather has forced them to do so. By contrast, south- populations of a number of wader species overwinter- west densities of Eurasian Oystercatcher, Bar-tailed ing in south-west Britain does indeed vary relative to Godwit and Eurasian Curlew have not declined (Austin climatic conditions in a way consistent with our et al., 2000). These are the largest species considered hypothesis. That is, in milder winters a smaller (Brough, 1983) and thus probably the least susceptible proportion of each of these species winter in south- to cold weather. They may also be more capable of west Britain, a greater proportion remaining in the east reaching prey that has retreated deeper into the of Britain where the muddier sediments provide for sediment with colder conditions (Pienkowski, 1981). more profitable foraging. Furthermore, the onset of Common Redshank are especially site faithful and declines in south-west Britain and increases in eastern reluctant to relocate even during the most severe of England was coincident with a rise of several degrees weather conditions when mortality can be particularly centigrade in the winter temperatures of eastern high (Mitchell et al., 2000). Furthermore, recoveries of England as elsewhere. The inverse relationship be- ringed birds indicate that the Common Redshank and tween the strength of the association between distribu- Eurasian Oystercatchers wintering on the east and west tion and body-size is compelling evidence that this is a coasts of Britain have different origins. Those that causal relationship. This follows the prediction of winter in the east mainly arrive from breeding grounds Stevenson & Bryant, (2000) that the impacts of climate in northern Continental Europe while those that winter warming should be more evident in smaller-bodied in the west mainly arrive from breeding grounds in species than larger-bodied ones. Iceland or from the UK itself (Wernham et al., 2002). r 2005 Blackwell Publishing Ltd, Global Change Biology, 11, 31–38, 36 G. E. AUSTIN & M. M. REHFISCH

There is no evidence for this assortive winter distribu- Implications of global climatic change for overwinter tion for the other species being considered. Thus, the wader distributions benefits of low migration costs would not be relevant to We provide compelling evidence that climatic change UK breeding Eurasian Oystercatcher and Common has had a major impact on the winter distribution of a Redshank. Recoveries of ringed birds have also migratory fauna. The trend towards warmer, windier indicated that first-winter Eurasian Curlew winter and wetter winters in the UK is expected to continue further to the south and west than older birds into the foreseeable future. The United Kingdom (Wernham et al., 2002), which may reduce the likelihood Climatic Change Impacts Programme predicts that the of these first-winter birds settling on east coast mean annual temperature will rise by between 0.1 and estuaries. 0.3 1C per decade (to a greater extent in the south-east), that the annual precipitation will increase with a Mechanism for eastward shifts in populations greater proportion of this falling during the winter period and that the number of days with rain and There are two possible, nonexclusive, ways in which average seasonal wind speeds will increase (Hulme & the observed shifts in winter distribution may be driven Jenkins, 1998). by weather conditions. Firstly, young birds may be The relationships reported in this paper have the choosing estuaries during their first winter according to potential to drive substantial long-term shifts in wader the weather conditions that they experience while older distributions. The expected warmer winters are likely birds return to estuaries on which they previously to result in fewer waders in the south-west. The largest overwintered. Secondly, both young and old birds local sustained increases in wader numbers have may respond to severe weather by seeking milder occurred in the south-east and east of the Britain and conditions. we can only speculate as to the further increase that We suggest that following a westward shift in wader these estuaries can absorb. distribution during a severe winter there will be a tendency for an eastward reversal the following winter if the latter is less cold because a high proportion of Implications for conservation management arrivals in the UK will be first-winter birds that have yet to adopt a wintering site. To these birds, the These results have clear implications for the distribu- productive east coast estuaries where they first make tion of coastal waders and for conservation manage- landfall would be particularly attractive. First-winter ment in the face of global climatic change in Europe and birds would therefore be expected to settle on these probably elsewhere. A large proportion of the East sites in the first instance and, unless displaced by more Atlantic population of many of these species currently experienced birds where carrying capacity is limiting, overwinters on the UK coast (Rehfisch et al., 2003a) and can be expected to return to the same site in subsequent it is possible that similar trends are affecting other winters (e.g. Townshend, 1985). Consequently, numer- flyway populations. (The East Atlantic Flyway biogeo- ical losses on east coast sites in the previous year graphical unit is used to identify a geographical area because of mortality or emigration are likely to be within which populations of wintering waterbirds do redressed in a relatively mild winter by first-winter not experience significant emigration or immigration.) birds, regardless of whether older individuals that had If our assertions are correct, then it would be inappro- previously shifted towards the south-west adopt the priate to attempt to check or reverse the downward new location the following winter or return to their trends in the south-west through site management original wintering site. Loss of individuals from south- aiming to enhance the habitat for these species. Indeed, west Britain because of background mortality would be as a result of also being subject to fewer cold winters most likely redressed only following more severe estuaries in south-west Britain have probably also winter weather when birds are again forced westwards become more attractive to waders since the mid-1980s from the east. The above mechanism would therefore than they were previously, but not so relative to those in result in a gradual eastward shift in the absence of the east. During the past two decades, the numbers of a colder winters until estuaries have reached their range of wader species have decreased on estuaries in capacity for holding a species. Substantial increases in south-west Britain and these include cases where those numbers of many species in the east in recent decades species are designated features of special protection (Austin et al., 2000) suggest that in the past these areas (SPAs) (Stroud et al., 2001). These designations are estuaries would have been capable of supporting based on thresholds related to the proportion of the higher numbers of waders had weather conditions flyway population or proportion of the total numbers been favourable. wintering in Great Britain. There is now a distinct

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60 000 Europe. The latter may be particularly important for populations otherwise forced to undertake longer 50 000 migrations as their tundra breeding habitat shifts northwards with global climate change. 40 000

30 000 Acknowledgements 20 000 This study would have been impossible without the combined efforts of the counters who supply the data to the WeBS. Thanks Average midwinter count 10 000 are due to Steve Freeman and Jeremy Greenwood for statistical advice, John Lawton, Humphrey Crick and Mads Forchhammer 0 for encouragement and to Niall Burton, Rob Fuller, Jeremy 1975 1980 1985 1990 1995 2000 Greenwood and Rowena Langston for their comments during Winter the preparation of this manuscript. We would also like to thank Fig. 4 Average winter numbers (December–February) of Dun- the anonymous referees for their constructive comments on the manuscript. lin on the Severn Estuary Special Protection Area. The The WeBS partners funded this work. WeBS is funded by a international threshold for this species is 14 000 (dashed line). partnership of the British Trust for Ornithology, The Wildfowl & Data from Austin et al. (2004). Wetlands Trust, the Royal Society for the Protection of Birds and the Joint Nature Conservation Committee (on behalf of English Nature, Scottish Natural Heritage, Countryside Council for likelihood that numbers on some sites will drop below Wales and Environment & Heritage Service in Northern the thresholds upon which those designations were Ireland). based. For example, the Dunlin is a designated feature of the Severn Estuary SPA because numbers there exceed the 14 000 international threshold for this References species. However, although in the national context Austin GE, Jackson SF, Mellan HJ (2004) WeBS alerts 2000/2001: numbers of Dunlin have remained relatively stable changes in numbers of wintering waterbirds in the United Kingdom, since the mid-1980s, those on the Severn Estuary have its constituent countries, special protection areas (SPAs) and sites of been declining steadily and in the most recent winters special scientific interest (SSSIs). BTO Research Report No. 349 the overwinter average has dropped below this thresh- to the Wetland Bird Survey Partnership. British Trust for old (Fig. 4, based on data from Austin et al., 2004). Ornithology, Thetford, UK. While this species remains a designated feature of the Austin GE, Peachell I, Rehfisch MM (2000) Regional indexing of SPA under alternative criteria, the loss of internation- waders in Britain. Bird Study, 47, 352–371. ally important numbers is clearly of conservation con- Austin G, Rehfisch MM (2003) The likely impact of sea level rise cern and may diminish the perceived importance of the on waders (Charadrii) wintering on estuaries. Journal for site to the wider public. This is not an isolated example Nature Conservation, 11, 43–58. for in Wales, declines of 50% or more have been Austin G, Rehfisch MM, Holloway SJ et al. (1996) Estuary, sediments and shorebirds III. Predicting waterfowl densities on identified in seven species of waterbird (waders, ducks intertidal areas. BTO Research Report No. 160 to ETSU (T/04/ and geese) as compared with one species in England 00207/REP). British Trust for Ornithology, Thetford, UK. and two species in (Austin et al., 2004). Beebee TJC (1995) Amphibian breeding and climate. Nature, 374, However, the importance of estuaries may change 219–220. through time, both within Britain and elsewhere in Brough T 1983 Average weights of birds. Ministry of Agriculture, Europe. The south-east coast of Britain is already losing Fisheries and Food. a disproportionate amount of intertidal habitat as a Brown JL (1969) The buffer effect and productivity in tit result of sea-level rise exacerbated by isostatic adjust- populations. The American Naturalist, 103, 133–354. ment (realignment of the land surface following the Clark NA (1982) The effects of the severe weather in December retreat of the ice sheets at the end of the last ice age) and 1981 and January 1982 on waders in Britain. Wader Study coastal squeeze (loss of habitats between sea level and Bulletin, 34, 5–7. Coulson T, Catchpole EA, Albon SD et al. (2001) Age, sex, hard sea defences) (e.g. Austin & Rehfisch, 2003). It is, density, winter weather and population crashes in Soay sheep. therefore, conceivable that the increasing numbers of Science, 292, 1528–1531. overwintering waders on east coast estuaries will soon Crick HQP, Sparks TH (1999) Climate change related to egg- be displaced. The numbers of birds in south-west laying trends. Nature, 399, 423–424. Britain may take an upward turn although ultimately, Davidson NC, Clark NA (1985) The effects of severe weather in warmer winters may allow these populations to find January and February 1985 on waders in Britain. Wader Study suitable wintering grounds further east in continental Bulletin, 44, 10–16. r 2005 Blackwell Publishing Ltd, Global Change Biology, 11, 31–38, 38 G. E. AUSTIN & M. M. REHFISCH

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