Effects of Disturbance on Shorebirds: a Summary of Existing Knowledge from the Dutch Wadden Sea and Delta Area

Home , Bosch (island), Engelsmanplaat, Heerenveen, Mellum, Rottumeroog

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 studysituation at 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). roostsare 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 from the DutchWadden Sea andDelta area

locationwhere in summermany tourists were present.A comparable feature was found in Denmark where Bar- tailedGodwits also changed roosting sites, probably as OYSTE.RCA•'CHER ':"'•....'"'"1 '" III• I windkayaksuderJ I a resultof disturbancefrom hunting (Rudfeld 1990). The REDSHANK easternpart of the islandof Amelandconsists of a large .KNOT sandflat. Normally 10,000 Curlews roost here in summernights, whereas during the day onlysome .BAR-TA'II•ED.GODW.!: T hundredsare present. These birds use alternative .SH'EED'ucK day-timeroosts on the sandflatsof Engelsmanplaatand on the Frisiancoast. In winter,when considerably fewer 'CURLEW people visit that area, Curlews do roost on Ameland 0 1(•0 2•)0 3•)0 4•)0 5•)0 6•)0 (Kersten1975). Zwarts (1972) notedthat a traditional flight distance(m) Curlewroost on the saltmarshes was gradually Figure5. Distances(mean values in m and standarddeviations) at abandonedafter an increasein disturbanceby tourism. whichflocks of roostingwaders were recordedto take flight Ringingactivities on Vlielandon twosuccessive nights when approachedby a kayak or wind surfer. Data from the lead to severe disturbance of Redshank roosts. It took Jadebusen(German mainland coast) (from: Koepff & Dietrich 1986). fivedays before the normallyoccurring numbers were presentagain. Cannon-netting at a Curlewhigh-tide rooston the sameisland on twonights with a ninedays observationshave been madeon inlandfeeding geese intervalalso led to a temporarydeparture. In this case it (cf. Gerdes & Reepmeyer1983). took 2-3 weeks before the numbers were back to normalagain (Zegers 1973). Koepff& Dietrich(1986) studiedthe effectsof kayaks and wind surferson roostingwaders and Shelduck Tadornatadorna in the Jadebusen(German Wadden EFFECTS OF LEISURE ACTIVITIES ON FORAGING Sea). Althoughthere are somedifferences in species BIRDS compositionbetween this and the previouslymentioned study, more or less the same order of disturbance Althoughdata on flightdistances are available for some susceptibilitywas found(Figure 5). Kayaksand sailing tidalflats, it is impossibleto givestandard figures on this boats disturbed more often than motor boats and wind matter.Distances vary between sites and are dependent surfers.Kayaks have a smalldraught which enables on earlierexperiences (learning) in that particular them to come very closeto the high-tideroosts. In the location.In Denmark, for instance, Curlews show an KOnigshafen,Sylt, wind surfers had strongdisturbing extremewariness and have a flightdistance of 500 m, effectson dabblingducks. A few zigzagmovements of a probablybecause they are a huntedspecies in that singlesurfer were sufficientfor a completedeparture of country(Meltofte 1986). In the Mokbaai,Texel, a small all ducks present. In the same situation Brent Geese left baysurrounded by sea wallsand dunes, with a large the area when approachedat 300 m or less (K•sters & varietyof humanactivities (recreation, bait digging, von Raden 1986). angling,helicopter traffic, navy inflatables speeding throughthe channel,etc.) some Dunlinscan be Waders may leave their usualhigh-tide roosts as a approachedto within10-20 m or lesswithout any visible result of disturbance but the available data are disturbance.At the same time of the year Dunlinflocks somewhatdifficult to interpret.Boer et aL (1970) found feedingon the open mudflats east of the islandmay take on the Balgzand that Bar-tailed Godwits Limosa flightat a distanceof 100 or 200 m. Such differences lapponicaused two high-tideroosts. One was used were also foundat Terschelling,where birdsclose to the especiallyin summerand was situatedin a relatively sea wall (withrather frequent human activities) tolerated quietarea. The secondwas used mainlyin winterat a people walking on the tidal flats at shorter distancesthan Table3 Meanflight distances (inm) of wader species, when approached bypeople walking over the tidal flats near Terschelling (Dutch Wadden Sea)at variousdistances from the sea wall (data: Glimmerveen & Went 1984).

Species 200-300m fromsea 500-1,000m from mussel bed at 1,000 m wall sea wall from sea wall

Oystercatcher 79 113 77 Bar-tailed Godwit 101 138 - Curlew 140 196 102 Effectsof disturbanceon shorebirds:a summaryof existingknowledge from the DutchWadden Sea and Delta area

Table 4, Mean distancesand ranges(m) at whichbirds take flightwhen approachedby peoplewalking over the tidalflats (from:van der Meer 1985 (Delta area), Wolff et al. 1982 and Smit unpubl.(Wadden Sea)).

Species Delta Wadden Sea

Flight dist. Range Flight dist. Range

Curlew 211 124-299 339 225-550 Sheld uck 148 99-197 250 200-300 Grey Plover 124 106-142 - 50-150 Ringed Plover 121 80-162 - - Bat-tailed Godwit 107 88-127 219 150-225 Brent Goose 105 58-152 - - Oystercatcher 85 81-89 136 25-300 Dunlin 71 57-86 163 100-300 Turnstone 47 31-53 - 150-250 birdsforaging farther away from the sea-wall (Table 3). group (Figure 7). Using the informationon the distance The flightdistance is also influencedby the behaviourof at which birds take flight and additional informationon a person or group of people. One individualperson the time needed for recoverywe can calculate the size generally disturbsless than a group; dogs running of the area where birdsare temporarilyforced out. around are very disturbing.Bait diggers,working at the When a person or group crossesthe tidal flats of the same spot for longerperiods, are toleratedat shorter Wadden Sea from the mainlandto one of the islands(a distancesthan a walking person. popular sport in The Netherlandsin which tens of thousands participateeach summer) this information Birdstaking flightare the most obviousresult of also allows us to calculatethe loss of feeding area from disturbance.As in restingbirds (Figure 4), they often which the birdsare forced out. Van der Meer (1985) change their behaviourlong before they take flight. Van calculated the size of this area at: der Meer (1985) has shown that some birds do so at distances which are on average 30% greater than those surface = •r22 + 2r,.h,.s + 2(G-r,).h2.s at which they take flight. In Brent Geese it was as much as 95% (205 m and 105 m respectively).Using the in which: distances at which birds take flight we can simply computethe area where no birdsare present. The size of such an area will be •r 2. The results of these surface = area abandoned by foraging birds (in m•) calculationsare depicted in Figure 6. In general, the s = walking speed in m/s situationwill be more complex because people will r, = flight distance in metres (zone 1) move over the tidal flats. If this happens birds will leave r• = distance at which birds stopped feeding in metres from an area in front and on both sides of a person or (zone 2)

CURLEW REDSHANK DUNLIN OYSTERCATCHER GULLS 300-500 m 200-300 m 100-300 m 100-200 m 100-150 m 50 ha 28 ha 13 ha 7 ha 5 ha Figure 6. Theoreticalsize of areas withoutany birds for five shorebirdspecies, usingthe informationfrom the Wadden Sea from Table 4.

lO Effectsof disturbance on shorebirds: a summary of existingknowledge from the Dutch Wadden Sea and Delta area

, Similar data on the effects of other sources of h1 s disturbanceon feedingbirds are muchmore scanty.A 4 h2 s ; small motorboat near Terschelling,sailing at approximately10 km/h,caused Oystercatchers to walk awayat 95 m; at 50 m mostbirds stopped feeding or rl r2 flew off. Curlews were less tolerant and responded at 190 m. At 95 m most Curlewswalked away or stopped feeding.As in otherstudies Bar-tailed Godwits reacted at distancesin betweenthose of Oystercatchersand Curlews(Glimmerveen & Went 1984). ß walkingperson ---•,• direction Figure7. Theoreticalsize of the area withoutbirds, after disturbance froma singleperson or a groupwalking over the tidalflats. See EFFECTS OF SMALL AIRPLANES, JETS AND text for additionalinformation (from: Van der Meer 1985) HELICOPTERS

Visser(1986) extensivelystudied the behaviourof birds roostingat the Noordvaarder,Terschelling. This area hi = recoveryspeed from zone 1 in seconds faces militaryactivities (including a jet shootingrange, h2 = recovery speea trom zone :,' In seconas ne•icop[eractivities, [ransport vehicles, •l.c.) arid a varietyof activitieslinked with tourists and local For Oystercatchersin the Delta area for a person inhabitants(including people walkingwith or without walkingwith a speedof 1 m/s (whichequals 3.6 km/h) dogs,angling, cross-country motorcycles, etc.). The the followingfigures were measured: frequentpresence of jets and helicoptersallows for a comparisonof the effectsof the two aircrafttypes. r, = 85 m Figure8 showsthat helicoptersdisturb more frequently G = 120 m and over longerdistances than jets, even though h, = 900 seconds activitiesfrom jets are accompaniedby Shootingand h2= 300 seconds highsound levels. The relativelymild effects from militaryjets are also knownfrom other places (Boer For Oystercatchersthis calculation leads to a disturbed et aL 1970; de Roos 1972) and are shown in Table 5. area of 20 ha. For bait diggersthe disturbedarea will be From these data it appears that small civil aircraftcause somewherein betweenrI andr 2. Onceagain the much more disturbance.Again, there were clear disturbedarea will be •r 2, whichmeans that a bait differencesbetween species. Oystercatchers were diggeris surroundedby an area of 3.3 ha withoutany rather tolerant of aircraft disturbance; Curlews were less Oystercatchers. so.Other siJecies (like Bar-tailed Godwits) were in

I JET I HELICOPTER 100

•:•:•:•:73•:•-• 100iii?:!i!iiiiiii::iIIIIIII1,,,,,,,,,,, iiiiiiiili:•iii:•!,::::.,.!!!,.111:•:•ili:•:•111iii!"'"'"" ;;;•;;•;' i===?====?=?=?=?='"'"" llllllllllIIIIIIII ??ii:?•i?• !?i!i!i!!!:: i•...... ::::::-•::•:::

...... ::•:•:•:•:•:• ...... •::::::::::::::•: :::::::::•:::::::::: .:.:.:.:.:.:.:•.:.,. .• . :•:•:•:•:•:[:•:•:•:•. :::::::::::::::::::::

60 60 i!

::::::::::::::::::::: :•:•:•:•:•:?•:•:K: 40 40 :.::

::::::::::::::::::::: :::::;::.-::::.::;.• ::::::::::::.:::::,• ::::::::::::::::::::: :•:•:•.•:•.•:•.•:•: 80•• >rainlying I80 ::::::::::::::::::::: . 2O 2O

...... :...... :. •:'--•?:..• rea•ionsnoreactions I 0 0 I I I o ,.q 8 oø o o, 'T o•, oO, &,o &, õ, õ, Figure8. Distances(in m) at whichmilitary jets and helicopters caused disturbance among roosting Bar-tailed Godwits at the Noordvaarder, Terschelling.Data were collected from 1980-84 and represent925 (jets)and 100 (helicopters)potential cases of disturbance(data from Visser 1986).

11 Effectsof disturbanceon shorebirds:a summaryof existingknowledge from the DutchWadden Sea and Delta area

Table 5. Disturbanceof waders (total numberof hoursof observationtime, numberof flocksobserved, the frequencyat which flocksflew up and the % of flocks reacting)at the Noordvaarder(Terschelling) in summer (1980-84). Flock sizes were >100 (Oystercatcherand Bar-tailed Godwit)and >20 (Curlew).Disturbance was consideredto occurwhen more than 50% of the flockflew up. Altitudesof all aircraftwere below 300 m. (data from: Visser 1986).

Disturbancefrom Hours obs. n Flight % Average jet at <1200 m frequency duration(s)

July 15-September 15

Oystercatcher 320 2120 110 5 36 Bar-tailedgodwit 150 925 168 18 56 Curlew 50 299 48 16 50

July 16-December 1 and March 1-May 1

Oystercatcher 320 2120 110 5 36 Bar-tailedgodwit 150 925 168 18 56 Curlew 50 299 48 16 50

Disturbance from Hours obs. n Flight % Average helicopter at <250 m frequency duration (s)

July 15-September 15

Oystercatcher 320 108 29 27 38 Bar-tailed godwit 150 58 43 74 73 Curlew 50 23 12 52 65

July 16-December 1 and March 1-May 1

Oystercatcher 300 84 27 32 22 Bar-tailed godwit 200 62 44 71 41 Curlew 30 12 8 68 38

Disturbance from Hours obs. n Flight % Average helicopterat <1500 m frequency duration (s)

July 15-September 15

Oystercatcher 320 15 11 73 50 Bar-tailedgodwit 150 13 11 85 114 Curlew 50 7 6 86 83

July 16-December 1 and March 1-May 1

Oystercatcher 300 3 2 - 48 Bar-tailedgodwit 200 2 2 - 168 Curlew 30 ....

12 Effects of disturbance on shorebirds:a summary of existing knowledge from the Dutch Wadden Sea and Delta area between. Heinen (1986) found that a small aircraftflying respectively)caused more dramatic effects. After 45 over roosts of shorebirds on the East Friesian islands minutes only 67% and 87% of the originallypresent Juist,Wangerooge and Mellum(German Wadden Sea) Oystercatcherand Curlew numbers had returnedto the led to 'disturbedbehaviour' (varyingfrom looking up and study plot (Glimmerveen& Went 1984). Small and slow more frequent callingto taking flightand not returningto flying aircraftare consideredto be among the most the initialroosting place and 4 categoriesin between) in disturbingphenomena in the Wadden Sea. The 44-53% of the cases, dependingon species, altitude, behaviour of the plane and its altitude both govern the locationand aircrafttype. In her study, in which she reaction of the birds: flying high in a straight line leads to unfortunatelydid not specifythe distancesbetween smaller effects than flying low or with unpredictable roostingflocks and planes,jets disturbedmore often (in curves (Boer et aL 1970). There is some discussion 84% of all potentiallydisturbing situations) than small between authors on the altitude at which planes cause civilaircraft (56%) and motorgliders (50%), whereas no disturbance. According to the Werkgroep helicopterswere very disturbingindeed (100%). Brent Waddenzee (1975) there is still disturbancewhen an Geese were among the most strongly'reactingspecies aircraft passes at 1,000 m. Baptist & Meininger (1984) (64-92%), togetherwith Curlew(42-86%) and Redshank always registered disturbance at 150 m and found that (70%). Shelduck(42%) and Bar-tailedGodwit (38%) at 300 m there was still disturbance within a radius of reacted less often. Civil aircraftflying at an altitudeof 1,000 m. Glimmerveen & Went (1984) found that >300 m disturbedin 8%, those flying at 150-300 m in individualCurlews only partly reacted by taking flight. 66% and those flying <150 m in 70% of the cases. On one occasion they observed a Curlew which These figures are comparablewith those found by de pressed itself stiff to the ground when a plane came Vlas (1986) in the Dutch Wadden Sea. over at 450 m, in another case a Curlew only looked up rather frequently (altitude 360 m). The result of a Observations on the tidal flats east of the island of passing small airplane (altitude 150 m) is also shown in Texel, with jets from the Vlieland shooting range Figure 9. All Curlews flew up but had recommenced frequentlypassing directly over at altitudesof less than feeding after 7 minutes. In contrast to many other 100 m, showedthat foragingbirds generally did not studies the recovery time for Oystercatchers was much respond.Occasionally short reactionswere noted, longer:30 minutes. Using the model describedabove, varyingfrom lookingup, stoppingforaging for a few an aircraft passing over at 150 m, creates a disturbed seconds,to shortflights of 10-30 seconds.Occasionally area of more than 15,000 ha! (Van der Meer 1985). somewhatstronger reactions were noted, possiblyfrom birdswhich had recentlyarrived in the area (like Brent Ultra Lightaircraft are a new developmentin aviation Geese, shortlyafter their arrivalfrom the Siberian technology.Very little research on the effects of Ultra breedinggrounds) (Smit & Visser 1985). Lightshas been carried out so far, but our first impressionis that they are very disturbing,probably Experimentson tidal flats southof the islandof because of the low altitude at which these planes Terschellingshow that 10 minutesafter a single operate and the noise they produce. Numbers of disturbanceby a small plane at 360 m altitude bird roostingand foraging Bewick'sSwans Cygnus bewickii numbers were back at the same level as prior to the close to an Ultra Light airstripat Schouwen Duiveland disturbance.A plane passingtwice (at 450 and 360 m (Delta area) droppedfrom 1,400-4,300 in 1986-88 to only a few birds in 1989, after the strip had been used for one year (Brilman in Smit & Visser 1989).

EFFECTS OF MILITARY SHOOTING ACTIVITIES

Early studies showed strong effects of the Vileland and E 40 Terschellingshooting ranges on waders. Flocks of roostingKnot disappeared almost completely from the 30 island of Vlieland (Van der Baan et aL 1958). In another

20 study in the same area, Tanis (1962) found no response in roostingShelduck, Mallard Anas platyrhynchos,Eider 10 Somateria mollissimaand gulls, but waders all took flightafter the first shot. All Tringasmoved to more 13:00 13:15ß• 13:30 I 13;45I 14:00I 14:15 14:30 peaceful roosting sites and most Dunlin and Knot 'left'. time Some of these birds returned later that day but Figure9. The numberof foragingCurlews in the Zandkreek(Delta continued shooting forced others to leave again. area) before and after disturbanceof a small civil aircrafton 9 March. Effectson Oystercatcherswere much longerlasting Oystercatchers,Bar-tailed Godwits and Curlews (from:Van der Meer 1985). returnedto the previouslyused roostingarea. The total

13 Effects of disturbanceon shorebirds:a summary of existingknowledge from the Dutch Wadden Sea and Delta area

sound level (dB) 90 95 100 105 110 105 100 95

i i , i i i i i i 5(•0 3•0 2•)0 1(•0 •) 1(•0 2•)0 3•0 5•)0 distance (m)

Rgure 10. Soundlevels of a completelyloaded overflying jet at an altitudeof 50 m, in relationto the distanceat whichthe plane is flying(from Hoffmann, in KOsters&von Raden 1986).

numbers of ducks, waders and gulls in the heavily Wintermans(1991), studyingthe effectsfrom a shooting disturbedarea amounted to 69,000 prior to shooting range in the Marnewaard found no indicationsfor a sessions(Monday-Friday) and to 38,000 duringthe lower diversityon the tidal flats. This could be due to the shooting sessions. lower sound levels in that area, which were between 43- 87 dB(A). Apparently,shooting alone has limitedvisible At present, tank shootingat Vileland is combinedwith effects on feeding waders. However, very strongsound bombing and rocket shootingfrom jets for most of the levels, as in the Vileland and Sylt examples, incidental year. These activitiesyield sound levels of 84-100 dB(A) heavy shooting(Visser 1986) or sonic booms (Burger at 5 km from the firing range. Nowadays, the shooting 1981a,b) may lead to strong reactions. Earlier activitiesstill have disturbingeffects, the extent observationson Vileland (van Koersveld in Platteeuw depending on the time the shooting starts. When the 1986) suggest that reactionsare stronger if sound is roosts have already been established,the first shot may combined with visual disturbance. cause considerable numbers of birds to take flight. Some returnto the originalroosting site, whereas others select a site farther away from the shootingrange. If EFFECTS OF DISTURBANCE ON FOOD INTAKE AND shootingstarts when the birds are still arrivingfrom the BEHAVIOUR OF TERRITORIAL BIRDS feeding areas, such a resettlement more or less takes place automatically(Smit unpubl.). Theoreticallyit is Disturbance of non-territorial birds often forces all birds possiblethat some birds stay away from disturbed to leave the most heavily disturbed areas. areas altogether, to roost elsewhere on nearby islands. Consequently,they have to feed in higherdensities However, there is no information to confirm such elsewhere. This may affect their food intake. Zwarts behaviour. A comparison of the past and present (1980) and Goss-Custard(1980) showed that food s•tuationat Vileland shows that roosting birds still intake of waders decreased when bird densities respondto shootingactivities, despite the fact that increased, probablydue to a higher level of interactions shootinghas been going on there for about 40 years. between birds. More or less territorial waders react This may be due to the very high sound levels,or to the differently.Zegers (1973) showed that a single person use of differenttypes of ammunition,leading to strong or group of people may cause the departure of most or differences in sound levels. KSsters &von Raden all Redshanks, Oystercatchersand Curlews from their (1986) suggestthat continuousreactions of birdsto jet preferred feeding site, a mussel bed south of shootingand bombingat Sylt may be due to sound Schiermonnikoog(Figure 11). After such a disturbance levels exceedingthe pain thresholdat 120 dB(A) (van only 9% of the originallypresent number of birds could Son 1987). He registeredshort lasting sound levels of still be found in the study area. The speed at which 105 dB(A) at 100 m from overflyingjets (Figure 10). birds return to the preferredfeeding areas varies Very high sound levels were absent in another study between species. When forced out from preferred from the Dutch Wadden Sea. In contrastto experiences feeding areas such birdsoften simplywait untilthe of the previousstudies, Wintermans (1991) could not source of disturbancehas disappeared again. Beli•n & find effects of militaryshooting on waders roostingalong Van Brummen (1985) studiedfood intake and behaviour the Groningen coast. In his study area sound levels did of an individualOystercatcher from a hide, in a situation not exceed 55 dB(A). in which it was possibleto manipulatethe length of the disturbedperiod in a standardizedway. By carefully Shootingactivities at Vileland have littlevisible effects forcingout a single bird from its preferredfeeding site on feeding waders. Prey choice, behaviourand intake they were able to show that the intake-rate may drop to rates of Oystercatcherand Curlewswere not different almost zero, despite the fact that birds continueto feed on days with and withoutshooting (Smit 1986). He in the area to which the bird has gone. These results found indicationsthat the diversityof feeding shorebirds are confirmedby comparableexperiments described by south of Vileland was higher on days without shooting, Hooijmeijer(1991 ). Figure 12 shows that during suggestingthat some speciesare less tolerantof high disturbanceresting and walking become more dominant sound levels and move away from that area. in the behaviour.After disturbancefeeding became

14 Effects of disturbance on shorebirds:a summary of existing knowledge from the Dutch Wadden Sea and Delta area

1•.•)I OYSTERCATCHER REDSHANKI CURLEW

4O AVY ..... Figure 11. Effectsof heavydisturbance on the numberof Oystercatchers,Redshanks and Curlewsin studyplots on musselbeds closeto Schiermonnikoog.Light disturbance: dotted line, heavy disturbance:straight line. The mean number of birds priorto disturbanceis set at 100%, the figuresat the abscissaindicate 15 minuteintewals beforeand after disturbance(from: Zegers 1973). much more dominantagain. Figure 13 shows that the gulls and StarlingsSturnus vulgarisat airfieldswhere intake rate duringa recoveryperiod was much higher. startingand landingpatterns of planes are very In an undisturbedlow-tide period the food intake of the predictable, both in terms of sound levels and in same bird is rather similar over the whole period. movements (c.f. Burger 1981a,b).

In some areas in the Wadden Sea helicoptersor small HABITUATION civil aircraft may cause panic reaction among thousands of roostingor foraging birds (e.g. Van der Kamp Reijnen& Thissen(1987), studyingthe presenceof & Koopman 1989). In areas where planes are common breedingsongbirds along motorways,found reduced at least some habituation can be noted. As shown in densitiesfor some species, despite the predictability earlier in this paper, there are large differences in the and constancyof sound levelsand trafficactivity on the effects of jets and small civil aircraft between the motorways.Apparently, some birdsdo not 'get used' to studies from Heinen (1986) and Visser (1986). In the disturbance.This agrees withthe shootingrange effects Mokbaai, Texel, a high degree of habituation has on roostingwaders at Vileland.The absence of a visible occurred to standard helicopter movements. response by foragingwaders shows that in other Helicopters transportingcrew to offshore drilling situations habituation may take place. This process platforms pass over at a frequency of 2-3 per hour at ("learning")is probablyfacilitated by a more or less 100-300 m altitude. These activities do not appear to constantsupply of identicalstimuli. This may be the have strong effects on feeding and roostingwaders and reason for the presence of Lapwings Vanellusvaneflus, ducks. The same area is also used for activities from

Behaviour (% of time spent) Intake (rag AFDW/sec) 100 2.5

8O 2

6O 1.5

2O 0.5 o Before During After Before During After

• Agression • Resting [•] Walking • Feeding • Undisturbed • Disturbed Figure12. Behaviour(in % of timespent) of an individuallymarked Figure 13 Intake rates (mg Ash Free Dry Weight/s) of an individually Oystercatcher(LYCB) when forcedout from its 'territory'on a markedOystercatcher (LYCB) in an undisturbedlow tide period mussel bed in the Mokbaai, Texel. The behaviour was (artificiallycut intothree parts)and duringa low-tideperiod in registeredfor a wholelow tide period,the disturbancewas which the bird was forced out from its 'territory'on a mussel bed initiatedartificially (from: Hooijmeijer 1991). in the Mokbaai, Texel (from: Hooijmeijer 1991).

15 Effects of disturbanceon shorebirds:a summary of existing knowledgefrom the Dutch Wadden Sea and Delta area military helicopters,often at low altitudes. Despite a reactions whereas under normal conditionsthey would large degree of habituationto standard helicopter have much smaller effects (Smit unpubl.). movements, these activities may force birds to less disturbed parts of the bay, and some species (like These data suggestthat birdsare able to distinguish Cormorants Phalacrocorax carbo or Eiders) often betweentypes of planes.This is not really surprising.At temporarilyleave the area. Small aircraft passingover the Banc d'Arguin, Mauritania, waders also appeared to at altitudes of over 300 m have effects comparable to distinguishbetween predators. Fish-eating Ospreys those of civil helicopters.'Unusual' types of planes, Pandion haliaetus were observed to land in between however, which show up at low frequencies still have flocks of waders withoutany disturbance,whereas strong effects (Smit unpubl.). This has also been found potentially dangerous Marsh Harriers Circus to happen on Vlieland. Under normal conditions aeruginosusforced thousands of waders to take flight roostingbirds do not react severely when jets at the (Piersma 1982 and Smit unpubl.).Apparently, birds can Vliehors shooting range pass by at high speed. learn throughexperience that some potentially Relatively slowly flying A10 jets are much less common dangerous objects are not dangerous after all. An in that area and can force thousands of birds to take unknownor infrequentlyoccurring object, however, will flight for several minutes.This aircraft type is able to fly be regarded with caution. at a very low speed, can make very short curves and carries very strong machine guns. Consequently,its behaviour and sound productionis very differentfrom FACILITATION 'ordinary' jets, which are much more common there (Smit & Visser 1985). Also in the Mokbaai the birds The data presentedso far suggestthat high levels of could be more easily disturbedafter the appearance of disturbancelead to highertolerance levels. This may be an unusual aircraft type. After such an event, an true in some cases but cannot be considered a rule. overflyingGrey Heron Ardea cinerea or Great Black- The opposite(referred to as facilitation)may also occur. backed Gull Larus marinus may even cause panic Figure 14 depictsthe numberof potentiallydisturbing activitiesat the Noordvaarder,Terschelling, from 15 July - 15 September(summer) and I March - 1 May and 15 September- I December (spring/autumn)and the I SUMMER I SPRINGand AUTUMN actual amount of disturbanceresulting from these. In 10 summer the number of potentiallydisturbing stimuli from .c: 8 militaryactivities is only slightlylarger than in springand i• ::•:;:;•½:•- .'? .....• autumn.Their effect, however,is much larger.This is due to a cumulation of effects of disturbance from :'::2::: :i:i:!:i:!:i differentsources. Leisure activities alone already may :-:-:::::::; .... have a large disturbingeffect. Combinedwith military activitiesthey lead to disturbancelevels far exceeding

:."..:.:.'..', .:.:.:.:.:.: the effects each activityalone would have had. .;.;.;.;.;...... ;..... :.:;:.:.:.: ' •:•:•:•:::•:•:::•':':':':':':' !:i:i:i:i:! Comparableresults were foundon Sylt, Germany, •i!.-"'•(::i!ii;•;::::::::::::: •' 20 where overflyingjets appearedto have larger effects E:•:•:•. .-..... when prior to these activitieswind surfers had been • 40 present in the area (K•sters &von Raden 1986). These :::::::.':.': •::i•.:2:!::! ::::::::::: reactionsare comparable to the Heron and Black- .• 60 - •!•:!:•i.•i•i':-:-:-:-:-: ::::::::::: iii.!.!•-• ::i.iii ._ '"•'•;•;%::!:!:!:'•! ':":':':':'""'""'" backed Gull example from the Mokbaai, described -o 80 .,-. :,..-.• military previously. :-.'-• disturbance -o 100 i:i• / othermainlysources,recreation A summary of all factorsdetermining the behaviourof a ._ :•::•::• •i•!i!i!':i' :iiii•d.'.•:;::•:.•::::•...... single bird after disturbanceis given in Figure 15. Birds o) 120 respondas a functionof earlier experiences, reactions • 140 of other birds nearby and severalfactors determining , , either their willingnessto remain at the same place or act as a motivationto leave for another place. Activity rhythmsand the availabilityof food and rest are importantfactors influencing the decisionto remainat the same site. Birdsfeeling hungrywill sooner leave Figure 14. Number of potential6isturbance cases per •our an6 the high-tideroosts than well-fed birdsthat have just arrived time these cases actually resulte6 in 6isturbance(se½on6s per hour) in summer and spring/autumnat the Noordvaarder, from the feeding areas (Visser 1986). Protection Terschellingfor Oystercatcher,Bar-tailed Godwit and Curlew through nature protectionmeasures or the presence of (data: Visser 1986). a flock of conspecificsmay also play a role. Additionally,

16 Effectsof disturbanceon shorebirds:a summary of existingknowledge from the Dutch Wadden Sea and Delta area

Disturbance

I I I I ; ; Deds•; to move ebewbem i I i I

Reactionof a singleb•rd

Food Protection

Figure15. Reactionsof a singlebird on disturbanceand the factorspotentially affecting its behaviour(modified after K0sters&von Raden 1986)

a bird may postpone an immediate reaction in order to higher densities.Another questionis whether minimizeits energy expenditure(referred to as 'slow non-territorialbirds are really non-territorial.From decisionmaking'). Activity rhythms and the presenceof several studieswe know that at least some species of suitablealternative feeding or roostingsites may induce non-territorialbirds neverthelessdo show a high a decision to move elsewhere. Birds may respond site-fidelitywithin a winter and between years, at least directlyto disturbance,for instancefrom a shot in a at the roostingplaces (Furness & Galbraith 1980, previouslyundisturbed area. In many cases, however, Symonds et al. 1984). This could mean that these birds birds will not respond immediatelybut use earlier do not distributethemselves over the tidal flats as freely experiencesand the behaviourof other birds in the as we may think. Consequently,regular disturbance in same area as a filter mechanism.Warning signals part of their preferredfeeding area could affect their (withoutany furtherdisturbance) may also lead to food intake more than when they would freely distribute behaviouralresponses. over large areas. To what extent will be extremely difficultto measure. Birds may also be able to compensateby feedinglonger or by more frequentor THE USE OF PRESENTLY AVAILABLE DATA AND longerfeeding at night.Cage experimentsduring which FUTURE NEEDS the lengthof the low-tideperiod was artificially manipulatedhave shown that (territorial)Oystercatchers This contributionshows that in the past 20 years there are able to compensatefor lost feeding time by has been much study of the effects of disturbance.We increasingtheir efficiency(Swennen et aL 1989). Of could ask ourselves, however, whether these data are course we do not know to what extent this is possiblein the tools we actually need to understandthe effects on the field and at what price. Oystercatcherscould run a a bird. In the case of disturbance to non-territorial birds, higher risk of bill damage, other species could be more we also need to knowthe consequencesof feeding in vulnerableto predatorsbecause of reducedvigilance.

17 Effects of disturbance on shorebirds:a summary of existing knowledgefrom the Dutch Wadden Sea and Delta area

Frequent disturbancemay force waders to abandon REFERENCES traditionalhigh-tide roosts. This is demonstratedin the Dee estuary (Mitchell et aL 1988) where Bar-tailed Baan, G. van der, Blok A., Nijhoff, P. & Swennen, K. 1958. Een Godwits declined 99%, Knots 79% and Dunlins 81%. In inleidendonderzoek naar de betrekkingentussen wadvogels en bodemfauna. Bondsuitgeverij Vogelwerkgroep NJN, this case the birds continued to use the traditional Amsterdam: 28 pp. feeding areas. This behaviouralchange involvedhigher energy costs, because the birds had to fly an extra 40 Baptist,H. & Meininger,P. 1984. Ornithologischeverkenningen van de km each tidal cycle. Heavy disturbancecan also lead to Voordelta van Z.W. Nederland, 1975-1983. Rijkswaterstaat reportDDM-83.19, Middelburg:85 pp. a total departurefrom feeding sites. This is probably happeningin Denmark where suitablewetlands do not Beli•n, E. & Brummen,W. van 1985. Het gedragvan Scholekstersop harbourany Curlews, probablydue to large flight twee fourageergebiedenen bij verstoring.Unpubl. report SOL distancesand warinessas a resultof hunting(Meltofte Utrecht/RIN Texel: 63 pp.

1986). The consequencesof such banishmentare Blankestijn,S., Hoenderdos, A., Hoogeboom, F., Oerlemans, M., largely unknownand are part of a major applied Oosting,B. & Veugelers, G. 1986. Seizoensverbredingin de ecologicalquestion: 'how many birds can an estuary recreatie en verstoringvan Wulp en Scholeksterop hoogwater- support'(c.f. Sutherland& Goss-Custard1991, vluchtplaatsenop Terschelling.Report ProjectgroepWadden, L.H. Wageningen:261 pp. Lambeck 1991, Meire 1991 ). Boer, P., Brenkelen, W. van, Monsees, G.R., Mulder, Th. & Viies, K. In this contributionwe have restricted ourselves very van der 1970. Het Balgzand, bedreigdgebied. Wetensch. much to visible reactions.What is really happeningto Meded. KNNV 86:112 pp. birds is a much more complicatedproblem. Heart-rates Boere, G.C. 1975. De betekenisvan het internationaleWaddengebied of Eiders and Oystercatchersappear to increase voor de vogelbescherming. Waddenbull.10: 244-246. considerablywhen incubatingbirds are approachedby man or a helicopter, despite the fact that the birds Burger, J. 1981a. The effect of human activityon birds at a coastal showed no visible reaction (Gabrielsen 1987, Hf•ppop & bay. BioL Conserv. 21: 231-241. Hagen 1990). The implicationson energy of disturbance Burger,J. 1981b. Behaviouralresponses of HerringGulls Larus are not yet clear but indicatethat the effects can be argentatusto aircraftnoise. Environm.Poll. (Series A) 24: much larger than would appear from the studies 177-184. described in this paper. Furhess, R.W. & Galbraith, H. 1980. Numbers,passage and local The previousexamples of what we stilldo not know may movements of Redshanks Tringa totanuson the Clyde estuary as shown by dye-marking. Wader Study Group Bull. 29:19-22. give too pessimistica pictureof our knowledgeof disturbance.Clearly, there are many limitationson what Gabrielsen,G.W. 1987. Reaksjonerp•t menneskeligeforstyrrelser hos we can do with the existingdata. Nevertheless,they can aerfugl, svalbardrypeog krykke i egg/ungeperioden.V•r be usefultools for modellingbird reactions.They can also Fuglefauna 10: 153-158. be well appliedby policymakers or for localprotection Gerdes, K. & H. Reepmeyer, 1983. Zur rAumlichenVerteilung 6•ber- measures(such as keepingvisitors to coastalnature winterender Saat- und BIAssgAnse(Anser fabalis und A. reservesat a sufficientdistance from the high-tide albifrons)in AbhAngigkeitvon naturschutzschAdlichenund roosts).However, the availabledata are not yet suitable f0rdernden EinflC•ssen.Vogelwelt 104: 54-67. for answersto key questionslike 'what are the effectsof Glimmerveen, U. & Went, W. 1984. Dosis-effectrelatie bij verstoring disturbanceon energybudgets of a bird'or 'whatare the van wadvogels. Unpubl. report Inst. Landschapsoecologieen repercussionsof beingin a bad conditionon survivalor Natuurbeheer R.U. Utrecht: 59 pp. reproductionrate'. Such questionswill involvemore studieson energyloss, the costs(or lowerintake) of Goss-Custard, J.D. 1980. Competition for food and interference among waders. Ardea 68: 31-52. feedingat alternativefeeding sites and the consequencesof highercosts or lowerintake rates on Heinen, F. 1986. Untersuchungfiber den Einflussdes Flugverkehrs body-condition. auf br6•tendeund rastende K6•stenv0gelan ausgew•hlten Stellen des nieders•chsischen Wattenmeergebietes. Unpubl. report (Diplomarbeit) UniversityEssen: 82 pp.

Hooijmeijer,J.C.E.W. 1991. Onderzoek naar de invloedvan verstoring ACKNOWLEDGEMENTS op het gedrag en het voorkomenvan Scholeksters (Haematopusostralegus) in de Mokbaai op Texel. Internal RIN Thanks are due to all studentswho gave so much time report91/27, Texel/R.U. Groningen,103 pp. and persistenceto producethe basic data discussedin H0ppop, O. & Hagen, K. 1990. Der Einflussvon St0rungenauf this paper. Norbert Dankers and Rob Lambeck Wildtieream Beispielder HerzschlagrateBrC•tender commentedcritically on the manuscript.Arian Griffioen Austernfischer(Haematopus ostralegus). Vogelwarte 35: producedsome of the figures. 301-310.

18 Effectsof disturbanceon shorebirds:a summaryof existingknowledge from the Dutch Wadden Sea and Delta area

Kamp, J. van der & Koopman, E. 1989. Rottumeroog, verslag Smit, C.J., Lambeck, R.H.D. & Wolff, W.J. 1987. Threats to coastal bewaking 1988. ReportStaatsbosbeheer, Groningen: 54 pp. winteringand staging areas of waders. Wader Study Group Bull. 49, Suppl./IWRB Special Publ. 7:105-113. Kersten, M. 1975. Wulp. In: Ameland-verslag.Verlag van het wadvo- gelonderzoekop Ameland in 1972 en 1973:110-139. Smit, C.J. & Visser, G.J.M. 1985. Studies on the effects of military Jeugdbondenvoor Natuurstudie,Nijmegen. activities on shorebirds in the Wadden Sea. Proc. CCMS-Seminarof flora and fauna in militarytraining areas, Koepff,C. & Dietrich,K. 1986. St0rungenvon K0stenv0gelndurch 1984, Soesterberg,The Netherlands:34-51. Wasserfahrzeuge.Vogelwarte 33: 232-248. Smit, C.J. & Visser, G.J.M. 1989. Verstoringvan vogels door Kooy, A., Koersveld,S. & Suy, M. 1975. De invloedvan recreatie en vliegverkeer, met name door ultra-lichtevliegtuigen. RIN report andere verstoringsbronnenop de avifauna van het eiland 89/11, Texel: 12 pp. Vlieland. Unpubl. reportVakgroep NatuurbeheerL.H. Wageningen, Nr. 335:40 pp. Son, R.J.J.H. van 1987. Lawaai en dieren. Report Wetenschapsw•nkel UniversityAmsterdam: 35 pp. K0sters,E. & Raden, H. von 1986 Zum Einflussvon Tiefflug, Schiessbetriebund anderenanthropogenen St0rungen auf VOgel Sutherland,W.J. & Goss-Custard,J.D., 1991. Predictingthe im Wattenmeer bei Lyst/Sylt.Vogel und Luftverkehr6-2: 75-89. consequence of habitat loss on shorebird populations.Acta XX Congr. Intern. Omithol., New Zealand 1991: 2199-2207. Lambeck, R.H.D., 1991. Changes in abundance, distributionand mor- talityof winteringOystercatchers after habitatloss in the Delta Swennen, C., Leopold, M.F. & Bruijn, L.L.M. de 1989. Time-stressed area, SW Netherlands.Acta XX Congr. Intern. Omithol., New Oystercatchers, Haematopus ostralegus,can increase their Zealand 1991:2208-2218. intake rate. Anim. Behav. 38: 8-22.

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Platteeuw, M. 1986. Effecten van geluidhinderdoor militaire Wolff, W.J., Reijnders, P.J.H. & Smit, C.J. 1982. The effects of activiteitenop gedragen ecologievan wadvogels.RIN report recreation on the Wadden Sea ecosystem: many questions 86/13, Texel: 50 pp. but few answers. In: G. Luck & H. Michaelis (eds.), SchriftenreiheM.E.L.F., Reihe A: Angew. Wissensch. Nr. 275 Reijnen,M.J.S.M., & Thissen,J.B.M. 1987. Effectsfrom road trafficon 85-107. breeding-birdpopulations in woodland.Ann. rep. Research InstituteNature Management 1986:121-132. Wolff, W.J. & Smit, C.J. 1984. The Dutch Wadden Sea. Pp 238-253 in: P.R. Evans, J.D. Goss-Custard& W.G. Hale (eds.), Coastal Roos, G. Th. de 1972. De invloed van rekreatie en andere veron- waders and wildfowlin winter.Cambridge Univ. Press, trustingenop de broed- en trekvogelsin het Cambridge. Staatsnatuurreservaat'Kroonspolders' op het eftand Vlieland. Report 186 Vakgroep Natuurbeheer,L.U. Wageningen. Zegers, P.M. 1973. Invloedvan verstoringop het gedragvan wadvogels. Waddenbull.8(3): 3-7. Rudfeld, L. 1990.25 •rs beskyttelseaf Vadehavet. Miljoministeriet, Skov- og Naturstyrelsen,Kobenhavn. Zwarts, L. 1972. Verstoringvan wadvogels.Waddenbufi. 7(3): 7-12.

Smit, C.J. 1986. Ori•nterend onderzoeknaar veranderingenin gedrag Zwarts, L. 1980. Intra- and inter-specificcompetition for space in en aantallen van wadvogelsonder invloed van schietoefenin- estuarine birds in a one-prey situation. Proc. 17th Int. Ornithol. gen. RIN report86/18, Texel: 28 pp. Congr., Berlin 1978:145-150.