Effects of Disturbance on Shorebirds: a Summary of Existing Knowledge from the Dutch Wadden Sea and Delta Area
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Effectsof disturbanceon shorebirds:a summaryof existingknowledge from the Dutch Wadden Sea and Delta area Effects of disturbance on shorebirds: a summary of existing knowledge from the Dutch Wadden Sea and Delta area Cor J. Smit & George J.M. Visser Smit,C.J. & Visser,G.J.M. 1993. Effectsof disturbanceon shorebirds:a summaryof existing knowledgefrom the DutchWadden Sea and Delta area. Wader Study GroupBull. 68: 6-19. The extent to which shorebirdsare disturbedby various activities is discussed, with reference to studies carried out on the Wadden Sea and Delta area. The effects of leisure activities on foragingand roostingbirds are discussed. The effectsof small airplanes,jets and helicopters are also considered, as are the effects of disturbance on food intake and behaviour of territorial birds. Frequentdisturbance may force waders to abandontraditional high-tide roosts. The implicationsof disturbanceon energyare notyet clearbut indicatethat the effectscan be larger than would appear from the studies described. Cor J. Smit & George J.M. Visser, DLO-Insfitutefor Forestryand Nature Research, P.O. Box 167, 1790 AD Den Burg, The Netherlands. INTRODUCTION man (Figure 1). Over the past 20 years the scale of most of these (potential) conflicts has been studied The Wadden Sea and Delta area are both wetlands in the Dutch part of the Wadden Sea and the Delta of outstandingimportance for many bird species area and, more recently, also in the Danish and ecologically dependent on intertidal habitats (Wolff & German Wadden Sea. The more recent investigations Smit 1984; Leeuwis eta/, 1984). At the same time have focussed on whether limits should be set to these areas are intensively used for a great variety of human presence in an area or whether certain human activities, In some cases this leads or may activities should be banned from (parts of) an lead to conflicts between the interests of birds and area. Disturbance can be defined as 'any situation in which a bird behaves differentlyfrom its preferred behaviour' (Boere 1975) or 'any situation in which human activities cause a bird to behave differently from the behaviour it would exhibit without the presence of that activity' (Oranjewoud1982). In this contributionwe will Interactions birds / man restrictourselves to disturbancecaused by human activities:disturbance from natural causes (weather, predators)has not been studiedin detail and will only Resting birds Foraging birds briefly be addressed. We will not discuss the effects on Tourism - walking Bait digging breeding birds and mainly cover Dutch studies from - surfing, sailing Walking over mudflats coastal sites (with some informationfrom the German Farming Civil aircrafts part of the Wadden Sea). Outside The Netherlands Hunting, egg collecting Military activities Military activities Fisheries very little is known of this work. This is very Leisure boats comprehensible: with the exception of some preliminarydata on part of the problem (Smit & Visser 1985) and some rather brief summaries (Wolff et al. 1982, Smit et aL 1987), the results have been presented in not easily available reports from institutes,government agencies and universities. Access to these reportsis also hampered by language F•gure 1. Human activitiesactually or potentiallyconflicting with the barriers: nearly all informationis published in Dutch. interestsof restingand foragingshorebirds in the Dutch This paper is an attempt to summarize briefly the Wadden Sea and Delta area results of these studies. Effectsof disturbanceon shorebirds:a summaryof existingknowledge from the Dutch Wadden Sea and Delta area DENMARK K6ninc SYLT GERMANY WANGEROOGE JUlST iSL/• t•'D MELLUM. SCHIERMONNIKOOG ENG ELS MAN PLAAT Bosch AMELAND TERSCHELLIN( Jadebuse VLIELAND Marnewaard TEXEL GERMANY Mokbaa Balgzand - THE NETHERLANDS Figure 2. Map of the Dutch part of the Wadden Sea with locationof sites referredto in the text. EFFECTS OF LEISURE ACTIVITIES ON BIRDS distances.Cars, agriculturalactivities and dogs caused RESTING AT HIGH-TIDE ROOSTS less disturbance(Blankestijn et al. 1986). There are also differencesbetween species as illustratedby the flight High-tide roosts may be encountered in many places distanceswhen birds are approached by walking people along the bordersof the Wadden Sea and the Delta (Figure 3). Golden Plovers Pluvialisapricaria are fairly estuaries. On the mainland coast of the Wadden Sea tolerant,but Curlew Numeniusarquata and Redshank (Figure 2) shorebirdsroost on man-made saltmarshes. Tringa totanustend to take flight at more than twice as In most of the Wadden Sea these areas are not great a distance. Detailed summer observationsat intensivelyused for agriculturalpurposes, and have a rather low degree of human disturbance.On the shores of the Wadden Sea barrier islands and in the Delta area the disturbancefrequency is generally much higher, but GOLDEN PLOVER from most places quantitativedata on the scale of the BLACK:H:•::ADEDGULL HERRING GULL problem are lacking. --- ... --.- .::..... .......... OYSTERCATCHEB BAR-TAILED GODWIT Flocks of shorebirdsmay be disturbedby a variety of human activities,though natural causes (such as .... •.:.:.......:::::•:.... ..:::::::::::::::::::::::::::::::::::: REDSHANK predators)may also take an importantshare. Table 1 I I I I I I I I I I presentsthe reasonsfor disturbance,as registeredon 0 20 40 60 80 100 120 Terschelling.In particularsmall aircraftand tourists flight distance (m) walkingare importantsources of disturbance;cattle or Figure 3. Distances (mean values in m and 95% confidencelimits) at people with a highlypredictable behaviour (like farmers) whichflocks of roostingwaders and gullswere recordedto take flightwhen approachedby walkingpeople. Data from are less so. Table 2 shows that small aircraft and people Terschelling,July- September 1981 (data: Tensen & van Zoest walkingaround also cause birdsto take flightat large 1983). Effectsof disturbanceon shorebirds:a summaryof existingknowledge from the DutchWadden Sea and Delta area Table 1. Disturbancefrequency, expressed as a percentageof the total amountof visibledisturbance in one study situationat high-tideroosts in a cultivatedgrassland area at Terschelling.The studywas carriedout in July-September1981, when relativelymany tourists on bicycles were presenton the island.Most of these have a very predictablebehaviour through their preferencefor metalledcycle paths(data: Tensen & van Zoest 1983). Source of disturbance Curlew Bar-tailed Godwit Oystercatcher Gulls Small aircraft 39 23 18 27 Walking person(s) 31 32 65 17 Agriculturalactivities 10 8 4 7 Cows I 2 13 1 Cyclist(s) - I - - Natural 11 16 - 24 Unknown reason 8 18 - 24 Terschellingshow that walkingpeople within250 m of difficultto explain. At the comparativelyundisturbed roostingOystercatchers Haematopus ostralegus caused Banc d'Arguin,Mauritania (where flightdistances for flocksto fly in 57% of the cases. As a mean, these birds most species are smallerthan in the Wadden Sea) were 38 secondsper houron the wing (mean of 320 flocks of winteringOystercatchers fly up at 400-500 m observationhours). Curlews flew up in 76% of the cases. (Smit unpubl.) whereas in the Wadden Sea they are a On averagethese birdsflew 57 sec/h (mean over 50 rather tolerant species (Figure 3, Table 2). Birds hours)(Visser 1986). Figure4 showsthat beforeflying roostingin cultivatedgrasslands with a certain amount up, the behaviourof roostingbirds may already have of human activitycan often be approached to closer been considerablyaffected: looking up and walking distances than those roostingin remote salt marshes away become more dominantas distancesget smaller. (withinthe same area and time of the year). Curlews roostingin cultivatedgrassland areas at Terschelling Weather conditionspartly determine flight distances. could be approached to approximately 100 m, whereas Kersten (1975) reportsthat Curlews can be more easily on the salt marshes on same island the flight distance disturbedduring rainy weather. A..t the same time the was 200 m (Tensen & van Zoest 1983). roosts are less compact, smaller and distributedover larger areas. Several studiesshow that large flocksare Hunting may increase flight distances for non-target more easilydisturbed (Zwarts 1972; Kooy et al. 1975). species as well as target species. By the end of Flightdistances are very much time and location September, Brent Geese Branta bernicla in Denmark dependent. These differencesare sometimesrather take flight at 210 m; by the end of October (after the start of the huntingseason) the mean flight distance has increasedto 370 m (Rudfeld 1990). Comparable Behaviour within the flock (in %) 80 Table 2. Mean distances(m) at which flocks of Oystematchersand Curlews flew up when approachedby various sourcesof 60 disturbance.Data from flocks roostingin cultivatedgrasslands on the islandof Terschelling(data: Blankestijnet al. 1986). Source of disturbance Oystercatcher Curlew Small aircraft 500 - Walking person (s) 82 213 200 I • 50-75 75-100 100-125 125-150 150-175 175-200 Helicopter - 200 Dietance flock to walking pereon (m.) Car 106 188 Egg collector 46 140 • Birdswalking • Birdslooking up • Birdssleeping Farmer/Agricultural Figure 4. Behaviourof flocksof Curlews when approachedby walking activities 60 129 people, in relation to the distance to the flocks. Data were collected in standardized experimental situations,using flocks Dog(s) - 90 of Curlews roostingin cultivatedgrasslands (data: Blankestijn Cattle 10 - et aL 1986). Effe(•tsof disturbanceon shorebirds: a summary of existingknowledge