The Effects of Car Traffic on Breeding Bird Populations in Woodland

0047486
Journalof
Applied Ecology
1995, 32,
187-202
The effectsof car trafficon breedingbird populations
in woodland.III. Reductionofdensityin relationto the
proximity
of main roads
RIEN
REIJNEN*,
JOHAN
THISSEN*t
RUUD
FOPPEN*,
CAJO
TER
BRAAK*t
and
* DLO-Institutefor Forestryand NatureResearch,Departmentof Landscape Ecology, PO Box 23, 6700 AA
Wageningen,The Netherlandsand tIDLO-Agricultural
MathematicsGroup, PO Box 100, 6700 AC
Wageningen,The Netherlands
Summary
1. This studyinvestigatedtheeffectof car traffic
on thebreedingdensityofbirdsin
deciduous and coniferouswoodland, and the importanceof noise and visibilityof
cars as possible factorsaffecting
density.
2. Of the 43 species analysed in both woodland types,26 species (60%) showed
evidence of reduced densityadjacent to roads (based on analysiswithWilcoxon
signed-rankstestand regression).
3. Regressionmodels withnoise load as the only independentvariable gave the
best overall results.Calculated 'effectdistances'(the distancefromthe road up to
where a reduced densitywas present)based on these regressionsvaried between
speciesfrom40- 1500m fora road with10000 carsper day to 70-2800 m fora road
with60000 carsper day (120 kmh-' and 70% amountofwoodland alongtheroad).
For a zone of 250 m fromthe road the reductionof the densityvaried from20 to
98%.
4. Whenvisibility
ofcarswas controlledfor,thenumberof speciesshowingdensity
reductionswas muchhigheron plotswitha highnoise load thanon ones witha low
noise load. When noise conditionswere held constant,however,there was no
differencein bird densitiesbetweenplots withhighand low visibilityof cars.
5. It is argued that noise load is probably the most importantcause of the
reduced densities.Visibilityof cars, directmortalityand pollutionare considered
unimportant.
6. The resultsof thisstudystressthe importanceof consideringthe effectof car
traffic
on the breedingdensityof birdsin planningand constructing
main roads.
road planning
Key-words:noise load, visibilityof cars, pollution,road mortality,
and management.
Journal of Applied Ecology (1995) 32, 187-202
Introduction
187
The effectof car traffic
on birdshas receivedmuch
attention,but most of the studies are focused on
road victims(e.g. Hodson & Snow 1965; Bergmann
1974; Fullhaas et al. 1989; van den Tempel 1993).
However, froma conservationpoint of view, it is
much more interestingto know the consequences
at the populationlevel. This has been poorlyinvestigated. The few available studies (woodland and
* Present address: National ReferenceCentre for
Nature,Forestand Landscape,PO Box 30, 6700 AA
Wageningen,
The Netherlands.
open field)point out thatseveral territorial
species
show a lower breedingdensityin areas adjacent to
roads thanin controlareas furtheraway (e.g. Clark
& Karr 1979; Raty 1979;Ferris1979;van der Zande,
ter Keurs & van der Weijden 1980; Adams & Geis
1981; Reijnen & Thissen 1987; Illner 1992a). In
some species higherdensitieswere found adjacent
to roads (Ferris1979; Clark & Karr 1979; Adams &
Geis 1981), butthereis evidencethatthiswas due to
different
conditionsof the vegetationstructure
close
to the road.
Estimatedeffectdistances(the distancefromthe
road up to wherea lower densitycan be observed)
0047487
188
Main roads and
densityof birdsin
woodland
Fig. 1. Main roads in The Netherlands(bold lines) and locationof paired plots in deciduous woodland (black dots) and
coniferouswoodland (open dots).
in open fieldhabitat extend to more than 1000m
(van der Zande et al. 1980) and in woodland up
to several hundredmetres(Rity 1979; Reijnen &
Thissen 1987). Since there are indicationsthat the
reductionofthedensityin the disturbedzone can be
ratherlarge (Rdty 1979; van der Zande et al. 1980;
Reijnen & Thissen 1987), this mightpoint to an
importanteffecton breedingbird populations.
How car trafficcauses densitiesto be reduced is
unknown.It has been generallyassumed that the
increaseof the mortalitydue to road traffic
will be
too small to cause a significantdecrease of the
densityor the population'ssize (e.g. van der Zande
etal. 1980; Leedy & Adams 1982; Ellenberg,Muller
& Stottele1981). This impliesthat possible causes
are probably related to emission of matter and
energy by road traffic,such as pollution, visual
stimuliand noise (van derZande etal. 1980; Reijnen
& Thissen 1987). For woodland it is assumed that
noise load, in particular,mightplay an important
role (Reijnen & Thissen 1987; Reijnen & Foppen
1994).
The objectiveofthisstudywas to determinewhich
woodland species showed lower breedingdensities
next to roads, over whichdistancessuch an effect
occurred and to which degree the densitieswere
lower. Furthermore,
the assumptionwas testedthat
noise is the mostcriticalcause of the effect.
Methods
STUDY
AREAS
AND
SITE
SELECTION
The studywas carriedout in areas withdeciduous
woodlandand coniferouswoodlandcrossedby main
roads, scatteredover the Netherlands.The traffic
densityof roads in areas withdeciduous woodland
was 8000-61000 (mean 30334) cars per day and
in areas with coniferouswoodland 29000-69000
(mean 45 319) cars per day. In deciduouswoodland
38 pairedplotsand in coniferouswoodland 17 paired
plots were selected (Fig. 1). One plot of a pair was
situatedadjacent to the road (road plot) and one
plot at a distanceof on average400 m (controlplot).
Withineach pair of plots habitat differenceswere
controlledfor (see section of methods 'Control of
0047488
189
R. Reijnen et al.
Table 1. Habitat characteristics
of road and controlplotsin deciduousand coniferouswoodland (mean values). Statistical
significance
is based on Wilcoxonsigned-rankstest
Deciduous woodland (n = 38)
Coniferouswoodland (n = 17)
Estimate
Road
Control
Sign.
Road
Control
Sign.
Cover of vegetationlayers(%)
<25 cm
25-50cm
50-100 cm
1-2 m
2-5 m
5-10m
10-20 m
>20 m
59
31
30
19
42
55
49
14
55
35
28
21
41
51
53
20
NS
NS
NS
NS
NS
NS
NS
72
63
33
3
25
56
37
75
65
33
8
27
49
33
NS
NS
NS
NS
NS
NS
NS
Treecharacteristics
Circumference
(cm)
Distance between(m)
Height (m)
87
3-1
16-4
86
2-8
17-0
NS
NS
NS
63
18-8
12-5
63
20-4
12-6
NS
NS
NS
Size and edge
Size (ha)
Borderlinewithopen area (%)
4-04
53
4-24
53
NS
NS
7-20
55
8-20
46
NS
NS
NS=P>G1G:
** P<OO1.
other factors that influence breeding density').
Because available studiesin woodland indicatethat
effectdistances can be rather large (Raty 1979;
Reijnen& Thissen1987), some or partofthecontrol
plots mightstill be influencedby car traffic.For
practicalreasonsthesecontrolplotswerenotlocated
fartheraway fromthe road. To obtainreliablebird
densitiesa minimumplot area of 1-5ha was taken
for deciduous woodland and 5 ha for coniferous
woodland(based on Hustingsetal. 1985,see Table 1
formean size of plots). Investigationswere carried
out in 1987 (deciduous woodland) and 1988 (coniferouswoodland).
Oak (Quercus robur L.) was the dominanttree
species in 30 of the 38 paired plots of deciduous
woodland,poplar(Populus sp.) in five,beech (Fagus
sylvaticaL.) in threeand alder (Alnus glutinosaL.)
in two. The developmentand floristiccomposition
of shruband herbaceouslayersalso variedbetween
the paired plots. In all coniferouswoodland plots,
pine (mostlyPinus sylvestris
L.) was the dominant
tree species and a shrub layer was almost absent.
The herbaceous layer was characterizedby species
of poor soil conditions, of which Deschampsia
flexuosa(L.) Trin. was mostabundant.
BREEDING
**
DENSITY
Breeding bird densities were measured as the
numberofterritories
perarea unitusingthemapping
methodaccordingto Hustingset al. (1985). In the
mapping method the number of territoriesof a
species in an area is derived from all individual
ofterritorial
behaviourmade on several
registrations
visitsin the field. However, because of the rather
small size of the plots, for some species that have
large territories,the number of registrationswas
used to calculate densities.To calculate the total
densityof all species combined, the densities of
species which were based on registrationswere
dividedby two.
Everyplot of a pair was visited12 timesfromthe
beginningofMarchtilltheend ofJune.The distance
of 50 m at which an observershould approach all
partsof the plot (Hustingset al. 1985) was reduced
to 25 m in order to minimizethe maskingeffectof
trafficnoise on bird song. For territorieswhich
overlappedthe boundariesof a plot, only the proportionof theterritory
withinthe plot (based on the
percentage of registrations)was included in the
calculationof density.
TRAFFIC
LOAD
To measurethe traffic
load of plots or partof plots
(see analysis)we used parametersforthe noise load
and forthe visibilityof cars. Other possible causes,
such as pollutionand road mortality,could not be
measuredquantitatively.
The noise load was estimatedusing an existing
mathematicalmodel,whichexpressesthenoise level
in dB(A) as the 24-hour value of the equivalent
noise level (Moerkerken & Middendorp 1981).
Calculated noise levels refer to points and the
most relevantparametersin our studywere: traffic
density,speed and type of cars, shortestdistance
fromthe receiverpoint to the road and heightof
road above groundlevel. Other parameterssuch as
road surfaceand groundsurfacedid notvaryand for
the receiverheight,1 m was taken (other heights
0047489
190
Main roads and
densityof birdsin
woodland
50 40 -
30
0
2
a
200
l
l
l|3
400
600
800
Distancefromthe road Cm)
1000
Fig. 2. Estimationof thenoise load ofcar traffic
along a highwaywith50 000 cars per day and a speed of cars of 120kmhbased on Moerkerken& Middendorp(1981) and Huisman (1990) (see Methods forfurtherexplanation). 1 = open field;
2 = proportionof woodland along the road 05; 3 = proportionof woodland along the road 1 0.
were investigated,
but the calculatedvalues were all
stronglycorrelatedwiththe 1 m data, r = >0-90).
Because the model does not take into account
the noise reducingeffectof woodland, which can
be very important(Huisman 1990; Huisman &
Attenborough1991), thecalculatednoiselevelswere
adapted by usinga 'woodland effectterm'according
to Huisman (1990). This 'woodland effectterm'is a
functionof woodland type, the shortestdistance
betweenroad and receiverpoint,thereceiverheight
and the proportionof woodland between receiver
pointand theroad (PW). It is specificforroad traffic
noise and ranges from 0 to 12 dB(A) for most
woodland types(deciduous and pine) and from0 to
16 dB(A) fordense coniferouswoodland.The maximum reductionis reached at 200 m fromthe road
(see Fig. 2). Because changesin noise transmission
between the road and the receiverpoint are most
importantin a sightangle fromthereceiverpointto
the road (Moerkerken& Middendorp1981), assessing of PW was restrictedto thissightangle (set at
143 degrees).
To obtaina mean noise level fora plotor partof a
plot, the value of one representativepoint was
taken. This pointwas situatedin the middleof the
line parallelto the road at a distanceof 10(log DL-Iog
DS)/2 [log distanceis chosen,because noise levels in
dB(A) have a logarithmic
scale], in whichDL is the
largestdistanceand DS the smallestdistanceof the
plot fromthe road. Trafficdata were available in
reportsof the Ministryof Traffic,Public Worksand
Water Management(trafficdensity,speed of cars,
type of cars). The distancefromthe representative
pointto the road and PW were derivedfromcartographicmaps. The heightof the road above ground
level and the woodland typewere estimatedin the
field.
of cars of plotsor partof
To measurethevisibility
plots,a simplemethodwas developed. The distance
fromthe road where the trafficcould not be seen
anymore,was thebasic estimate.Thiswas measured
in early spring(leaves absent) and in late spring
(leaves present). Assuming that the visibilityof
cars decreases with the distance to the road, the
relativevalue for a plot (between 0 = no visibility
and 1 = visibilityin the verge of the road) was
calculated as the proportionof the plot influenced
by visibilitymultiplicatedby the mean level of the
visibilityin the influencedpart of the plot (see
Fig. 3). Differencesin traffic
densityand typeofcars
were not considered. It was assumed that when
trafficis more or less continuous(>10000-20000
carsper day), thevisualexperienceoftraffic
remains
constant.
CONTROL
INFLUENCE
OF
OTHER
BREEDING
FACTORS
THAT
DENSITY
the effectof road traffic,
To identify
we chose plots
that were as similaras possible in other variables
that influencebreeding density. Because of the
paired design, the similarity
was only requiredfor
the plots of one pair. Variables taken into account
werearea ofplots,vegetationstructure,
surrounding
landscape and management. Only plots along
roads used by trafficfor more than 5 years were
considered.
To assess the similarityof vegetationstructure
betweenthe road and controlplots of each pair the
followingmeasurementsweremade. The vegetation
structurewas measured along randomly chosen
transectswitha lengthof 100m. To obtain values
forthe vegetationcover, every5 m the presenceof
vegetationwas scored at eightheightclasses. Also
the distanceto the nearesttree fromthese points,
and the heightand circumference
of thistree were
established.The proportionof the boundaryof the
plot borderingopen fieldwas used to characterize
the edge.
In deciduouswoodland onlyone characteristic
of
the vegetation structure,the cover of the layer
>20 m, showeda significant
difference
betweenroad
and controlplots(Table 1). However,it is notlikely
that this caused significant
differencesin breeding
0047490
191
Studyplot
R. Reijnenet al.
0-8
o
06
V -----*MV
DL-DS*M
MV
0-2
0-
DS
DM
DN DL
Distancefromthe road
Fig. 3. Estimationof a relativevalue forthe visibilityof cars (V) in woodland (see also methods). DS= mean smallest
distanceof plot to the road; DN = mean largestdistanceof plot to the road; DL = mean distancefromthe road where
of
traffic
cannotbe seen anymore; DM = mean distancefromtheroad of thepartof theplot thatis influencedbyvisibility
cars; MV = mean value forthe visibilityin the influencedpartof the plot (is value at DM). Because DS was alwaysvery
in thevisibility
ofcarsin thezone betweentheroadsideand DS were neglectedwithrespectto
close to theroad, differences
the visibilityparameter.
densitiesbetweenroad and controlplots. The cover
of the layer >20 m was of minor importancefor
the structureof the deciduous woodlands, and the
differencebetween road and controlplots was not
in the heightof the trees.
supportedby a difference
There were no differencesof vegetationstructure
betweenroad and controlplotsin coniferouswoodland plots.
Since species were not always presentin all the
pairedplotsof a series,additionaltestswere carried
out on subsets of paired plots. The results were
similaras compared with the whole dataset. Significantdifferences
between road and controlplots
wereonlypresentindeciduouswoodlandandrestricted
to the cover of the layer>20 m.
ANALYSIS
In accordancewiththe studydesign(paired plots),
the data were analysed at firstwith the Wilcoxon
test.A non-parametric
signed-ranks
testwas chosen
forrobustness.The basic analysiswas a comparison
of densitiesbetweenroad and controlplots. Moreover, to detect possible effectson the densityat a
veryshortdistancefromthe road, large road plots
(at least twice the required minimumsize) were
dividedintotwopartsthatwerenearto and farfrom
the road. This was only done for road plots that
had a very homogeneous vegetation structure.
Because we expected lower densitiesclose to the
in densitywere tested one-sided.
road, differences
Deciduous woodlandand coniferouswoodlandwere
treated as separate series of paired plots and an
analysisforindividualspecies was carriedout if the
numberof territorieswas >10 or the number of
registrations
>20.
In order to obtain informationabout the size
of the effecton the densityand to determinethe
relative importance of noise load and visibility
of cars, a supplementaryanalysiswas carried out
usinga regressionmodelwithnoiseload and visibility
of cars as independentvariables. Since both measurementsof visibilityof cars were stronglycorrelated with each other (r = 0-95), only visibility
of cars in early spring(withoutleaves) was used
in the regressionanalysis.Althoughthe correlation
between noise load and visibilityof cars was also
high (r = 0 63), usingboth explanatoryvariablesin
the regressionmodels was considered acceptable.
For a more precise measurementof the effect
distance,large homogeneousplotswere splitin two
or sometimesmoresubplots(dependingon thesize).
or
As dependentvariable the numberof territories
was taken and to model densities,the
registrations
logarithmof the area of the (sub) plotswas included
in the model as an offset.Because counts tend
to follow a Poisson-likedistributionin which the
varianceis proportionalto themean, thenumbersof
territories
weremodelledby Poisson regressionwith
correctionforpossibleoverdispersion(Jongman,ter
Braak & van Tongeren 1987; see F-test below).
Includinga factorforpairs in the model accounted
betweenpairsof plots(whichmaybe
fordifferences
due to differencesin vegetationstructure,etc.). If
load follows
of birdsto traffic
the level of sensitivity
a Gaussian distribution,the relationshipbetween
numbers and trafficload is sigmoidal. However,
theoreticallythe most ideal combinationof fitting
a sigmoidalrelationwitha Poisson regressionand
a factor for pairs was very difficultto compute.
Instead, a thresholdmodel was used: below the
thresholdthereis no change in bird density;above
the thresholdan equal percentageof the number
of territoriesdisappears per unit of noise load.
The thresholdvalue and the decrease factorwere
estimated by log linear Poisson regressionwith
0047491
192
Main roads and
densityof birdsin
woodland
simultaneousconfidenceregion for the threshold
and the decrease factor.
To get a furtherindicationof the importanceof
noise load and visibilityof cars, subsets of paired
plots were created in whichthe road plots differed
A
\
a)
in one explanatoryvariable but not in the other
(see Table 2). This was onlypossible fordeciduous
woodland. If noise load is the mostimportantcause
B
of reduced densities, one would expect that the
effect
on the densityin the subsetwitha highnoise
T
R
load is much largerthan in the subset with a low
Noise load in dB(A)
noise load. Between the subsets that differedin
of thedecreasefactorof the density visibilityof cars, differencesin the effecton the
Fig. 4. Estimation
usinga threshold densityshouldthenbe muchsmalleror absent. The
based on loglinearPoissonregression
value in dB (A),
model (see methods).T = threshold
analysiswas done withthe Wilcoxon signed-ranks
R = noiseload in dB (A) in thevergeof theroad. The
test
and regression.
= areaofA/(areaofA + B).
decreasefactor
Because of thesmallsize of the data set, besides a
level of
level of P < 0*05,a significance
a numericalsearch procedure for the best-fitting significance
P < 0.10 was considered. We used a comparisonthresholdvalue. For the estimationof the decrease
of the effectof the
factorsee Fig. 4. The significance
wise significancelevel, to avoid loss of power and
1989)
noiselevelwas testedwiththeF-test(Jorgensen
therebyaccept thatwe are likelyto marktoo many
withtwo degreesof freedom(one forthe threshold species as showinga significance
change.
value and one forthe slope) in the numeratorand,
therefore,had to be two-sided. However, in the
Results
absence of a threshold,the test was one-sided.
Over-dispersionwas indicatedif the mean deviance
DENSITY
IN DECIDUOUS
WOODLAND
was largerthan 1. In such cases the mean deviance
Of the 50 species presentin the 38 paired plots, 41
was used as denominatorof the F-statistic,other(see Table 3) weresufficiently
wise the value 1 was used. An approximate90%
numerousforanalysis.
By usingthe Wilcoxonsigned-ranks
test,the density
confidenceinterval for the thresholdvalue was
of 12 of the41 species was reducedclose to the road
constructedas a by productof our numericalsearch
thresholdvalue. The
(at P < O10, six species at P O< 05, Table 3). For
procedurefor the best-fitting
three of these 12 species there was only an effect
intervalcontainsthe thresholdvalues whichyielded
on the densitywhen comparingparts of the road
deviancevalues thatexceeded theminimumresidual
deviance by no more than sF, wheres is the mean
plots that were near to and far fromthe road (at
P < 010, one species at p < 0-05, Table 3). Despite
deviance(set to 1 ifs was smallerthan1) at thebestthe negativeeffecton severalspecies, the densityof
fittingthresholdvalue and F is the 90% point of
all species combinedwas not significantly
an F-distributionwith one degree of freedom in
reduced
the numeratorand the degrees of freedomof the
close to the road (Table 3).
With regressionthe model with only noise load
residual in the denominator.Because some plots
reductionof the densityfor
weresplitintosubplots,someindependencebetween
resultedin a significant
data was introduced.As a result,the confidence seven of the 12 species (at P < 0. 10, fivespecies at
P < 0-05, Table 4) whichshowed an effectwiththe
intervalfor the thresholdvalue may be somewhat
paired test and also for four additionalspecies (at
too narrow. We did not succeed in obtaining a
Table 2. Trafficload of road plotsforsubsetsof paired plotsin deciduouswoodland (mean values). Statisticalsignificance
is based on Wilcoxonsignedrankstest
Subset of plots
n
Cars per day
Noise load
Visibilityof cars
Noise load
High
Low
19
19
33179
27488
58
48
0-46
0 35
NS
Visibilityof cars
High
Low
19
19
30562
30106
53
52
NS
0-61
0-21
NS=P>0
10; *** P<O 001.
0047492
193
R. Reijnen et al.
Table 3. Density of breeding birds in road (R) and control (C) plots of deciduous woodland. Species for which a
comparisonof densitieswithinroad plotswas carriedout are markedwithparenthesesand resultsare onlyshownifthey
were significant'
NT
R
C
Sign.
Species
NP
Anas platyrhynchos
10
16
0-10
0-05
NS
Buteo buteo(
18
64
0-16
0-21
NS
Phasanius colchicus
13
60
0-13
0-22
*
Cuculus canorus
13
35
0-04
0-16
*
Scolopaxrusticoka
11
42
0.11
0-09
NS
Columba oenas (
Columbapalumbus 0
14
30
37
186
0-08
0-59
0-12
0-54
NS
NS
Picus viridus
Dendrocopusminor
Dendrocopusmajor 0
7
10
28
27
11
85
0-05
0-02
0-25
0-12
0-05
0-25
NS
NS'
8
33
0-09
0-08
NS
0
troglodytus
Troglodytus
Prunellamodularis0
34
16
289
41
0-91
0-16
0-96
0-14
NS
NS
Erithacusrubecula0
Phoenicurusphoenicurus
Turdusmerula(
Turdusphilomelos()
34
10
38
24
357
17
350
112
1-09
0-05
1-15
0-29
1.00
0-06
1-04
0-32
NS
NS
NS
NS
Acrocephaluspalustris
Hippolais icterina
Sylviaborn (
Sylviaatricapilla0
Phylloscopussibilatrix
Phylloscopuscollybita(
Phylloscopustrochilus(
Regulusregulus
5
7
28
35
10
35
26
7
11
16
175
172
22
222
264
23
0-04
0-11
0-70
0-56
0-04
0-87
0-70
0-05
0-15
0-15
0-82
0-62
0-10
0-90
0-98
0-07
t
NS
NS
NS
NS
Muscicapa striata0
Ficedula hypoleuca0
26
13
85
27
0-27
0-09
0-24
0-06
NS
NS
Aegithaloscaudata 0
Parus montanus
Parus palustris0
Parus caeruleus0
Parus major (
25
18
23
33
37
39
31
65
252
504
0-17
0-09
0-17
0-79
1-63
0-13
0-19
0-20
0-75
1-45
NS
NS
NS
NS
NS
Sittaeuropaea 0
0
Certhiabrachydactyla
18
30
65
142
0-22
0-44
0-17
0-44
NS
NS'
Oriolusoriolus
10
33
0-04
0-20
t
Ganrulusglandarius(
Pica pica (
Corvuscorone0
29
17
29
146
66
185
0-50
0-20
0-83
0-40
0-23
0-71
NS
NS'
NS
Sturnusvulgaris0
20
121
0-41
0-45
NS
Coccothraustes
coccothraustes
Fringillacoelebs (
10
32
18
238
0-03
0-72
0-06
0-85
NS
All species combined
38
4493
14 76
15-53
NS
Anthustrivialis
*
*
**
t
t
lower densityin partof road plots near to the road (N) thanin partof road plots furtheraway fromthe
Significantly
P <0-10 (N = 0 50, F = 0.66), Pica pica
road (F). DendrocopusmajorP < 0-01 (N = 0-21,F = 0.41), Certhiabrachydactyla
P<010 (N =0-33, F =0-56).
For species in bold the data are
or registrations.
NP = numberof paired plotsinvolved;NT = totalnumberof territories
test.NS = P > 0-10; t P < 0-10;
is based on Wilcoxonsigned-ranks
Statisticalsignificance
based on numberof registrations.
* P<0-05;
** P<0-01.
0047493
194
Main roads and
densityof birdsin
woodland
ofcars (V) in deciduouswoodlandusinga
Table 4. Significant
regressionsofbreedingdensityon noise load (N) or visibility
thresholdmodel (see Methods). For species in bold the data are based on numberof registrations
Species
N/V
df
Buteo buteo
N
1/45
2-88
Phasanius colchicus
N
V
2/28
1/29
5 15
2-92
Scolopax rusticola
N
2/28
Cuculus canorus
N
V
Dendrocopusminor
F
T(CFL)
C
Sign.
?23 (?-50)
-0 050
*
53 (45-58)
0 (?-7)
-0 253
-0 009
*
*
2-53
56 (53-62)
-2-191
t
2/31
1/32
8-28
18-60
49 (40-52)
0 (?-24)
-1 143
-0-064
**
N
V
2/23
2/23
2-84
3-81
50 (43-56)
46 (33-59)
-3 207
-1 191
t
Phylloscopussibilatrix
Phylloscopustrochilus
N
N
V
1/24
2/61
2/61
5 41
4-30
4-30
?26 (?-62)
39 (?-60)
62 (44-81)
-0 052
-0 034
-0077
*
*
*
Oriolusoriolus
N
V
1/27
1/27
14-0
13-0
?23 (?-39)
0 (?-57)
-0 121
-0 038
Pica pica
N
2/43
2 95
Coccothraustes
coccothraustes
N
1/30
3 30
Fringillacoelebs
N
1/66
All species combined
N
1/81
*
-0-086
t
?26 (?-56)
-0-067
*
2 51
?23 (?-60)
-0-013
t
1 98
?23 (?-60)
-0 004
t
46 (40-56)
df, degrees of freedom;F, change in F-statistic(see methods) afteradding noise load or visibilityof cars; T (CFL),
thresholdvalue in dB (A) and 90% confidencelimits(? means thata lower limitcould not be calculated); C, regression
coefficient.
NS=P>O-10. t P<O-10. * P<005: ** P<O0O1; *** P<O0OO1.
P<O*10, two species at P<O05, Table 4). Moreover,therewas an effecton thedensityof all species
combined(at P < 0-10,Table 4). Addingvisibility
of
cars to the model and assuminginteractionbetween
noise load and visibilityof cars, did not improve
thesemodels. If noise load was replacedby visibility
of cars, onlyfivespecies withsignificant
regressions
remained (at P < 0-05, Table 4) and the effecton
the densityof all species combined disappeared.
So the model with only noise load gives the best
overallresults.For thefivespecies showingan effect
withnoise load and visibilityof cars, the fitof the
regressionmodels was similar.This means that,in
these species, the effectcan eitherbe explainedby
noise load or visibilityof cars.
Effect distances (threshold value in m) were
Table 5. Decrease factor(see Methods)of thebreedingdensityforspecies and all speciescombinedin deciduouswoodland
in thezone betweentheroad and the thresholdvalue and in a fixedzone of 250 m fromtheroad. Based on regressionswith
noise load as explanatoryvariable. For species in bold the data are based on numberof registrations
Decrease factorup to 250m
Species
Decrease factor
up to threshold
10000
cars per day
60000
carsper day
Buteo buteo
Phasanius colchicus
Scolopax rusticola
Cuculus canorus
Dendrocopusminor
Phylloscopussibilatrix
Phylloscopustrochilus
Oriolusorioles
Pica pica
Coccothraustes
coccothraustes
Fringillacoelebs
0 62
0 77
0 97
0 96
0 98
0 61
0-38
0 82
0 58
0 68
025
0 76
0 14
0 14
0 25
0 24
0 73
0 28
0.95
0 20
0 81
0 32
0 81
0 28
0 27
0 52
0 43
0 79
0 45
0 98
0 49
0-86
0 36
All species combined
0-09
0-11
0 13
0047494
S+
195
R. Reijnen et al.
S-
Sco/opaxrust/co/C
Phosonius co/chicus
Dendrocopus minor
Cuculus canorus
Pica pica
l
Phyl/. troch//us
[_
_
_
_
C. coccothroustes |
Phy//.sibi/atrix
Fringil/a coelebs
_
_
_
_i/
_
)
_
_
_
_
_
Buteo buteo
Oriolusori/ous
All species combined
10 000
1000
100
10
Distance fromthe road (i)
Fig. 5. Effectdistances(m) withconfidencelimitsin deciduouswoodland for10000 cars per day (------) and 60000 cars
). The car speed was fixedat 120kmh-1 and theamountofwoodlandalongtheroad at 70%. Effectdistances
per day (---*--were derivedfromthe regressionwithnoise load as explanatoryvariable and a conversionof noise load to distancein m
(see Methods). S = mean rangeof visibilityof cars withleaves (+) or withoutleaves (-).
intensities
estimatedfornoise load only.Two traffic
were chosen thatcoveredthe whole rangeof intensitiespresentin the data set; 10000 and 60000 cars
per day. Othervariableswhichinfluencedthe effect
distancewere kept constant,such as speed of cars
(120kmh-1) and amount of woodland along the
road (70% = mean value for the plots). For 10000
cars per day the effectdistancesof species varied
fromabout 40 to 1500m, and for60 000 cars per day
from70 to 2800m (Fig. 5). However,theconfidence
limitswere verylarge,especiallyforthe large effect
distances.The decrease factorof the densityin the
zone betweenthe road and the thresholdvalue was
in most species veryhigh (median 0-69) (Table 5).
Calculated fora fixedzone of 250m adjacent to the
(10 000
road, the decrease factorwas stillsignificant
cars per day, median 0-33; 60 000 cars per day,
median 0.49) (Table 5).
regressionwas carriedout withnoise load only. Of
thefivespecies whichshowedan effect(at P < 0-10,
two species at P < 0-05, Table 7), two also had an
effectwiththe paired test. Again therewas also a
clear effecton the densityof all species combined
(at P<0-5, Table 7).
Effectdistances and decrease factorswere calculated using the same trafficdata and amount of
woodlandalongtheroad as fordeciduouswoodland.
The effectdistances of species varied from50 to
790 m for10000 carsper day and from100 to 1750m
for60 000 cars per day (Fig. 6). The decrease factor
of the densityin the zone between the road and
the thresholdvalue was in most species veryhigh
(median 0.61) and remainedimportantif calculated
fora fixedzone of 250 m (10 000 carsday-': median
0-23; 60000 carsday-': median 0-60) (Table 8).
COMPARISON
DENSITY
IN
CONIFEROUS
WOODLAND
Of the28 speciesoccurringin the 17 pairedplots,18
numerousto
species (see Table 6) were sufficiently
allow an analysis. By using the Wilcoxon signedrankstestonlyfourspeciesshoweda reduceddensity
close to the road (at P < 0-10,3 species at P < 0-05,
Table 6), but therewas a clear effecton the density
of all species combined(at P < 0-05, Table 6).
Because therewas a strongcorrelationbetween
noise load and visibilityof cars (r = >0-90), the
PAIRED
OR
PLOTS
VISIBILITY
DENSITIES
OF
DIFFERING
OF
IN
IN
SUBSETS
NOISE
OF
LOAD
CARS
In the two subsets of paired plots that differedin
noise load, but not in visibilityof cars, 28 species
could be compared.WiththeWilcoxonsigned-ranks
testninespecies showeda lowerdensityclose to the
road in thesubsetwitha highnoise load (at P < 0-10,
four at P < 0-05, Table 9) and only two in the
subsetwitha low noise load (at P < 0-05, Table 9).
Moreover, in the subset with a low noise load all
0047495
196
Main roads and
densityof birdsin
woodland
Table6. Densityofbreeding
birdsinroad(R) andcontrol
(C) plotsof coniferous
woodland.Speciesforwhicha
comparison
ofdensities
within
roadplotswascarriedout
aremarkedwithparentheses
andresults
areonlyshownif
theyweresignificant'
Species
NP NT R
Columba palumbus ()
11
48
Anthustrivialis
(
10
30 0-13 0-23 NS
()
Troglodytus
troglodytes
14
37
0 16 0-22 NS
Erithacusrubecula()
Turdusmerula
16
13
55
25
0-32 0 26 NS
0-09 0-15 NS
Sylviaatricapilla
Phylloscopuscollybita()
Phylloscopustrochilus
Regulusregulus(
5
8
8
14
11
23
24
41
0.04
0-10
0 11
0 22
C
Sign.
0-32 0-63 *
of cars (at P<0-10, two at P<0-05, Table 11). In
the regressionanalysis,withvisibilityof cars as the
explanatoryvariable, the number of species that
showed a negativeeffecton the densitywas two in
plotswithhighvisibility(at P < 0-10, one species at
P < 0-05,Table 12) and one in low visibility
plots(at
P < 0-10, Table 12). In none of these cases was an
effecton the densityof all species combinedfound.
Discussion
REDUCTION
OF
DENSITIES
Of the 43 species that could be analysed in the
paired plotsof deciduousand coniferouswoodland,
26 species (60%) showed evidence of a reduced
*
density(Wilcoxonsigned-ranks
testand regression).
*l
Althoughone can argue that thereis some uncertaintyforspecies thathad a weaklysignificant
effect
Parus montanus
12
19 0-06 0 11 NS
(only P<0-10), there still remain 18 species with
()
Paruscristatus
16 48 0 29 0-33 NS
Parus ater(
15
29 0-17 0-29 t
a significanteffectat P < 0-05 (= 42%). This is
Parus caeruleus
8
12 0 05 0 09 NS
considerablymore than the few available studies
Parus major ()
17
49 0-26 0-27 NS
indicate(Ferris 1979; Adams & Geis 1981; Reijnen
& Thissen 1987). Nine of the 10 species which
Garrulusglandarius
13
39 0-12 0-32 NS
showed an effectin the studyof Reijnen & Thissen
Pica pica (
10
59 0-31 0-26 NS
Corvuscorone(
11 108 0 42 0-38 NS
(1987), also did so in this study. So, the results
indicate that along busy roads, car trafficcausing
Fringillacoelebs (
13
77 0 45 0-52 NS
reduceddensitiesofbreedingbirdsin woodland,is a
generalphenomenon.This is reinforcedby the fact
0 17 256 2-35 3.39 *
All speciescombined
thatspeciesofverydifferent
taxonomicgroupswere
l Alsosignificant
lowerdensity
inpartofroadplotsnear
affected.
to theroad (N) thanin partof roadplotsfurther
away
Of the few available studies that give rough
fromthe road (F). Regulus regulusP< 0.01 (N =0-04,
estimatesof the effectdistanceand the size of the
F= 0 22).
effecton the density,only the studyof Reijnen &
NP = number of paired plots involved, NT= total
Thissen (1987) allows an appropriatecomparison
number
of territories
or registrations.
For speciesin bold
thedata are basedon number
of registrations.
Statistical with our results of deciduous woodland. They
significance
is based on Wilcoxonsigned-ranks
test.
investigatedsimilar species along a highwaywith
I
NS=P>0*10;
P<0 10; * P<0-05, ** P<0 01.
45 000 cars/dayand estimateda mean effectdistance
of about 500 m and a decrease factorof the density
species had an effectat a veryshortdistance(comfor all species combined of 0-15. Although the
parisonof densitieswithinroad plots), whilein the
decreasefactorforall speciescombinedis somewhat
subset with a high noise load this was only the
higherthanin thisstudy,the effectdistanceis much
case in three of the nine species (Table 9). With
lower. These differences
could be due to the rough
regression, using noise load as the explanatory and descriptivemethodused by Reijnen & Thissen
variable, negativeeffectson the densitywere only
(1987) (comparisonof densitiesin plots at different
foundin the subsetof paired plotswitha highnoise
distancesfromthe road). However, the difference
load (seven species at P < 0-05, Table 10). This
in the effectdistanceis not necessarilysignificant,
supports the results obtained with the Wilcoxon
sincethevalue of Reijnen & Thissen(1987) remains
test.Furthermore,
forthedensityof all
signed-ranks
withinour confidencelimits(see Fig. 5). Moreover,
species combined,therewas only a negativeeffect our largesteffectdistancesmightbe overestimated,
in the subset of paired plots witha highnoise load
because they were obtained from regressionsin
(at P < 0-05, Tables 9 and 10).
whicha threshold
was absent.The absenceof a thresFor the twosubsetsof pairedplotsthatdifferedin
hold could be due to the relativelysmall numbers
visibilityof cars but not in noise load, no clear
of observationsat a greaterdistancefromthe road.
patternarose. Here, 26 species could be compared.
With the Wilcoxon signed-rankstest, only three
POSSiBLE
CAUSES
OF REDUCED
DENSITIES
species showed a lowered densityclose to the road
witha highvisibility
of cars (at P < 0-10,one species
The fact that significantrelationshipswere found
at P < 0-05,Table 11) and threewitha low visibility betweentraffic
relatedfactorsand breedingdensity
0 05
0.11
0 25
0 36
NS
NS
0047496
197
R. Reijnenet al.
S
T: trog/odytus
Anthus frivia/is
Regulus regulus
Porus coeruleus
Phyll. trochl/us
All species combined
I
I
I
I I'I I
1
10
I I
'
I
I
I
1
11
100
I
I
I
I I I I
1000
Distance
from the road
10 000
(in)
Fig. 6. Effectdistances(m) withconfidencelimitsin coniferouswoodland for10000 cars per day (---i---) and 60000 cars
per day (---*---).The car speed was fixedat 120kmh-1 and theamountofwoodlandalongtheroad at 70%. Effectdistances
were derivedfromthe regressionwithnoise load as explanatoryvariable and a conversionof noise load to distancein m
(see Methods). S = mean rangeof visibilityof cars.
suggestthat car trafficand not the presence of a
road per se, is the main cause of the observed effects
(see also van der Zande et al. 1980; Illner 1992a).
This is supportedby the relativelysmall numbersof
reduceddensitiesin the subsetof pairedplotswitha
low noise load.
This studysuggeststhatthe effectof car traffic
on
breedingbird densitiesin woodland can be largely
explainedby noise load. For subsetsof paired plots
whichdifferedin noise load, but not in visibilityof
cars, a larger number of species had a reduced
densitywith a high noise load than with a low
noise load. On the otherhand, therewere no clear
differencesbetween subsets of paired plots which
differedin visibilityof cars but not in noise load.
Furthermore,the few significant
relationshipswith
visibility,when noise load is held constant,are all
for species in which noise levels produce models
which are at least as good. Althoughthe absolute
numbersof species involved are actuallynot very
great, the noise load hypothesisis in particular
supportedby the factthateffectson the densityof
all species combinedwere onlyfoundin the subset
of paired plotswitha highnoise load. Moreover,of
all (sub)sets of paired plots, the numberof species
thathad reduceddensitieswas highestin thissubset.
Table 7. Significantregressionsof breedingdensityon noise load in coniferouswoodland using a thresholdmodel (see
Methods)
Species
df
F
Anthustrivialis
2/23
4-12
Troglodytes
troglodytes
2/25
2-72
Phylloscopustrochilus
Regulusregulus
1/17
2/28
3-00
3-54
Parus caeruleus
1/19
2-46
All species combined
2/30
5-84
T(CFL)
C
Sign.
52 (47-58)
-2 618
*
58 (46-59)
-6-846
t
?29 (?-52)
44 (?-56)
-0 043
-0-080
t
?29 (?-59)
-0-057
t
-0-042
**
48 (38-56)
*
df,degreesof freedom;F, changein F-statistic(see methods)afteraddingnoise load;
T(CFL), thresholdvalue in dB (A) and 90% confidencelimits(? meansthata lowerlimit
could not be calculated); C, regressioncoefficient.t P<0-10; * P<0-05; ** P<0-01.
0047497
198
Main roads and
densityof birdsin
woodland
Table 8. Decrease factor(see Methods) of the breeding densityfor species and all species combined in coniferous
woodland in the zone between the road and the thresholdvalue and in a fixedzone of 250 fromthe road. Based on
regressionswithnoise load as explanatoryvariable
Decrease factorup to 250 m
Species
Decrease factor
up to threshold
10000 cars per day
60 000 cars per day
Anthustrivialis
0-98
0 20
0 39
Troglodytus
troglodytes
0.99
0 11
0-23
Phylloscopustrochilus
Regulusregulus
0-53
0-58
0-62
0-69
0 60
Parus caeruleus
0 61
0-23
0 78
All species combined
0-35
0 10
0 22
One can pointout thatthemeasurementof visibility
of cars mightgive some uncertainty,
because differences in traffic
densitywere not takeninto account.
However, it is unlikelythat this would be very
important,since trafficdensitieswere ratherhigh
and quite similarin all subsets of paired plots in
deciduouswoodland (Table 2).
Because other possible causes such as collisions
and pollution(e.g. van derZande etal. 1980; Reijnen
& Thissen1987) werenotconsideredin our analysis,
thenoise load is notnecessarilytheonlycause of the
reduceddensities.However,thereis some evidence
that these other factorsare not veryimportant.It
is assumed that an increase of mortalitydue to
road traffic
is notveryimportantin causingreduced
densitiesnear the road (e.g. Leedy & Adams 1982;
Ellenberget al. 1981). This is plausible for species
whichhave small territories
and thatare less likely
to flyacross roads (+60% of the studied species),
whichis supportedby a studyof Reijnen & Foppen
(1994). They observed equal survivalrates of male
willow warblers Phylloscopus trochilus (which
showed a clear decreased densitynear the road in
our plot series) close to a highwayand in areas at a
distance of several hundredmetres. On the other
hand, a studyby Sargeant(1981) indicatesthatalso
in species withlarge territories
(ducks), road mortality(0-2% of breedingpopulations)cannotbe very
importantin reducing densities. Only for owls,
in particularbarn owl Tyto alba, there is some
indication that road mortalitymight influence
population levels (van den Tempel 1993; Illner
1992b). However,owls were not presentin our data
set.
Pollutioncaused by road traffic
can affectabundance and size ofinsectsadjacentto roads (Przybylski
1979; Bolsinger & Flickinger 1989) and therefore
mighthave an effecton densitiesof breedingbirds
by reducing the availabilityof insects as a food
source. However,the reportedrangeof the effectis
verysmall(up to 50 m fromtheroad in a ratheropen
area). Moreover, of the 27 insectivorousspecies
investigatedin thisstudy,only 16 species showed a
reduced densityof which13 had a maximumeffect
distancebetween 100 and 1500m.
Pollutioncan also cause increasedlevels of toxic
substancesin birds. This has been shown forlead,
butthelevelswerefarbelow thetoxiclevel (Lowell,
Best & Prather 1977; Grue et al. 1986), and an
effecton reproductionand mortalitycould not be
established(Lowell et al. 1977).
There is also a more general indication that
pollutionby car trafficis not very importantas a
cause of reduceddensities.L.P. Kuggeleijn(personal
communication)estimatedthat in woodland concentrationsof exhaust gases and other pollutants
reach background levels within 50 m from a
highwaywithdense traffic
(40-50 000 cars per day).
The majorityof the species in this study (75%),
however,showedmaximumeffectdistancesbetween
100 and 1500m (see Figs 5 & 6).
Very littleis knownabout how noise load could
cause reduced densities of breeding birds. An
obvious explanation would be disturbanceof the
communicationpattern between birds (see also
Martens, Foppen & Huisman 1985). Reijnen &
Foppen (1994) found that male willow warblers
Phylloscopustrochilus
close to a highwayexperienced
in attractingor keeping a female and
difficulties
moved away fromthe road in the followingyear. It
was argued that distortionof the song mightbe a
possiblecause ofthiseffect.In thehazelhenBonasia
bonasia it was assumed that the decrease of the
breedingdensityin partlyurbanizedareas was due
to a more permanentincrease of the noise level,
which increases predation by hamperinghearing
alarm calls (Scherzinger 1973). However, there
are some indicationsthat disturbanceof the communicationpatternis not a general mechanismin
causingreduceddensities.Trafficnoise in woodland
has a wide frequencyband of at least 100Hz to
about 10 kHz withthe highestsound pressurelevels
0047498
199
R. Reijnenet al.
Table 9. Densityof breedingbirdsin road (R) and control(C) plotsof deciduouswoodlandwithhighand low noise load.
Species forwhicha comparisonof densitieswithinroad plotswas carriedout are markedwithparenthesesand resultsare
onlyshowniftheywere significant
(see footnote).NP = numberof paired plots involved.For species in bold the data are
based on numberof registrations.
Statisticalsignificance
is based on Wilcoxonsigned-ranks
test
Noise load high
Species
NP
R
Noise load low
C
Sign.
NP
R
C
Sign.
Phasanius colchicus
9
0 07
0-28
**
5
019
0 17
NS
Buteo buteo
9
0.15
0-12
NS
8
0-17
0-29
NS
Scolopax rusticola
6
0-12
0 07
NS
5
0 11
0 11
NS
Colomba oenas
Columbapalumbus ()
8
14
0 09
0 53
0-08
0 49
NS
NS
6
16
0-06
0-62
0.15
0.55
NS
NS
Dendrocopusmajor ()
11
022
0-25
NS'
17
0 28
0 25
NS'
Troglodytus
troglodytus(
Prunellamodularis
17
9
0 70
0.19
0 78
0-18
NS
NS
17
7
1 11
0-12
1-14
0-10
NS
NS
Erithacusrubecula(
Turdusmerula()
Turdusphilomelos(
16
19
13
1-15
1-07
040
1.11
0.99
0-41
NS
NS
NS'
18
19
10
1 00
1-24
0-20
0 90
1 10
0-25
NS
NS
NS
Sylviaborn (
Sylviaatricapilla
Phylloscopuscollybita()
Phylloscopustrochilus()
14
17
18
13
0-62
0-48
0-78
0-69
1-13
0-72
094
1-12
*
t
t
**
14
17
17
13
0-78
0-65
096
0-72
0-51
0 57
0-86
0-83
NS
NS
NS
NS
Muscicapa striata()
13
0-26
0-24
NS
13
0-28
0-25
NS
Aegithaloscaudata
Parus montanus
Parus palustris
Parus caeruleus()
Parus major ()
12
8
11
17
19
0-14
0 10
0.20
0-78
1-58
0-16
0-14
0-25
0 74
1-54
NS
NS
f
NS
NS
13
10
12
16
18
0.19
0-08
0-14
0 79
1-67
0 10
0-23
0.15
0 75
1-36
NS
NS
NS
NS
NS
Sittaeuropaea
Certhiabrachydactyla
9
15
0-21
0-42
0-18
0-44
NS
NS'
9
15
0-23
0 46
0-16
0 44
NS
NS'
Garrulusglandarius0
Pica pica (
Corvuscorone
15
8
14
053
0-15
0-82
0-42
0-16
0-46
NS
NS
NS
14
9
15
0 47
0 24
0-84
0 38
0 30
0 97
NS
NS
NS
Sturnusvulgaris
10
0 45
0.59
NS
10
0-36
0-31
NS
Fringillacoelebs
16
0-61
0-87
NS
16
0-82
0-83
NS
All species combined0
19
13 3
15 8
19
14 1
13 3
NS
1 Significant
lowerdensityin partof road plotsnear to theroad
(N) thanin partof road plotsfurther
away fromtheroad
(F). Dendrocopusmajor P < 0-05 (noise load high,N = 0 10, F = 0.25) and P < 0.01 (noise load low, N = 0-17, F = 0.45),
TurdusphilomelosP < 0 10 (N = 0-49, F = 0 79), Certhiabrachydactyla
P < 0 10 (noise load high,N = 0 44, F = 0.64) and
P < 0 05 (noise load low, N= 057, F = 0*83).
NS = P> 0 10; U P< 0-10; * P< 005; ** P< 0-01.
between 100-200Hz and 0 5-4kHz (Huisman
& Attenborough1991). So, if disturbanceof communication is an important mechanism, those
species producingsongs and calls witha frequency
band similarto these highestsound pressurelevels
oftraffic
noisewouldbe expectedto be morevulnerable. Frequencybandsofsongand calls ofbirdswere
taken from Bergmann & Helb (1982). However,
there was no clear pattern(chi-squaredtest, 2df,
P>O.10, Table 13), but the numbersare low and
hardlyallow statisticaltesting.Because there are
indicationsthat noise load due to car trafficcan
cause stress in birds (Helb & Huppop 1991), an
alternativeor, more probably, a supplementary
explanationcould be thatbirdsavoid road habitats
because of thisaspect (cf. Reijnen & Foppen 1994).
0047499
200
Main roads and
densityof birdsin
woodland
Table 10. Significantregressionof breedingdensityon noise load in deciduous woodland withhigh noise load using a
thresholdmodel (see Methods). For species in bold the data are based on numberof registrations
Species
df
F
T (CFL)
C
Sign.
Phasanius colchicus
Scolopax rusticola
Sylviaborin
Sylviaatricapilla
Phylloscopustrochilus
Pica pica
Fringillacoelebs
All species combined
2/15
2/11
1/27
1/37
2/29
2/22
1/33
1/38
5-28
3 99
3-24
4-20
5-73
3-45
3-39
5 30
40 (?-58)
56 (55-61)
?23 (?-54)
?23 (?-48)
38 (?-50)
46 (33-56)
?23 (?-56)
?23 (?-48)
-0 099
-2-515
-0-024
-0-036
-0-046
-0 092
-0 019
-0-008
*
*
*
*
**
*
*
*
df, degrees of freedom;F, change in F-statistic(see methods) afteradding noise
load; T(CFL), thresholdvalue in dB(A) and 90% confidencelimits(? means that a
* P < 0 05; ** P < 0.01.
lower limitcould not be calculated); C, regressioncoefficient.
of
Table 11. Densityof breedingbirdsin road (R) and control(C) plotsof deciduouswoodlandwithhighand low visibility
cars. Species forwhicha comparisonof densitieswithinroad plotswas carriedout are markedwithparenthesesand results
l
are only shownif theywere significant
Visibilityof cars low
Visibilityof cars high
Species
NP
R
C
Sign.
NP
R
C
Sign.
Phasanius colchicus
5
0-21
0-24
NS
8
0 05
0-20
**
Buteo buteo
9
0-18
0-25
NS
8
0 13
0-16
NS
Columbapalumbus ()
15
055
0 59
NS
15
0-63
0-49
NS
Dendrocopusmajor ()
16
032
0-28
NS
12
0-17
0-21
NS
(
troglodytes
Troglodytus
Prunellamodularis
17
8
0-92
0-16
0-96
0-13
NS
NS
17
8
0-89
0-15
0 90
0-16
NS
NS
Erithacusrubecula()
Turdusmerula(
Turdusphilomelos()
20
19
15
1-21
1-36
026
1-26
1-10
0 33
NS
NS
NS
15
19
9
0-76
0 94
0-33
0 81
0-98
0-32
NS
NS
NS
Sylviaborn ()
Sylviaatricapilla(
Phylloscopuscollybita()
Phylloscopustrochilus()
13
18
17
12
0 54
050
0 74
026
0-48
061
0-85
0-66
NS
tI
NS
**1
15
16
18
14
0-86
0-63
1-00
1-14
1-16
0-67
0-96
140
NS
NS
NS
t
Muscicapa striata()
14
0 35
0-28
NS
13
0 41
0-41
NS
Aegithaloscaudata
Parus palustris
Parus montanus
Parus caeruleus(
Parus major (
13
15
7
18
19
0-18
0-25
0-08
0 94
1-83
0-14
0-32
0-13
0-86
1 58
NS
NS
NS
NS
NS
12
8
11
15
19
0-15
0 10
0-10
0-63
1-28
0-12
0-08
0-25
0-64
1-28
NS
NS
NS
NS
NS
Sittaeuropaea
Certhiabrachydactyla
(
11
18
0-35
053
0-22
0-62
NS
NS'
7
12
0 09
0 35
0-12
0-26
NS
NS
Garrulusglandarius0
Pica pica (
Corvuscorone0
15
7
14
0-51
0-16
0 54
0-42
0-15
0 59
NS
NS
NS
14
10
15
0*50
0-23
1-07
0-38
0-31
0-83
NS
NS
NS
Sturnusvulgaris
14
0-68
0-67
NS
6
0-14
0-22
NS
Fringillacoelebs
17
0-82
0 97
NS
15
0-61
0 73
NS
All species combined 0
15
13-7
NS
16
12-5
NS
13-0
11-7
away fromthe road
lowerdensityin partof road plotsnear to theroad (N) thanin partof road plotsfurther
lSignificant
Phylloscopus
Sylvia atricapillaP <0-10 (N =0-45, F =063),
(F). Dendrocopus major P K0*01 (N =0-18, F =045),
P <0-10 (N = 0.41, F =0.64) .
brachydactyla
P <0 05 (N = 0-.17,F =0.84), Certhia
trochilus
NP = numberof paired plots involved.For species in bold the data are based on numberof registrations.Statistical
P< 010; * P<0 005; ** P< 0.01.
is based on Wilcoxonsigned-rankstest. NS =P> 010;
significance
0047500
201
R. Reijnenet al.
of cars in deciduouswoodlandwithhigh(H) and low (L)
regressionsof breedingdensityon visibility
Table 12. Significant
of cars usinga thresholdmodel (see Methods)
visibility
Species
H/L
DF
F
T (CFL)
C
Sign.
Phasanius colchicus
Phylloscopustrochilus
Fringillacoelebs
H
H
L
2/7
1/27
1/33
3-38
12-16
2-74
50 (11-71)
0 (?-41)
0 (?-57)
-0-058
-0-017
-0 01
t
**
t
of cars; T (CFL), thresholdvalue in
df,degreesof freedom;F, changein F-statistic(see Methods) afteraddingvisibility
dB(A) and 90% confidencelimits(? meansthata lowerlimitcould notbe calculated); C, regressioncoefficient.I P < 0 10;
** P < 0.01.
Table 13. Reductionof densityof species relatedto coverageof thefrequencyspectrumof song and calls by thefrequency
noise (see also Discussion)
spectrumof traffic
Numberof species with
POPULATION
PRACTICAL
Frequencyspectrumof traffic
noise, and of song and calls
of the studiedbird species
Reduced density
No effecton density
Similar
Partlysimilar
Not similar
9
7
10
6
7
4
CONSEQUENCES
AND
IMPLICATIONS
Because the analysiswas based on the hypothesis
that there is a negative relationshipbetween car
and breedingdensity,possiblepositiverelationtraffic
ships could not be detected. One mightargue that
species which are not affectedby road trafficcan
reach higherdensitiesifrelatedspecies are affected
(due to competition).The presenceof road victims
could also favour feeding conditions of carrion
eaters,such as manycorvidspecies (e.g. Ellenberg
et al. 1981). This would diminishthe overall effect
on the breeding density.However, such patterns
could not be detected in our data. One corvid
species,themagpiePica pica, even showeda negative
effecton the breedingdensity.Moreover, in both
woodland types the total density of all species
combinedwas lowered close to the road. The fact
thatin deciduouswoodland onlya weak significance
was found, could be due to the large number of
paired plotswitha low noise load close to the road.
For the subset of paired plots with a high noise
load the densityof all species combinedwas clearly
reduced (Tables 9 & 10).
The available studiesalso do not indicatepositive
Althoughhigherdensitiesclose
effectsof car traffic.
to roads were found in some studies (Ferris 1979;
Clark & Karr 1979; Adams & Geis 1981), in thefirst
in the
two studiesthiswas explainedby differences
vegetationstructure.For the study of Adams &
Geis (1981) thiswas shownby a roughrecalculation
of the data. They investigatedbreeding densities
of species in transects perpendicular to roads,
and observed lower densitiesclose to the road in
seven species and higherdensitiesin nine species.
Excludingthe partof the transectsclose to the road
(which had differenthabitat features),six species
had lowered densitiestowardsthe road and higher
densitiestowardstheroad were no longerobserved.
Since road trafficaffectsa large proportionof
breedingbird species in woodland and causes significantreductionsin densityup to large distances
fromthe road, the effectis probablyimportantin
breedingbird populationsin largerareas.
affecting
This is reinforcedby the assumptionthat lower
densitiesindicate a lower habitat quality (van der
Zande et al. 1980; Reijnen & Thissen 1987), which
was verifiedfor the willow warbler (Reijnen &
Foppen 1994). Because differencesin densitywill
in quality(e.g
not alwaysequally reflectdifferences
van Horne 1983;Bernstein,Krebs& Kacelnik1991),
thereductionof habitatqualityalong roads mightbe
underestimated.
The resultsof thisstudystressthe importanceof
on the breeding
consideringthe effectof car traffic
densityof birds in designingnew and modifying
existingmajor roads. If noise load is the most
importantcause of reduced densities,it is possible
noise-muffling
to reducetheseeffectsbyconstructing
devices along the roads. However, to justifysuch
evidence
itis necessaryto providefurther
expenditure
that noise load is the main cause of the reduced
densities of many bird species adjacent to busy
roads.
Acknowledgements
We thank Carl Derks, Ton Eggenhuizen, Henk
Meeuwsen and Jaap Mekel for theirassistancein
the field,S.B. den Ouden and Henk Meeuwsen for
preparingthe data, the manylandownersforkindly
us to work on theirproperty,and Paul
permitting
Opdam, Helias Udo de Haes and two anonymous
0047501
202
Main roads and
densityof birdsin
woodland
refereesfor their many valuable remarkson the
manuscript.The studywas supportedby the Road
and HydraulicEngineeringDivision of the Ministry
ofTransport,PublicWorksand WaterManagement.
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Received26 October1993; revisionreceived1 March 1994