0046437
Effectsof disturbanceon shorebirds:a summaryof existingknowledgefrom 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 disturbance
on shorebirds:
a summaryof existing
knowledgefromthe DutchWadden Sea and Delta area. Wader Study GroupBull.68: 6-19.
The extent to which shorebirdsare disturbedby various activitiesis discussed,with reference to
studies carried out on the Wadden Sea and Delta area. The effects of leisure activities on
foragingand roostingbirdsare discussed. The effectsof smallairplanes,jets and helicopters
are also considered, as are the effects of disturbance on food intake and behaviour of territorial
birds. Frequentdisturbancemay force waders to abandontraditionalhigh-tideroosts. The
implications
of disturbance
on energyare notyet clearbutindicatethatthe 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
The Wadden
Sea and Delta area are both wetlands
man (Figure 1). Over the past 20 years the scale of
most of these (potential)conflictshas been studied
in the Dutch part of the Wadden Sea and the Delta
area and, more recently, also in the Danish and
German Wadden Sea. The more recent investigations
of outstandingimportancefor many bird species
ecologically dependent on intertidal habitats (Wolff &
Smit 1984; Leeuwis eta/, 1984). At the same time
these areas are intensively used for a great variety of
human activities, In some cases this leads or may
activities should be banned from (parts of) an
lead to conflicts
area.
between
the interests
Interactions
birds
of birds and
/ man
Resting birds
Foraging birds
Tourism - walking
- surfing, sailing
Bait digging
Walking over mudflats
Farming
Civil aircrafts
Hunting, egg collecting
Military activities
Military activities
Fisheries
Leisure
boats
F•gure 1. Human activitiesactuallyor potentiallyconflictingwith the
interestsof restingand foragingshorebirdsin the Dutch
Wadden
Sea and Delta area
have focussed
on whether
limits should be set to
human presence in an area or whether certain
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 differentlyfrom the
behaviour it would exhibit without the presence of that
activity' (Oranjewoud1982). In this contributionwe will
restrictourselvesto disturbancecaused by human
activities:disturbancefrom natural causes (weather,
predators)has not been studiedin detail and will only
briefly be addressed. We will not discuss the effects on
breeding birds and mainly cover Dutch studiesfrom
coastal sites (with some informationfrom the German
part of the Wadden Sea). Outside The Netherlands
very little is known of this work. This is very
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,governmentagencies and universities.
Access to these reportsis also hampered by language
barriers: nearly all informationis published in Dutch.
This paper is an attempt to summarize briefly the
results
of these
studies.
0046438
Effectsof disturbanceon shorebirds:a summaryof existingknowledgefrom 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
THE
Balgzand -
NETHERLANDS
Figure 2. Map of the Dutch part of the Wadden Sea with locationof sites referredto in the text.
EFFECTS
OF LEISURE
ACTIVITIES
RESTING
AT HIGH-TIDE
ROOSTS
ON BIRDS
High-tide roosts may be encounteredin many places
along the bordersof the Wadden Sea and the Delta
estuaries.
On the mainland coast of the Wadden
Sea
(Figure 2) shorebirdsrooston man-made saltmarshes.
In most of the Wadden
Sea these areas are not
distances.Cars, agriculturalactivitiesand dogs caused
less disturbance(Blankestijnet al. 1986). There are also
differencesbetween species as illustratedby the flight
distanceswhen birdsare approached by walking people
(Figure3). Golden PloversPluvialisapricariaare fairly
tolerant,but Curlew Numeniusarquata and Redshank
Tringa totanustend to take flightat more than twice as
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
from most places quantitativedata on the scale of the
problem are lacking.
Flocksof shorebirdsmay be disturbedby a variety of
human activities,though natural causes (such as
predators)may also take an importantshare. Table 1
presentsthe reasonsfor disturbance,as registeredon
Terschelling.In particularsmall aircraftand tourists
walkingare importantsourcesof disturbance;cattle or
peoplewith a highlypredictablebehaviour(like farmers)
are less so. Table 2 shows that small aircraft and people
walkingaroundalso cause birdsto take flightat large
GOLDEN
PLOVER
BLACK:H:•::ADED
GULL
HERRING
--- ...
--.- .::.....
GULL
..........
OYSTERCATCHEB
BAR-TAILED
....
•.:.:.......:::::•:
....
GODWIT
..::::::::::::::::::::::::::::::::::::
REDSHANK
I
0
I
I
20
40
I
I
60
I
I
I
80
I
100
I
120
flight distance (m)
Figure 3. Distances(mean values in m and 95% confidencelimits) at
whichflocksof roostingwadersand gullswere recordedto take
flightwhen approachedby walkingpeople. Data from
Terschelling,July- September 1981 (data: Tensen & van Zoest
1983).
0046439
Effectsof disturbanceon shorebirds:a summaryof existingknowledgefrom the DutchWadden Sea and Delta area
Table 1. Disturbancefrequency,expressedas a percentageof the total amountof visibledisturbancein one studysituationat high-tideroosts
in a cultivatedgrasslandarea at Terschelling.
The studywas carriedout in July-September
1981,whenrelativelymanytouristson bicycles
were presenton the island.Most of these have a very predictablebehaviourthroughtheirpreferencefor metalledcyclepaths(data:
Tensen & van Zoest 1983).
Curlew
Bar-tailed Godwit
Small aircraft
39
Walkingperson(s)
Agriculturalactivities
31
10
I
Source
of disturbance
Oystercatcher
Gulls
23
18
27
32
8
65
4
17
7
2
13
I
-
-
11
16
-
24
8
18
-
24
Cows
Cyclist(s)
-
Natural
Unknown
reason
Terschellingshowthat walkingpeople within250 m of
roostingOystercatchersHaematopusostraleguscaused
flocksto fly in 57% of the cases. As a mean, these birds
were 38 secondsper houron the wing (mean of 320
observationhours).Curlewsflew up in 76% of the cases.
On averagethese birdsflew 57 sec/h (meanover50
hours)(Visser1986). Figure4 showsthat beforeflying
up, the behaviourof roostingbirdsmay already have
been considerablyaffected:lookingup and walking
away become more dominantas distancesget smaller.
Weather conditionspartlydetermineflightdistances.
Kersten (1975) reportsthat Curlews can be more easily
disturbed
during
rainyweather.
A..t
thesametimethe
roostsare less compact, smaller and distributedover
largerareas. Several studiesshow that large flocksare
more easilydisturbed(Zwarts1972; Kooyet al. 1975).
Flightdistancesare very muchtime and location
dependent.These differencesare sometimesrather
Behaviour
within
the flock
1
difficultto explain. At the comparativelyundisturbed
Banc d'Arguin,Mauritania(where flightdistancesfor
most species are smallerthan in the Wadden Sea)
flocks of winteringOystercatchersfly up at 400-500 m
(Smit unpubl.)whereas in the Wadden Sea they are a
rather tolerantspecies(Figure 3, Table 2). Birds
roostingin cultivatedgrasslandswith a certain amount
of human activitycan often be approached to closer
distancesthan those roostingin remote salt marshes
(withinthe same area and time of the year). Curlews
roostingin cultivatedgrasslandareas at Terschelling
could be approached to approximately 100 m, whereas
on the salt marshes on same island the flight distance
was 200 m (Tensen & van Zoest 1983).
Hunting may increase flightdistances for non-target
species as well as target species. By the end of
September, Brent Geese Branta bernicla in Denmark
take flightat 210 m; by the end of October (after the
start of the huntingseason) the mean flightdistance
has increasedto 370 m (Rudfeld 1990). Comparable
(in %)
80
Table 2. Mean distances(m) at which flocksof Oystematchersand
Curlewsflew up when approachedby varioussourcesof
disturbance.Data fromflocks roostingin cultivatedgrasslands
on the islandof Terschelling(data: Blankestijnet al. 1986).
60
Source of disturbance
20
I •
Small aircraft
0
50-75
75-100
100-125
125-150
150-175
175-200
Car
Dietance flock to walking pereon (m.)
•
Birdswalking •
Birdslookingup
•
Walking person (s)
Helicopter
Birdssleeping
Figure 4. Behaviourof flocksof Curlews when approachedby walking
people, in relationto the distance to the flocks. Data were
collected in standardizedexperimental situations,using flocks
of Curlews roostingin cultivatedgrasslands(data: Blankestijn
et aL 1986).
Egg collector
Farmer/Agricultural
activities
Oystercatcher
500
82
106
Curlew
-
213
200
188
46
140
60
129
Dog(s)
-
90
Cattle
10
-
0046440
Effe(•ts
of disturbance
onshorebirds:
a summary
of existing
knowledge
fromtheDutchWaddenSeaandDeltaarea
locationwherein summermanytouristswere present.A
comparable feature was found in Denmark where BarOYSTE.RCA•'CHER ':"'•
....'"'"1
'"
I• I wind
kayak
I
suderJ
II
REDSHANK
tailedGodwitsalsochangedroosting
sites,probablyas
a resultof disturbance
fromhunting(Rudfeld1990).The
easternpartof the islandof Amelandconsists
of a large
sandflat. Normally10,000 Curlews roost here in
.KNOT
summernights,whereasduringthe day onlysome
.BAR-TA'II•ED
.GODW.!:
T
hundredsare present. These birds use alternative
.SH'EED'ucK
day-timeroostsonthe sandflatsof Engelsmanplaat
and
on the Frisiancoast.In winter,whenconsiderably
fewer
'CURLEW
people visit that area, Curlews do roost on Ameland
0
1(•0 2•)0 3•)0 4•)0 5•)0 6•)0
flight distance(m)
Figure5. Distances
(meanvaluesin m andstandarddeviations)
at
whichflocksof roostingwaderswere recordedto takeflight
when approachedby a kayak or wind surfer.Data from the
Jadebusen(Germanmainlandcoast)(from:Koepff& Dietrich
1986).
observations
havebeenmadeon inlandfeedinggeese
(cf. Gerdes& Reepmeyer1983).
(Kersten1975). Zwarts(1972) notedthat a traditional
Curlewrooston thesaltmarsheswasgradually
abandonedafteran increasein disturbance
by tourism.
Ringingactivities
onVlielandontwosuccessive
nights
lead to severe disturbance of Redshank roosts. It took
fivedaysbeforethe normallyoccurring
numberswere
presentagain.Cannon-netting
at a Curlewhigh-tide
rooston the sameislandon twonightswitha ninedays
intervalalso led to a temporarydeparture.In this case it
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
Tadornatadornain the Jadebusen(GermanWadden
Sea). Althoughthereare somedifferencesin species
compositionbetweenthisand the previouslymentioned
EFFECTS OF LEISURE ACTIVITIES ON FORAGING
BIRDS
study, more or less the same order of disturbance
Althoughdata on flightdistancesare availablefor some
susceptibility
was found(Figure5). Kayaksand sailing
tidalflats,it is impossible
to givestandardfiguresonthis
matter.Distancesvarybetweensitesandare dependent
on earlierexperiences(learning)in that particular
boats disturbed more often than motor boats and wind
surfers.Kayakshave a smalldraughtwhichenables
them to come very closeto the high-tideroosts.In the
KOnigshafen,
Sylt,windsurfershad strongdisturbing
effectson dabblingducks.A few zigzagmovementsof a
singlesurferwere sufficientfor a completedepartureof
all ducks present. In the same situationBrent Geese left
the area whenapproachedat 300 m or less(K•sters &
von Raden 1986).
location.In Denmark, for instance,Curlews show an
extremewarinessand have a flightdistanceof 500 m,
probablybecausetheyare a huntedspeciesin that
country(Meltofte1986). In the Mokbaai,Texel, a small
baysurrounded
byseawallsanddunes,witha large
varietyof humanactivities
(recreation,
baitdigging,
angling,helicopter
traffic,navyinflatables
speeding
throughthe channel,etc.) some Dunlinscan be
Waders may leave their usualhigh-tideroostsas a
somewhatdifficultto interpret.Boeret aL (1970) found
approached
to within10-20m or lesswithoutanyvisible
disturbance.
At the sametimeof the year Dunlinflocks
feedingontheopenmudflats
eastoftheislandmaytake
on the Balgzand that Bar-tailedGodwits Limosa
lapponicausedtwo high-tideroosts.One was used
flightat a distanceof 100 or 200 m. Such differences
were also foundat Terschelling,where birdscloseto the
especiallyin summerandwassituatedin a relatively
sea wall(withratherfrequenthumanactivities)
tolerated
quietarea. The secondwas used mainlyin winterat a
people walking on the tidal flats at shorter distancesthan
result of disturbance but the available data are
Table3 Meanflight
distances
(inm)ofwader
species,
when
approached
bypeople
walking
overthetidalflatsnearTerschelling
(Dutch
Wadden
Sea)at various
distances
fromtheseawall(data:Glimmerveen
& Went1984).
Species
200-300m fromsea
wall
Oystercatcher
500-1,000m from
sea wall
mussel bed at 1,000 m
from sea wall
79
113
77
Bar-tailed Godwit
101
138
-
Curlew
140
196
102
0046441
Effectsof disturbanceon shorebirds:a summaryof existingknowledgefrom the DutchWadden Sea and Delta area
Table 4, Mean distancesand ranges(m) at whichbirdstake flightwhen approachedby peoplewalkingover the tidalflats (from:van der Meer
1985 (Deltaarea), Wolffet al. 1982 and Smit unpubl.(WaddenSea)).
Species
Delta
Wadden Sea
Flight dist.
Range
Flightdist.
Range
Curlew
211
124-299
339
225-550
Sheld uck
148
99-197
250
200-300
Grey Plover
Ringed Plover
124
121
106-142
80-162
-
Bat-tailed
107
88-127
219
150-225
105
58-152
-
-
Godwit
Brent Goose
50-150
-
Oystercatcher
85
81-89
136
Dunlin
71
57-86
163
100-300
Turnstone
47
31-53
-
150-250
birdsforagingfartheraway from the sea-wall (Table 3).
The flightdistanceis also influencedby the behaviourof
a person or groupof people. One individualperson
generally disturbsless than a group; dogs running
aroundare very disturbing.Bait diggers,workingat the
same spotfor longerperiods,are toleratedat shorter
distancesthan a walking person.
Birdstakingflightare the most obviousresultof
disturbance.As in restingbirds(Figure 4), they often
change their behaviourlong before they take flight.Van
der Meer (1985) has shown that some birds do so at
distances which are on average 30% greater than those
at which they take flight. In Brent Geese it was as much
as 95% (205 m and 105 m respectively).Using the
distancesat 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
calculationsare depictedin Figure 6. In general, the
situationwill be more complex because people will
move over the tidal flats. If this happens birdswill leave
from an area in front and on both sides of a person or
25-300
group (Figure 7). Using the informationon the distance
at which birds take flight and additional informationon
the time needed for recoverywe can calculate the size
of the area where birdsare temporarilyforced out.
When a person or group crossesthe tidal flats of the
Wadden Sea from the mainlandto one of the islands(a
popular sport in The Netherlandsin which tens of
thousands participateeach summer) this information
also allows us to calculatethe lossof feeding area from
whichthe birdsare forced out. Van der Meer (1985)
calculated
the size of this area at:
surface= •r22+ 2r,.h,.s + 2(G-r,).h2.s
in which:
surface = area abandoned by foraging birds (in m•)
s = walking speed in m/s
r, = flight distance in metres (zone 1)
r• = distance at which birdsstoppedfeeding in metres
(zone 2)
CURLEW
REDSHANK
DUNLIN
OYSTERCATCHER
GULLS
300-500 m
50 ha
200-300 m
28 ha
100-300 m
13 ha
100-200 m
7 ha
100-150 m
5 ha
Figure 6. Theoreticalsize of areas withoutany birdsfor five shorebirdspecies, usingthe informationfrom the Wadden Sea from Table 4.
lO
0046442
Effects
ofdisturbance
onshorebirds:
a summary
of existing
knowledge
fromtheDutchWaddenSeaandDeltaarea
,
Similar data on the effects of other sources of
h1 s
disturbanceon feedingbirdsare muchmorescanty.A
small motorboat near Terschelling,sailingat
4 h2s ;
approximately
10 km/h,causedOystercatchers
to walk
awayat 95 m;at 50 m mostbirdsstoppedfeedingor
rl
r2
flew off. Curlews were less tolerant and responded at
190 m. At 95 m mostCurlewswalked away or stopped
feeding.As inotherstudiesBar-tailed
Godwitsreacted
ß
at distancesin betweenthoseof Oystercatchersand
Curlews(Glimmerveen& Went 1984).
walking
person
---•,•
direction
Figure7. Theoreticalsize of the area withoutbirds,afterdisturbance
froma singlepersonor a groupwalkingoverthe tidalflats.See
text for additionalinformation(from:Van der Meer 1985)
EFFECTS OF SMALL AIRPLANES, JETS AND
HELICOPTERS
Visser(1986)extensively
studiedthe behaviourof birds
roosting
at the Noordvaarder,
Terschelling.
Thisarea
faces militaryactivities(includinga jet shootingrange,
ne•icop[eractivities,[ransportvehicles,•l.c.) arid a
varietyof activitieslinkedwithtouristsand local
inhabitants(includingpeoplewalkingwith or without
hi = recoveryspeed from zone 1 in seconds
h2 = recovery speea trom zone :,' In seconas
For Oystercatchers
in the Deltaarea for a person
walkingwitha speedof 1 m/s (whichequals3.6 km/h)
dogs,angling,cross-country
motorcycles,
etc.).The
frequentpresenceof jets and helicopters
allowsfor a
comparisonof the effectsof the two aircrafttypes.
Figure8 showsthat helicoptersdisturbmorefrequently
the followingfigureswere measured:
r, = 85 m
G = 120 m
h, = 900 seconds
h2= 300 seconds
ForOystercatchers
thiscalculationleadsto a disturbed
area of 20 ha. For bait diggersthe disturbedarea will be
somewhere
in betweenrI andr2. Onceagainthe
disturbed
areawillbe •r 2, whichmeansthata bait
and over longerdistancesthan jets, even though
activitiesfromjets are accompaniedby Shootingand
highsoundlevels.The relativelymildeffectsfrom
militaryjets are also knownfrom otherplaces (Boer
et aL 1970; de Roos 1972) and are shown in Table 5.
From these data it appears that small civilaircraftcause
much more disturbance.Again, there were clear
differencesbetweenspecies.Oystercatcherswere
diggeris surroundedby an area of 3.3 ha withoutany
rather tolerant of aircraft disturbance; Curlews were less
Oystercatchers.
so.OthersiJecies
(likeBar-tailed
Godwits)werein
I
JET
100
IIIIIII1,,,,,,,,,,,
"'"'""
'"'""
IIIIIIII
iii?:!i!iiiiiii::i
iiiiiiiili:•iii:•!
,::::.,.!!!,.111:•:•ili:•:•111iii!
;;;•;;•;'
i===?====?=?=?=?=
llllllllll
??ii:?•i?•
!?i!i!i!!!::
i•.................
I
100
HELICOPTER
•:•:•:•:73•:•-•
::::::-•::•:::
..........
::•:•:•:•:•:•
....................
.•
•::::::::::::::•:
:::::::::•::::::::::
.
.:.:.:.:.:.:.:•.:.,.
:•:•:•:•:•:[:•:•:•:•.
80•• >rain
lying
I80
60
60
40
40
:::::::::::::::::::::
i!
:::::::::::::::::::::
:•:•:•:•:•:?•:•:K:
:.::
:::::::::::::::::::::
:::::;::.-::::.::;.•
::::::::::::.:::::,•
:::::::::::::::::::::
:•:•:•.•:•.•:•.•:•:
:::::::::::::::::::::
2O
.
2O
. ...............
:.........:.
0
0
,.q
8
, 'T
o
o
•,
ø,
o
o oO&,o
&,
õ,
õ,
rea•ions
•:'--•?:..•
no
reactions
I
I
I
I
o
Figure8. Distances
(inm) at whichmilitary
jetsandhelicopters
causeddisturbance
amongroosting
Bar-tailed
Godwitsat the Noordvaarder,
Terschelling.
Datawerecollected
from1980-84andrepresent925 (jets)and 100 (helicopters)
potentialcasesof disturbance
(datafrom
Visser 1986).
11
0046443
Effectsof disturbanceon shorebirds:a summaryof existingknowledgefrom the DutchWadden Sea and Delta area
Table 5. Disturbanceof waders (total numberof hoursof observationtime, numberof flocksobserved,the frequencyat whichflocksflew 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).Disturbancewas consideredto occurwhen morethan50% of the flockflew up. Altitudesof all aircraftwere below
300 m. (data from: Visser 1986).
Disturbancefrom
Hours obs.
n
jet at <1200 m
Flight
%
frequency
Average
duration(s)
July 15-September 15
Oystercatcher
Bar-tailedgodwit
Curlew
320
150
2120
925
110
168
5
18
36
56
50
299
48
16
50
July 16-December 1 and March 1-May 1
Oystercatcher
Bar-tailedgodwit
Curlew
Disturbance
from
320
150
2120
925
110
168
5
18
36
56
50
299
48
16
50
Hours obs.
n
Flight
frequency
%
Average
duration (s)
helicopterat <250 m
July 15-September 15
Oystercatcher
Bar-tailedgodwit
Curlew
320
150
108
58
29
43
27
74
38
73
50
23
12
52
65
July 16-December 1 and March 1-May 1
Oystercatcher
Bar-tailedgodwit
Curlew
Disturbance
from
300
200
84
62
27
44
32
71
22
41
30
12
8
68
38
Hours obs.
n
Flight
frequency
%
Average
duration (s)
helicopterat <1500 m
July 15-September 15
Oystercatcher
Bar-tailedgodwit
Curlew
320
150
15
13
11
11
73
85
50
114
50
7
6
86
83
July 16-December 1 and March 1-May 1
Oystercatcher
Bar-tailedgodwit
Curlew
12
300
200
3
2
30
....
2
2
-
48
168
0046444
Effects of disturbanceon shorebirds:a summary of existing knowledge from the Dutch Wadden Sea and Delta area
between. Heinen (1986) foundthat a small aircraftflying
over roosts of shorebirds on the East Friesian islands
Juist,Wangeroogeand Mellum(GermanWadden Sea)
led to 'disturbedbehaviour'(varyingfrom lookingup and
more frequent callingto takingflightand not returningto
the initialroostingplace and 4 categoriesin between) in
44-53% of the cases, dependingon species, altitude,
locationand aircrafttype. In her study, in which she
unfortunatelydid not specifythe distancesbetween
roostingflocksand planes,jets disturbedmore often (in
84% of all potentiallydisturbingsituations)than small
civilaircraft(56%) and motorgliders(50%), whereas
helicopterswere very disturbingindeed (100%). Brent
Geese were among the most strongly'reactingspecies
respectively)caused more dramatic effects. After 45
minutes only 67% and 87% of the originallypresent
Oystercatcherand Curlew numbershad returnedto the
study plot (Glimmerveen& Went 1984). Small and slow
flyingaircraftare consideredto be among the most
disturbingphenomena in the Wadden Sea. The
behaviour of the plane and its altitude both govern the
reaction of the birds:flying high in a straight line leads to
smaller effects than flying low or with unpredictable
curves (Boer et aL 1970). There is some discussion
between authors on the altitude at which planes cause
no disturbance. According to the Werkgroep
Waddenzee (1975) there is still disturbancewhen an
aircraft passes at 1,000 m. Baptist & Meininger (1984)
always registered disturbance at 150 m and found that
(64-92%), togetherwithCurlew(42-86%) and Redshank
at 300 m there was still disturbance within a radius of
(70%). Shelduck(42%) and Bar-tailedGodwit(38%)
1,000 m. Glimmerveen & Went (1984) found that
reacted less often. Civil aircraftflyingat an altitudeof
individualCurlews only partly reacted by taking flight.
>300 m disturbedin 8%, those flyingat 150-300 m in
On one occasion they observed a Curlew which
66% and those flying<150 m in 70% of the cases.
pressed itself stiff to the ground when a plane came
These figures are comparablewith those found by de
over at 450 m, in another case a Curlew only looked up
Vlas (1986) in the DutchWadden Sea.
Observations
on the tidal flats east of the island of
Texel, with jets from the Vlieland shootingrange
frequentlypassingdirectlyover at altitudesof less than
100 m, showedthat foragingbirdsgenerallydid not
respond.Occasionallyshortreactionswere noted,
varyingfrom lookingup, stoppingforagingfor a few
seconds,to shortflightsof 10-30 seconds.Occasionally
somewhatstrongerreactionswere noted, possiblyfrom
birdswhich had recentlyarrived in the area (like Brent
Geese, shortlyafter their arrivalfrom the Siberian
breedinggrounds)(Smit & Visser 1985).
Experimentson tidal flats southof the islandof
Terschellingshow that 10 minutesafter a single
disturbanceby a small plane at 360 m altitudebird
numbers were back at the same level as prior to the
disturbance.A plane passingtwice (at 450 and 360 m
rather frequently (altitude 360 m). The result of a
passing small airplane (altitude 150 m) is also shown in
Figure 9. All Curlews flew up but had recommenced
feeding after 7 minutes. In contrast to many other
studies the recovery time for Oystercatchers was much
longer:30 minutes. Usingthe model describedabove,
an aircraft passing over at 150 m, creates a disturbed
area of more than 15,000 ha! (Van der Meer 1985).
Ultra Lightaircraftare a new developmentin aviation
technology.Very little research on the effects of Ultra
Lightshas been carried out so far, but our first
impressionis that they are very disturbing,probably
because of the low altitude at which these planes
operate and the noise they produce. Numbers of
roostingand foragingBewick'sSwans Cygnusbewickii
close to an Ultra Lightairstripat Schouwen Duiveland
(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).
70
EFFECTS
E 40
30
20
10
13:00
ß
•I I I
13:15
13:30
13;45
14:00
14:15
14:30
time
Figure9. The numberof foragingCurlewsin the Zandkreek(Delta
area) before and after disturbanceof a small civil aircrafton 9
March. Effectson Oystercatcherswere much longerlasting
(from:Van der Meer 1985).
OF MILITARY
SHOOTING
ACTIVITIES
Early studies showed strong effects of the Vileland and
Terschellingshooting ranges on waders. Flocks of
roostingKnot disappeared almost completelyfrom the
island of Vlieland (Van der Baan et aL 1958). In another
study in the same area, Tanis (1962) found no response
in roostingShelduck,MallardAnas platyrhynchos,Eider
Somateria mollissimaand gulls, but waders all took
flightafter the first shot.All Tringasmovedto more
peaceful roostingsites and most Dunlin and Knot 'left'.
Some of these birds returned later that day but
continued shooting forced others to leave again.
Oystercatchers,Bar-tailedGodwitsand Curlews
returnedto the previouslyused roostingarea. The total
13
0046445
Effectsof disturbanceon shorebirds:a summaryof existingknowledgefrom the DutchWadden Sea and Delta area
sound level (dB)
90
i
95
100
i
5(•0
,
105
i
3•0
110
105
•)
1(•0
i
2•)0
100
i
1(•0
95
i
2•)0
i
3•0
i
5•)0
distance (m)
Rgure 10. Soundlevelsof a completelyloadedoverflyingjet 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
disturbedarea amounted to 69,000 prior to shooting
sessions(Monday-Friday)and to 38,000 duringthe
shooting sessions.
At present, tank shootingat Vileland is combinedwith
bombingand rocket shootingfrom jets for most of the
year. These activitiesyield sound levels of 84-100 dB(A)
at 5 km from the firing range. Nowadays, the shooting
activitiesstill have disturbingeffects, the extent
depending on the time the shooting starts. When the
roostshave already been established,the first shot may
cause considerable numbers of birds to take flight.
Some returnto the originalroostingsite, whereas others
select a site farther away from the shootingrange. If
shootingstarts when the birds are still arrivingfrom the
feeding areas, such a resettlement more or less takes
place automatically(Smit unpubl.). Theoreticallyit is
possiblethat some birds stay away from disturbed
areas altogether,to roostelsewhere on nearby islands.
However, there is no information to confirm such
behaviour. A comparison of the past and present
s•tuationat Vileland shows that roostingbirds still
respondto shootingactivities,despitethe fact that
shootinghas been goingon there for about 40 years.
This may be due to the very high soundlevels,or to the
use of differenttypes of ammunition,leadingto strong
differences
in sound levels. KSsters &von
Raden
(1986) suggestthat continuousreactionsof birdsto jet
shootingand bombingat Sylt may be due to sound
levels exceedingthe pain thresholdat 120 dB(A) (van
Son 1987). He registeredshort lastingsound levels of
105 dB(A) at 100 m from overflyingjets (Figure 10).
Very high sound levels were absent in another study
from the Dutch Wadden Sea. In contrastto experiences
of the previousstudies,Wintermans (1991) could not
find effects of militaryshootingon waders roostingalong
the Groningencoast. In his study area sound levels did
not exceed 55 dB(A).
Shootingactivitiesat Vileland have littlevisibleeffects
on feeding waders. Prey choice, behaviourand intake
rates of Oystercatcherand Curlewswere not different
on days withand withoutshooting(Smit 1986). He
found indicationsthat the diversityof feeding shorebirds
south of Vileland was higher on days without shooting,
suggestingthat some speciesare lesstolerantof high
sound levels and move away from that area.
14
Wintermans(1991), studyingthe effectsfrom a shooting
range in the Marnewaard found no indicationsfor a
lower diversityon the tidal flats. This could be due to the
lower sound levels in that area, which were between 43-
87 dB(A). Apparently,shootingalone has limitedvisible
effects on feedingwaders. However, very strongsound
levels, as in the Vileland and Sylt examples, incidental
heavy shooting(Visser 1986) or sonic booms (Burger
1981a,b) may lead to strong reactions. Earlier
observationson Vileland (van Koersveld in Platteeuw
1986) suggestthat reactionsare stronger if sound is
combined
with visual disturbance.
EFFECTS
OF DISTURBANCE
BEHAVIOUR
OF TERRITORIAL
Disturbance
of non-territorial
ON FOOD
BIRDS
INTAKE
AND
birds often forces all birds
to leave the most heavily disturbed areas.
Consequently,they have to feed in higherdensities
elsewhere. This may affect their food intake. Zwarts
(1980) and Goss-Custard(1980) showed that food
intake of waders
decreased
when bird densities
increased, probablydue to a higher level of interactions
between
birds. More or less territorial waders
react
differently.Zegers (1973) showed that a single person
or group of people may cause the departure of most or
all Redshanks, Oystercatchersand Curlews from their
preferred feeding site, a mussel bed south of
Schiermonnikoog(Figure 11). After such a disturbance
only 9% of the originallypresent number of birdscould
still be found in the study area. The speed at which
birds returnto the preferredfeeding areas varies
between species. When forced out from preferred
feeding areas such birdsoften simplywait untilthe
source of disturbancehas disappeared again. Beli•n &
Van Brummen (1985) studiedfood intake and behaviour
of an individualOystercatcherfrom a hide, in a situation
in which it was possibleto manipulatethe lengthof the
disturbedperiod in a standardizedway. By carefully
forcingout a singlebirdfrom its preferredfeedingsite
they were able to show that the intake-rate may drop to
almost zero, despite the fact that birds continueto feed
in the area to which the bird has gone. These results
are confirmedby comparableexperimentsdescribedby
Hooijmeijer(1991 ). Figure 12 shows that during
disturbancerestingand walking become more dominant
in the behaviour.After disturbancefeeding became
0046446
Effects of disturbanceon shorebirds:a summary of existing knowledgefrom the Dutch Wadden Sea and Delta area
1•.•)
I OYSTERCATCHER
REDSHANK
I
CURLEW
AVY
4O
.....
Figure11. Effectsof heavydisturbanceon the numberof Oystercatchers,Redshanksand Curlewsin studyplotson musselbedscloseto
Schiermonnikoog.Lightdisturbance:dotted line, heavy disturbance:straightline. The mean numberof birds priorto disturbanceis set at
100%, the figuresat the abscissaindicate15 minuteintewalsbeforeand after disturbance(from:Zegers 1973).
much more dominantagain. Figure 13 shows that the
intake rate duringa recoveryperiodwas much higher.
In an undisturbedlow-tideperiodthe food intake of the
same bird is rather similar over the whole period.
HABITUATION
Reijnen& Thissen(1987), studyingthe presenceof
breedingsongbirdsalongmotorways,foundreduced
densitiesfor some species, despite the predictability
and constancyof soundlevelsand trafficactivityon the
motorways.Apparently,some birdsdo not 'get used' to
disturbance.This agreeswiththe shootingrange effects
on roostingwadersat Vileland.The absence of a visible
responseby foragingwaders shows that in other
situations habituationmay take place. This process
("learning")is probablyfacilitatedby a more or less
constantsupplyof identicalstimuli.This may be the
reason for the presenceof Lapwings Vanellusvaneflus,
gullsand StarlingsSturnusvulgarisat airfieldswhere
startingand landingpatternsof planes are very
predictable, both in terms of sound levels and in
movements (c.f. Burger 1981a,b).
In some areas in the Wadden Sea helicoptersor small
civil aircraft may cause panic reaction among thousands of roostingor foraging birds (e.g. Van der Kamp
& Koopman 1989). In areas where planes are common
at least some
habituation
can be noted.
As shown
in
earlier in this paper, there are large differences in the
effects of jets and small civil aircraft between the
studies from Heinen (1986) and Visser (1986). In the
Mokbaai, Texel, a high degree of habituation has
occurred to standard helicopter movements.
Helicopters transportingcrew to offshore drilling
platforms pass over at a frequency of 2-3 per hour at
100-300 m altitude. These activities do not appear to
have strong effects on feeding and roostingwaders and
ducks. The same area is also used for activities
from
Intake (rag AFDW/sec)
Behaviour (% of time spent)
2.5
100
8O
2
6O
1.5
4O
0.5
2O
Before
•
Agression •
During
Resting [•] Walking •
After
Feeding
Figure12. Behaviour(in% of timespent)of an individually
marked
Oystercatcher(LYCB)whenforcedout fromits 'territory'on a
mussel bed in the Mokbaai, Texel. The behaviour was
registeredfora wholelowtide period,the disturbance
was
initiatedartificially
(from:Hooijmeijer1991).
o
Before
During
•
Undisturbed
•
After
Disturbed
Figure 13 Intake rates (mg Ash Free Dry Weight/s)of an individually
markedOystercatcher(LYCB) in an undisturbedlow tide period
(artificiallycut intothree parts)and duringa low-tideperiodin
which the bird was forced out from its 'territory'on a mussel bed
in the Mokbaai, Texel (from: Hooijmeijer 1991).
15
0046447
Effects of disturbanceon shorebirds:a summary of existingknowledgefrom the Dutch Wadden Sea and Delta area
military helicopters,often at low altitudes. Despite a
large degree of habituationto standard helicopter
movements, these activities may force birds to less
disturbed parts of the bay, and some species (like
Cormorants Phalacrocorax carbo or Eiders) often
temporarilyleave the area. Small aircraft passingover
at altitudes of over 300 m have effects comparable to
those of civil helicopters.'Unusual' types of planes,
however, which show up at low frequencies still have
strong effects (Smit unpubl.). This has also been found
to happen on Vlieland. Under normal conditions
roostingbirds do not react severely when jets at the
Vliehors shooting range pass by at high speed.
Relatively slowly flying A10 jets are much less common
in that area and can force thousands
of birds to take
flightfor several minutes.This aircrafttype is able to fly
at a very low speed, can make very short curves and
carries very strong machine guns. Consequently,its
behaviourand sound productionis very differentfrom
'ordinary'jets, which are much more common there
(Smit & Visser 1985). Also in the Mokbaai the birds
could be more easily disturbedafter the appearance of
an unusualaircrafttype. After such an event, an
overflyingGrey Heron Ardea cinerea or Great Blackbacked Gull Larus marinus may even cause panic
10
.c:
I
SUMMER
I SPRING
and
AUTUMN
8
i• ::•:;:;•½:•- .'? .....•
:'::2:::
'
•'
20
•
40
.•
60
:."..:.:.'..',
.:.:.:.:.:.:
.;.;.;.;.;.
......;.....
....
:.:;:.:.:.:
•:•:•:•:::•:•:::•'
:':':':':':' !:i:i:i:i:!
•i!.-"'•(::i!ii;•;:
::::::::::::
E:•:•:•.
.-.....
:::::::.':.':
-
._
-o
.,-.
•::i•
.:2:!::!
:::::::::::
•!•:!:•i.•i•i'
:-:-:-:-:-:
:::::::::::
'"•'•;•;%::!:!:!:'•!
':":':':':'
""'""'"
iii.!.!•-• ::i.ii
i
80
These data suggestthat birdsare able to distinguish
betweentypes of planes.This is not reallysurprising.At
the Banc d'Arguin, Mauritania,waders also appeared to
distinguishbetween predators. Fish-eatingOspreys
Pandion
haliaetus
were observed
to land in between
flocksof waders withoutany disturbance,whereas
potentiallydangerous Marsh Harriers Circus
aeruginosusforcedthousandsof waders to take flight
(Piersma 1982 and Smit unpubl.).Apparently,birdscan
learn throughexperiencethat some potentially
dangerous objects are not dangerousafter all. An
unknownor infrequentlyoccurringobject, however, will
be regarded with caution.
FACILITATION
The data presentedso far suggestthat high levels of
disturbancelead to highertolerance levels. This may be
true in some cases but cannot be considered
:,..-.•military
a rule.
The opposite(referredto as facilitation)may also occur.
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
actual amount of disturbanceresultingfrom these. In
summer the numberof potentiallydisturbingstimulifrom
militaryactivitiesis only slightlylargerthan in springand
autumn.Their effect, however,is much larger.This is
due to a cumulation of effects of disturbance
:i:i:!:i:!:i
:-:-:::::::;
reactionswhereas under normal conditionsthey would
have much smaller effects (Smit unpubl.).
from
differentsources.Leisureactivitiesalone already may
have a large disturbingeffect.Combinedwith military
activitiesthey lead to disturbancelevelsfar exceeding
the effects each activityalone would have had.
Comparableresultswere foundon Sylt, Germany,
where overflyingjets appearedto have largereffects
when prior to these activitieswind surfers had been
present in the area (K•sters &von Raden 1986). These
reactionsare comparableto the Heron and Blackbacked Gull example from the Mokbaai, described
previously.
:-.'-• disturbance
-o 100
:•::•::• •i•!i!i!':i'
._
mainly
recreation
i:i•/ other
sources,
:iiii•d.'.•:;::•:.•:
:::•
.........
o) 120
•
140
,
,
Figure 14. Number of potential6isturbancecases per •our an6 the
time these cases actuallyresulte6 in 6isturbance(se½on6sper
hour) in summer and spring/autumnat the Noordvaarder,
Terschellingfor Oystercatcher,Bar-tailedGodwitand Curlew
(data: Visser 1986).
16
A summaryof all factorsdeterminingthe behaviourof a
single bird after disturbanceis given in Figure 15. Birds
respondas a functionof earlier experiences,reactions
of other birdsnearbyand severalfactorsdetermining
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
importantfactorsinfluencingthe decisionto remainat
the same site. Birdsfeelinghungrywill soonerleave
high-tiderooststhan well-fed birdsthat have just arrived
from the feedingareas (Visser 1986). Protection
through nature protectionmeasures or the presence of
a flockof conspecificsmay also play a role. Additionally,
0046448
Effectsof disturbanceon shorebirds:a summaryof existingknowledgefrom the Dutch Wadden Sea and Delta area
Disturbance
I
I
I
I
;
;
Deds•;
to move
ebewbem
i
I
i
I
Reactionof a singleb•rd
Protection
Food
Figure15. Reactionsof a singlebirdon disturbanceand the factorspotentiallyaffectingits behaviour(modifiedafter K0sters&von Raden 1986)
a bird may postponean immediate reaction in order to
minimizeits energy expenditure(referredto as 'slow
decisionmaking').Activityrhythmsand the presenceof
suitablealternativefeeding or roostingsites may induce
a decision to move elsewhere. Birds may respond
directlyto disturbance,for instancefrom a shot in a
previouslyundisturbedarea. In many cases, however,
birdswill not respond immediatelybut use earlier
experiencesand the behaviourof other birds in the
same area as a filter mechanism.Warning signals
(withoutany furtherdisturbance)may also lead to
behaviouralresponses.
THE USE OF PRESENTLY
FUTURE NEEDS
AVAILABLE
DATA AND
This contributionshows that in the past 20 years there
has been much study of the effects of disturbance.We
could ask ourselves, however, whether these data are
the tools we actually need to understandthe effects on
a bird. In the case of disturbance to non-territorial birds,
we also need to knowthe consequencesof feeding in
higher densities.Anotherquestionis whether
non-territorialbirds are really non-territorial.From
several studieswe knowthat at least some species of
non-territorialbirdsneverthelessdo show a high
site-fidelitywithina winter and between years, at least
at the roostingplaces (Furness & Galbraith 1980,
Symonds et al. 1984). This could mean that these birds
do not distributethemselvesover the tidal flats as freely
as we may think.Consequently,regulardisturbancein
part of their preferredfeedingarea couldaffect their
food intake more than when they would freely distribute
over large areas. To what extent will be extremely
difficultto measure. Birds may also be able to
compensateby feedinglongeror by morefrequentor
longerfeedingat night.Cage experimentsduringwhich
the lengthof the low-tideperiodwas artificially
manipulatedhave shownthat (territorial)Oystercatchers
are able to compensatefor lostfeeding time by
increasingtheir efficiency(Swennen et aL 1989). Of
course we do not know to what extent this is possiblein
the field and at what price. Oystercatcherscould run a
higherriskof bill damage, other speciescouldbe more
vulnerableto predatorsbecause of reducedvigilance.
17
0046449
Effects of disturbanceon shorebirds:a summary of existing knowledgefrom the Dutch Wadden Sea and Delta area
Frequentdisturbancemay force waders to abandon
traditionalhigh-tideroosts.This is demonstratedin the
Dee estuary (Mitchellet aL 1988) where Bar-tailed
Godwits declined 99%, Knots 79% and Dunlins 81%. In
this case the birds continued
to use the traditional
feeding areas. This behaviouralchange involvedhigher
energy costs, because the birds had to fly an extra 40
km each tidal cycle. Heavy disturbancecan also lead to
a total departurefrom feeding sites. This is probably
happeningin Denmark where suitablewetlandsdo not
harbourany Curlews, probablydue to large flight
distancesand warinessas a resultof hunting(Meltofte
1986). The consequencesof such banishmentare
largely unknownand are part of a major applied
ecologicalquestion:'how many birdscan an estuary
support'(c.f. Sutherland& Goss-Custard1991,
Lambeck 1991, Meire 1991).
In this contributionwe have restricted ourselves very
much to visible reactions.What is really happeningto
birds is a much more complicatedproblem. Heart-rates
of Eiders and Oystercatchersappear to increase
considerablywhen incubatingbirdsare approachedby
man or a helicopter, despite the fact that the birds
showed no visible reaction (Gabrielsen 1987, Hf•ppop&
Hagen 1990). The implicationson energyof disturbance
are not yet clear but indicatethat the effects can be
much larger than would appear from the studies
described in this paper.
The previousexamplesof what we stilldo not knowmay
givetoo pessimistica pictureof our knowledgeof
disturbance.Clearly,there are many limitationson what
we can do with the existingdata. Nevertheless,they can
be usefultoolsfor modellingbird reactions.They can also
be well appliedby policymakersor for localprotection
measures(suchas keepingvisitorsto coastalnature
reservesat a sufficientdistancefrom the high-tide
roosts).However,the availabledata are notyet suitable
for answersto key questionslike 'whatare the effectsof
disturbanceon energybudgetsof a bird'or 'whatare the
repercussions
of beingin a bad conditionon survivalor
reproduction
rate'. Suchquestionswill involvemore
studieson energyloss,the costs(or lowerintake)of
feedingat alternativefeedingsitesand the
consequencesof highercostsor lowerintakerateson
body-condition.
ACKNOWLEDGEMENTS
Thanks are due to all studentswho gave so much time
and persistenceto producethe basic data discussedin
this paper. NorbertDankers and Rob Lambeck
commentedcriticallyon the manuscript.Arian Griffioen
producedsome of the figures.
18
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