Goss-Custard& Verboven:Disturbance and feedingshorebirds on the Exe estuary

Disturbance and feeding shorebirds on the Exe estuary

J. D. Goss-Custard & N. Verboven

Goss-Custard, J.D. & Verboven, N. 1993. Disturbanceand feeding shorebirdson the Exe estuary. Wader Study Group Bull. 68.: 59-66.

The effects of disturbanceon shorebirdswintering on the Exe estuary are reviewed. The local level of disturbancevaries accordingto access and habitattype. Most people occur in sandy areas where, at low water, only a minorityof the birds of most species feed. By the time most people arrive on the recedingtide, most birdshave movedto their muddylow water feeding areas where they are littledisturbed. Disturbance can be intenseon the third major habitat, the mussel beds, where the most numerous shorebird is the Oystercatcher. However, disturbance levelsvary greatly betweenmussel beds, accordingto access. Few Oystercatchersfeed on two small intensivelydisturbed beds. Disturbanceis also common on two large beds near Cockwood and Exmouthand can reduce the rate at which the most vulnerableOystercatchers feed by as much as 33-50%. However the overall effect is much lower because so much feeding occurs when people are not present(on neap tides;at night;on the recedingand advancingtides). The birds also adapt to disturbanceby habituatingto the presence of stationarypeople, by movingto other less disturbedmussel beds, or by reschedulingtheir feeding routineduring the tidal cycle The increasinglevels of disturbanceover the last 10-15 years may have caused some redistnbu- tion of birdsbetween beds, with many birdsleaving the most intensivelydisturbed areas at Cockwood. Yet numbers have increased considerablyon the beds at which are also frequentlydisturbed. However there is no evidencethat the total numberof Oystercatcherson the mussel beds over the whole estuary have been reduced by the risinglevels of disturbance, their numbershave increasedin line with the rise in the whole Britishwintering population over the same period.

J. D. Goss-Custard,Institute of TerrestrialEcology, FurzebrookResearch Station, Wareham, Dorset BH20 5AS, U.K.

N. Verboven,Department of Nature Management, AgriculturalUniversity, Wageningen, The Netherlands.

INTRODUCTION

[] Mud flats This paper reportsobservations on the effect of :E• Sandflats disturbanceon shorebirdsduring a 15-year study on the Musselbeds

Exe estuary,. Over that period,the numbersof --- Boundary of the study area peopleon the estuaryover the low-waterperiod has increasednoticeably. Activities include dog-walking, casualand commercialshell-fishing, birdwatching and walking.Though most of the directobservations have Powderh been made while watchingOystercatchers Haernatopus ostra/eguson musselbeds, surveysof birddistribution \ elsewhere allow other habitats to be considered. •.. BUDDLEIGH Sand '• SALTERTON

The Exe supportsa generallytypical community of Bull HHI winteringand migratingshorebird species, including nationallysignificant numbers (>1%) of some species.A specialfeature is that it supportsa substantial Bite proportionof the Britishwintering populations of Black- tailed Godwits Limosa limosa and Avocets Recurvirostraavosetta. The mean peak number,with season at which it occursand percentageof British 0 I 2 mdes numbers in brackets, over recent years for each of the main speciesis as follows:Oystercatcher, 5,082 (winter, 1.8%); Ringedplover Charadrius hiaticula 454 (spring, Figure 1. The Exe estuary, showing the main intertidalhabitat types 1.5%), Grey plover Pluvialissquatarola, 323 (winter, and where the main wader roost occurs.

59 Goss-Custard& Verboven: Disturbance and feeding shorebirds onthe Exe estuary 2a Dunlin2ct••Grey •..•. Plover

2b 2d Redshank Bar-tailed Godwit

. • ,

28 2g Black-tailed Godwit

• o:'• '" [XMOUTH

2f 2h Curlew Shelduck

'½ [XMOUTH

Lm'• ,

F•gurewinter2.(a-h). 1986-87.The numbers ofeight wader species counted onthe Exe estuary atlow water innine sample areas (enclosed bydots) during the

6O Goss-Custard& Verboven: Disturbanceand feeding shorebirdson the Exe estuary

1.5%); Turnstone Arenaria interpres,211 (winter and SANDFLATS spring,0.5%); Curlew Numeniusarquata, 926 (winter and spring,0.9%); Black-tailedGodwit, 657 (winter, People are more attracted to sandflats than to the other 13.1%); Bar-tailedGodwit Limosa lapponica,494 habitat types. Bait-diggingand dog-walkingare common (winter,0.8%); Redshank Tringatotanus, 559 (0.8%); there throughoutthe year and in summer yachtsmen Knot Calidiscanutus 49 (<0.1%); Dunlin Calidrisalpina, and windsurfersvisit the sandy islands at the estuary 5,783 (1.3%); Avocet,310 (62.0%). mouth.

Features of the Exe estuary are shown in Figure 1. The The potential impact on the birds depends, in part, on main high-waterroost is on DawlishWarren, a sandy the numbersof birds usingthe sandflats.Mainly peninsulaacross the estuarymouth. Birds leave as the Oystercatchersuse the coarse sand and gravel flats at tide recedesand beginfeeding on the upper level flats the mouth where people occur regularly at low water in that expose first. Later, they disperseto other areas late summer and autumn. By mid-winter,where the where better feeding is found. This pattern is reversed birds are most hard-pressedfor food, few people occur on the advancing tide. There are approximately 30 there. Accordingly,the approximately 10% of the Exe musselbeds, of which only 10 are of significantsize and Estuary Oystercatcherpopulation that uses this area in containhigh densitiesof mussels.Mudflats are most winter is usuallyundisturbed for the short period (1-2 widespreadin the higherreaches of the estuarybut also hrs) for which these sandy areas are exposed at low occur at the higher shore levels in the lower reaches. water. The third major habitattype is the sand that occursin the lower reaches, includingjust outside the mouth. A greater variety of birds uses the generally less coarse Because the numbers of birds and people depend sandflatswithin the estuary itself.Ten surveyscovering largelyon the habitat,they are consideredin turn. most of the sandflatsand mudflatsthat expose at low water (mussel beds excluded) were made during the winter (October-March) 1986-87. The mean numbers A DUNL• B REDSHANK counted in each area are shown in Figure 2. Of the birds using both mudflatsand sandflats,the greatest 200 - 200- proportionin all species, except the Oystercatcher, occurred in areas dominated by mud. The proportionsof

100 - 100- the mean numbers (shown in brackets)counted over the winter on sandflats were as follows; Black-tailed Godwit, 0.3% (331); Redshank 10.2% (98); Dunlin, O- i O- i i 11.2% (2121); Shelduck Tadoma tadoma, 14.5% (76); Grey Plover, 16.9% (183); Bar-tailed Godwit, 19.2% (213); Curlew, 33.3% (231) and Oystercatcher,73.4% C GREY PLOVER D BAR-TALED GODWIT (274). All the Avocets feed in the softest mud in the upper reaches of the estuary.Thus, only a minorityof 100- lOO the populationsusing mud- and sandflats at low water are exposed to disturbanceby people on the sandflats.

50- A larger number of birds use high-level sandflatswhen the tide ebbs and flows and the preferred low-water O- feeding areas are covered by water. The main places are situated in the two bays in the south-west and south-eastcorners of the estuary (Figure 1). Cockle E CURIFW F OYSTERCATCHER Sand is a popularplace for dog-walkingand bait- digging,and people arrive very soon after the sandflats

100 - 200- begin to expose. However, wide and deep creeks prevent them reaching any but the inshore areas. By the time the more distantareas become accessible,many 50- birds have left to feed farther upriver.

• O' i ! A similarsequence of events occursin the high-level •4 8 12 100- (• 0 12 sandflatsin the Bite. Being less accessible,few dog-

HOURS AFTER HIGH WATER walkers go there. The main users are bait-diggersand anglers and some shell-fishermenwho maintain racks Figure3 (a-f). The numbersof six speciesof waders in the south-west cornerof the Exe estuary(the Bite) throughoutone wintertidal for cultivatingoysters along, or near to, the Low Water cycle. Mark (LWM). As countsthrough the tidal cycle on one

61 Goss-Custard& Verboven: Disturbanceand feeding shorebirdson the Exe estuary

4a occasion in winter illustrate(Figure 3), the largest 100 - numbers of birds occur on the receding tide. This extreme south-westcorner of the estuary is one of the 80- first to expose and, being close to the main roostson

Dawlish Warren, birds begin feeding there in large 60 ~ numbers. But by the time the bait-diggersand anglers arrive, most birds have moved elsewhere. By the time the oyster racks are exposed almost all of the waders 40- have left. Those that remain are spread over the large expanse of sandflats above the LWM and can easily 20" avoid people. Though not shown in Figure 3, many wildfowl,especially Wigeon Anas penelope and Brent • I i I I Geese, also use the Bite. They mainly occur in the 0 1 2 3 4 muddy areas at the top of the shore and feed on marine NUMBER OF PEOPLE grasses. As few people go there, disturbanceis infrequent. 4b I MUDFLATS 100I 80 Few people use the mudflatsin the upper half of the estuary where the majorityof most of the shorebirds 60 obtain their food at low water. Marines sometimes train over a small area in the north-east corner, but 40 infrequently.The main disturbanceoccurs along the west side, to the north of Powderham, where on the 20 receding tide people walk along the embankment, usuallylooking at birds.At low water, most birdsare too I I I I I 0 1 2 3 4 far out on the flats to be affected. NUMBER OF PEOPLE Elsewhere, small numbers of shell-fishermen maintain potsand slatesfor catching'peeler' crabs for bait. Since 4c the small numbers of people involvedwalk slowly and 800 are well dispersedover the flats, the disturbanceseems to beminor; birds normally fly only a shortdistance and 6OO quickly resume feeding.

400 Muddy areas are unattractivefor most leisureactivities, LIJ and they containfew harvestableanimals. This is fortunateas the main feedingareas of mostwaders in the 200 Exe are muddy.This spatialseparation between most people and most waders may be quite general on estuaries,except where boats are moored in muddy i I I places.In the Wash in east England,for example,five of 1000 2000 the eight most numerousshorebirds had a strong BIRD DENSITY preferencefor muddysites (Goss-Custard & Yates 1992). Figure 4, The effect of human disturbanceon the foragingbehaviour High levelsof potentialdisturbance in muddyareas can of Oystercatcherson Cockwoodbeds, (a) The effect of the occur in narrow creeks where people on the shore may number of people on the bed on the proportionof birdsfeeding be very close. But, even here, many birdsmay adapt, and (b) the densityof foragingbirds. (c) The effect of bird density on the rate of food intake of juvenile Oystercatchers thoughwith unknowneffects on localdensities. For (from Goss-Custard & Durel11987), example, there are many narrow creeks in south-west Englandwhere footpathsand roadsoccur immediately alongsideplaces where manyshorebirds, including MUSSEL BEDS territorialRedshank, feed throughoutthe winter. Unless new sportsor commercialfisheries arise that attracta Two of the smallest mussel beds on the Exe are seldom muchgreat number of peopleto mudflats,levels of used by birdsin daylightbecause people occurthere disturbancein manyof the birds,most importantfeeding almost continuously.One is at a high level on the shore, areas may not generallybe very significant. close to the pointof access at the northernend of the

62 Goss-Custard& Verboven:Disturbance and feedingshorebirds on the Exe estuary

Bite. The other is a low-level bed further to the north on 5b • '•00 the west side where anglers fish on most tides. Though 1.2 some birds do feed in these areas when people are • 75

present, the beds are too small to allow birds to avoid 0.8 ._ • 5c people so densitiesare much reduced. If present in sufficientnumbers, people certainlycan prevent birds 0.4 • 25 usingotherwise suitable feeding habitat. i ! ! i i i i 60 180 300 3.0 3.5 4.0 Some of the largest and most important mussel beds Time sincefirst exposure(min) Mean tide height (m) also attract many people. But because birds can usually 5c move sufficientlyfar (>75 m) to continuefeeding, some 160 o-,O 200 birds usually remain there throughoutlow water. The 120 ß , .• large beds most visited by people are those close to • 160 access pointsor those that do not require a long walk 80 over mud to reach and are reasonablyfirm underfoot. :• 120 The most disturbedare the two large beds lyingto the west of Exmouth,on the west (Cockwoodbeds) and ! i i i f east (Bull Hill beds) sides of the river. In general, the 60 180 300 i 2, , 4• i 6, i • ! 10i other large beds are only visitedby shell-fishermenwho Time sincefirst exposure (min) Numberof peopleon bed 3 usually limit their activities to the bed edges. Figure 5. The frequencyof occurrenceof people on mussel bed 4 at Cockwoodduring autumn and winter 1989. (a) The mean numbersof peopleon the bed throughoutthe tidal cycle on The Cockwoodand Bull Hill beds are used by people for neap (o) and spring(e) tides. (b) The proportionof days with dog-walking,birdwatching, casual cockle-, mussel- and humandisturbance according to the heightof the tide. The winkle-picking,angling and for launchingboats and mean tide heightis the mean of the highwaters occurring placing moorings.A few professionalshell-fishermen beforeand after the low-waterperiod. (c) The mean numbersof Oystercatcherson bed 4 throughoutthe tidalcycle on neap (o) pick winklesand, occasionally,collect mussels.The and spring (e) tides. (d) The numbersof birds on bed 4 in mussel beds are most used for recreationalpurposes in relationto the level of disturbanceon the adjacent bed 3 on the summer and early autumn, but people do occur there neap (o) and spring (•) tides; when people arrive on bed 3, throughoutthe winter, especiallyon the Cockwood birdsare regularlyseen flyingthe short distance to bed 4 where bird numbers consequentlyincrease. beds. In fact, these beds are the most disturbed on the estuary because they are so accessible. By contrast, the Bull Hill beds are 600 m from the shore and half of 1984, Goss-Custard& Durell 1988). Figure 4c illustrates them lie beyond a deep creek that can only be crossed how intake rate decreases as bird density rises in on foot around low water. juveniles, which are amongst the least dominant of Oystercatchers.The conclusiontherefore, is that as the Most birds on the mussel beds are Oystercatchers numbers of people increase on the mussel bed, most alongwith small numbersof Redshank,Curlew, birdsspend less time feedingand do so at a lower rate. Turnstoneand Greenshank Tringanebularia. Most of The importantquestion, though, is whether the reduced the several thousand Oystercatchersthat winter on the feedingopportunity actually has a significanteffect on Exe feed on mussels;very few eat the cockles,winkles the birds.The most convincingdemonstration would be and ragwormsthat also occuron the beds (Goss- that disturbance reduced the birds' fitness; that is, their Custard & Durell 1983). chancesof survivingthe winter,or to acquire energy reservesto migratesuccessfully or to maintaina good Studies on the northern half of the Cockwood beds enough body conditionto breed well in summer. As this (knownas bed 4) have identifiedsome of the effects cannot yet be examined, the indirectapproach of that people have on foragingOystercatchers. When assessingthe effect of these levels of disturbanceon disturbed,most Oystercatchersfly to another part of gross intake rate must be used instead. bed 4 where many then rest. As a consequence, more birdsstop feeding as the numbersof people on the bed Figure4a showsthat the presenceof three people increase(Figure 4a). Becausethose that do feed are (there are seldom more) reducesthe average proportion forced together into a smaller area, the densityof of birdsfeeding from approximately85% to 65%; that is, foragingbirds also increases(Figure 4b). the average bird feeds for 20-25% less of the time. Oystercatcherssteal musselsfrom each otherwith Figure 4b shows that average bird densitiesincrease increasingfrequency as the densityof foragingbirds, from 80 to 110 birdsper ha when three peopleare on and thus opportunityto steal from subordinate the bed. Figure 4c shows that such an increase in individuals,increases (Ens & Goss-Custard1984). As a densitywould decrease the intake rate of juvenilesby consequence,the intake ratesof sub-dominantbirds 13%, from approximately460 to 400 mg per 5 min of decreasesat high bird densities(Ens & Goss-Custard foraging.With three people on the bed, the average

63 Goss-Custard & Verboven: Disturbanceand feeding shorebirdson the Exe estuary

Table 1. Numbers of oystercatchers(_+ SE) in 1978-81 and 1988-91 on three groups of mussel beds with different levels, and increases, of disturbance over the decade.

Mussel beds Mean winter numbers: Percentage change Level, and change 1978/79-1980/81 1988/89-1990/91 in disturbance

Cockwood 311 + 16 198 _+22 -36 v high & increased Bull Hill 397 _+18 659 + 47 +66 high & increased Remainingbeds* 578 _+29 861 ñ 22 +49 v low throughout Total 1,286 ñ 6 1,718 ñ 30 +34 - National Index** 173 ñ 9 235 ñ 7 +36 -

* Beds 1, 20, 22, 25, 26 and 27 (in Goss-Custardet aL 1982) ** From the BTO BritishJanuary Index

juvenile would therefore feed for 75-80% of the time and thus be able to fill their crops before flyingto the roost at 85-90% the rate compared with when no people were so, in effect, extendingtheir foragingtime. This may be present. Its overall intake rate would therefore be why adult Oystercatcherson the Bull Hill mussel beds reduced by some 65-75%. increasedtheir intake rate at the end of the tidal cycle in September and October when disturbanceat low water This is an average value, of course, and there would was severe (Goss-Custard,Clarke & Durell 1984). be variation between individuals in the effect of However,young birdsdid not do so, perhapsbecause disturbance according to their age, feeding method they lacked the experience to know when it was and dominance (Goss-Custard & Durell 1988). A requiredor because high densitiesof feeding adults at decrease of a third perhaps even half, might occur in the end of the tidal cycle depressed their intake rates the most vulnerable groups with three people on the (Goss-Custard& Durell 1987; 1988). bed. However, such high levels of disturbance are untypicalfor three reasons. First, people are seldom If increasingdisturbance has significantlyaffected there during the first and last hour of the exposure Oystercatchersover the last 15 years, it shouldbe period (Figure 5a); this represents one-third and two- reflected in changes in their numbers. The Cockwood fifths of the exposure period on springand neap tides beds are most disturbedbecause access is so easy. respectively.Second, there is only seldom more than The Bull Hill beds are less disturbed because access to one person on the bed throughout the low-water half the area is only possiblefor two-three hours over period, especiallyon neap tides (Figure 5a). Third, the low water. Few people visit the five other major beds. If probabilitythat people will occur on the bed at some disturbance has had an effect, bird numbers would be point during the low-water period is much lower on expected to have decreased on the Cockwood and Bull neap tides (mean tide height <3.25 m) than on higher Hill beds, but not elsewhere. tides (Figure 5b). Fourth, people seldom occur on the beds at nights where Oystercatchersfeed throughout The total bird-daysper winter on the three groupsof the year, though at only 40% the daytime rate beds between 1976-77 and the present are shown in (Goss-Custard & Durell 1987). In other words, the Table 1. Bird-dayswere calculated from late average number of people on the bed throughoutthe September,when peak numbersoccur, throughto feeding time of the birds as a whole is considerably January,just before birds start migratingto the breeding less than three. grounds. Despite increased disturbance,Oystercatcher numbers have increased over the decade on the main When disturbance does occur, birds compensate by or 'priority'beds (Goss-Custardet aL 1982) combined movingelsewhere or by feeding at a greater rate during by a third, almost identicalto the increase record in undisturbedperiods of the day. Compared with neap Britain as a whole over the same period. tides, more birds leave the mussel beds to feed on other mussel beds at low water on springtides (Figure 5c), When differentmussel beds are compared, there is no partly in responseto the higherlevels of disturbance.In clear associationbetween changes in Oystercatcher the same way, birds move from adjacent bed 3 to bed 4 numbers and disturbance levels. Numbers have when large numbersof people occurthere (Figure 5d). decreased by a third on the Cockwood beds where the The birds may also rescheduletheir foragingthrough disturbance has been both the most intense and the tidal cycle. Captive Oystercatchershave been increased most. But numbers have increased by two- shown experimentallyto compensatefor lost foraging thirds on the Bull Hill beds which also have been time by raisingtheir intake rate at the end of the low- increasinglysubjected to disturbance.The increasein water feeding period (Swennen et al. 1989). Birds may Oystercatchernumbers on Bull Hill has been larger than

64 Goss-Custard& Verboven:Disturbance and feedingshorebirds on the Exe estuary that occurringon the remaining, largely undisturbed, birddensities. They may also rescheduletheir foraging beds. This probably happened because many birds routineby, for example, increasingtheir intake rates as displaced from Cockwood moved to Bull Hill, as studies the tide advances.Many can also feed in fieldsat high on marked individuals show. The counts therefore water, except when low temperatures render their earth- suggest no clear link between the increase in worm prey unavailable(Goss-Custard & Durell 1987). disturbance over the years, and its present level, and They can also habituateto people themselves,though the changes in bird numbers on the various mussel this dependscritically on the extentto whichthe people beds of the Exe. move about. Anglers and the local winkle and mussel pickersusually move ratherlittle; having found a Why have the changes in bird numbers over the decade suitable place, they remain there for much of the tidal been so different on the various beds? There is no clear cycle. After the initialdisturbance, the Oystercatchers association between the changes in the numbers and in settle down and even feed nearby. Such activitiesseem the food supply (unpublishedinformation), though to cause little more disturbance than the scares caused furthertests are requiredand are in progress. by the increasingnumbers of birdsof prey that hunt Oystercatchers may have been driven off the Cockwood over the estuary. Severe disturbancefrom people beds by Carrion Crows Corvuscorgne) which steal usually arises if several casual winkle- and cockle- mussels from Oystercatchers(Goss-Custard, Durell & pickers,along with peoplewalking their dogs or Ens 1982). However, there is no clear evidence of this birdwatching,roam over the musselbeds. This gives (unpublishedinformation), though further tests are also the birds littlechance to settle down or to get used to planned. It is possible that disturbance has reached a people, and so they leave. critical level on the Cockwood beds at which birds are driven away, and not on Bull Hill, but this is again With continuous intense disturbance of this kind, birds untested. may desert the mussel bed, as has virtuallyhappened on two smallbeds on the west side of the estuary.But with the much lower levelsof disturbancethat typically DISCUSSION occuron most beds, the effectson most birdsmight be insignificantbecause they can adapt theirforaging Oystercatchers behaviour. There is, in fact, no evidence from counts The increase in the winter numbers of Oystercatchers made on the Exe that the total numbers of on Bull Hill was surprisingin view of the high levels of Oystercatchershave been affected by the increased disturbance occurring there. However, our impression levels of disturbance over the last decade. Bird of disturbance levels on this bed, as on others, will be numbers on the main mussel beds of the Exe have highly exaggerated. Like us, most people visit the beds increased roughly in line with the rise in the British at low water spring tides in daylight. Yet the birds feed winteringpopulation. There may only have been some there all the time; on recedingand advancingspring local redistributionwithin the estuary.Numbers on the tides in daylight, throughoutdaylight neap tides and highlydisturbed beds at Cockwoodhave decreasedby throughoutboth neap and spring tides at night. Over a third,whereas they have increasedby between one- the winter as a whole, at least two-thirds of their and two-thirdselsewhere. Yet the largest increase feeding is done at times when people seldom occur on occurredon the disturbedBull Hill beds, underliningthe the beds. On the other hand, the mussels available at point that the frequency and intensityof disturbanceis the higher shore levels on the receding and advancing critical in determininghow seriouslythe birds are springtides and throughoutthe tidal cycle on neap affected.The mere presenceof people on the feeding tides contain less flesh than those available at the grounds is clearly not sufficientreason in itself for lower levels of the shore on spring tides (Goss-Custard believingthe disturbanceis deleteriouslyaffecting the & Durell 1987). Furthermore,birds feed at night at birds. under half the daytime rate (Goss-Custard & Durell 1987). In terms of its effectson the overallfeeding Despitethe absenceof any evidenceto suggestthat opportunitiesfor Oystercatchers,disturbance may Oystercatcher numbers on the Exe have decreased as occur for over half the effective time that the birds feed disturbance levels have increased, some vulnerable during the winter. sectionsof the populationmay now find it more difficult to obtain their food requirements.The first- and second- This would be serious if this meant that the birds were winterbirds are mostat riskof dyingin winter.Being actuallyprevented from feedingfor over half the time. amongstthe least efficientforagers and poorest But this is not so: Oystercatcherscan adapt to minimise competitorson the musselbeds, young birds may suffer the effects of disturbance.They can move some disproportionatelyfrom disturbance. This difficultywould distance to avoid people, though this could cause their be greatestin coldweather when energyrequirements food intake to decrease throughunfamiliarity with the are highyet the opportunitiesto feed- in fieldsat high area and increased interferenceresulting from elevated water, for example - are least. In the circumstances,

65 Goss-Custard& Verboven: Disturbanceand feeding shorebirdson the Exe estuary

REFERENCES disturbancemight have a more severe effect on some Oystercatchersthan merelyto redistributethem locally Ens, B.J. & Goss-Custard,J.D. 1984. Interferenceamong duringa three- to four-hourperiod over low water on oystercatchersHaematopus ostralegusfeeding on mussels springtides that fall in daylight.This couldbe significant, Mytilusedulis on the Exe estuary. J. Anim. Ecol. 53:217-231. because modellingshows that the overallpopulation Goss-Custard,J.D. & Durell, S.E.A. le V. dit. 1983. Individualand age size can be much affected by the winter mortalityrates differencesin the feeding ecologyof oystercatchers of youngbirds (Goss-Custard & Durell1990). However, Haematopus ostralegus winteringon the Exe estuary, Devon. the countsprovide no evidencethat disturbanceof Ibis 125: 155-171. youngbirds has actuallyreached the pointat whichthe Goss-Custard, J.D. & Durell, S.E.A. le V. dit. 1987. Age-related effects populationsize is being affected,despite the marked in oystercatchersHaematopus ostralegus feeding on mussels increase in human activitiesthat has taken place over Mytilusedulis. III. The effectof interferenceon overall intake rate J. Anim. Ecol. 56: 549-558. the same period. Goss-Custard, J.D. & Durell, S.E.A. le V. dit. 1988. The effect of Other shorebirds dominanceand feeding method on the intake rates of oystercatchers,Haematopus ostralegus,feeding on mussels, Many of the pointsmade on Oystercatchersapply to Mytilusedulis. J. Anim. Ecol. 57: 827-844. other speciesof waders.Our casual impressionof Goss-Custard, J.D. & Durell, S.E.A. le V. dit. 1990. Bird behaviour and disturbancelevels is again likelyto be exaggerated environmentalplanning. Ibis 132: 273-289. becausepeople tend to be presenton the feeding groundat similartimes. The manyoccasions when birds Goss-Custard, J.D., Durell, S.E.A. le V. dit & Ens, B.J. 1982. Individual feed undisturbed,at nightand on neap tides, for differencesin aggressivenessand food stealingamong winteringoystercatchers Haematopus ostralegus L. Anim. example,are just not noticed.Furthermore, in many Behav. 30:917-928. cases, birdsoccur on the flats at largelydifferent times, or in differentplaces, to people.Most waders on the Exe, Goss-Custard, J.D., Clarke, R.T. & Durell, S.E.A. le V. dit. 1984. Rates of food intake and aggressionof oystercatchers,Haematopus occuron muddyplaces which are leastpopular with ostralegus,on the most and least preferredmussel, Mytilus people.On the higherlevel sandflats, most birds have edulis,beds on the Exe estuary. J. Anim. Ecol. 53: 233-245. left before most people arrive.The birdsmay also be Goss-Custard,J.D., Durell,S.E.A. le V. dit., McGrorty,S. & Reading, able to adapt to the presenceof people,though this C.J. 1982. Use of mussel, Mytilusedulis, beds by mightbe lesseasily done in the smallerspecies which oystercatchers,Haematopus ostralegus,according to age and seem generallyto be morehard-pressed in winterthan populationsize. J. Anim. EcoL 51: 543-554. the largebirds, such as the Oystercatcher(Goss- Goss-Custard,J.D., Jenyon, R.A., Jones R.E., Newbery, P.E. & Custardet aL 1977). Nonetheless,on the Exe, there has Williams, R.B. 1977. The ecologyof the Wash. II. Seasonal been no trend in the peak winternumbers of any species variationin the feedingconditions of wadingbirds (Charadrii).J. recordedby the BTO EstuariesEnquiry over the last 15 Appl. Ecol. 701-719. years;the significancelevel of thecorrelations of bird Goss-Custard, J.D. & Yates, M. 1992. Towards predictingthe effect of numbersagainst year in the mainspecies varies further reclamationson the shorebirdsof the Wash. J. Appl. between 0.1 and 0.8. There is therefore no evidence that Ecol. 29 (in press). currentlevels of humandisturbance significantly affect Swennen, C., Leopold, M.F. & de Bruijn, L.L.L.M. 1989. Time-stressed the feeding,and thus numbers,of overwintering oystercatchers,Haematopus ostralegus, can increasetheir shorebirdson this apparentlyrather typical estuary. intake rate. Anim. Behav. 38: 8-22.

Bar-tailed godwit

66