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 birds leavingthe most intensivelydisturbed areas at Cockwood. Yet numbers have increased considerablyon the beds at Exmouth 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,Devon. 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 Lympstone 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 Dawlish Warren 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 mudflats that 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 mudflats in 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