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					J. Field Ornithol., 68(4):607-612

        THE      EFFECTS  OF PREDATION     RISK ON SCANNING                    AND
                       FLOCKING   BEHAVIOR     IN DUNLIN

                                            ANDRES BARBOSA •
                                               de       Evolutiva, CSIC.
                                    Departamento Ecologfa
                                       Museo Nacional    Ciencias Naturales
                                           C/Josg       Abascal,2
                                             28006 Madrid, Spain

Abstract.---Dunlin(Calid•s alpina) scanningand flocking behaviorwasstudied in two hab-
itats with different predation risk. Mean flock size, time spent scanning,and mean scan
duration were higher in rice fields (the lessprofitable and high-riskhabitat) than in littoral
habitats(the more profitable and low-riskhabitat), althoughvigilancerate wasnot different
between habitats.Increasedscanningand flocking behavior probablyincreaseefficencyin
predator avoidance.The results are explained as behavioral changesto ensure safetyin a
high predation-risksituationwhen starvationrisk is alsopresent.


Sinopsis.--Seestudi6 el comportamientode vigilanciay el tamafio de gruposde Calid•is
alpina, en dos habitatscon diferente riesgode depredaci6ny diferente rentabilidadalimen-
taria. Los resultadosmuestranque el tamafio medio del grupo, el tiempo total de vigilancia
y el tiempo medio de cadavigilancia,fue m•s alto en el arrozal (habitat menosrentabley
con m•s riesgo) queen el litoral (el habitat mssrentabley con menosriesgo).Sin embargo,
                                                              bitats.Se ha serialado incre-
la tasade vigilanciano fue diferente entre los dos tipos de hb,                     el
mento en los valoresde lasvariablesrelacionadas  con la vigilanciay el incrementodel grupo
como modo de aumentarla eficaciaantidepredatoria.      Los resultados            se
                                                                       obtenidos explican
               en                                             en
como cambios la conductaque incrementanla seguridad una situaci6nde alto riesgo
de depredaci6ncoincidente   con un alto riesgode muerte por inanici6n.

    Wintering birds should maximize their probabilityof survivingto the
next breedingseason                     depends foragingeffi-
                   (Caraco1979). Survival      on
ciencyand avoidanceof predation. Habitat patchesmay vary both in for-
agingprofitabilityand predationrisk,soa foragermusttrade energygains
against the risk of predation when deciding where to feed (Suhonen
1993). On the other hand, competition(inter- and intraspecific) known
to affect foraging-siteselectionin birds (Alatalo 1982) by restrictingin-
dividuals'use of more profitable habitat patches.
   Animalsoften use low-riskpatcheseven though safeareasare lessen-
ergeticallyprofitable (Anderson 1986, Caldwell 1986, Fergursonet al.
1988).At the Ebro Delta in Spain,Dunlin (Calidrisalpina) foragein both
littoral habitatsand rice fields,preferingthe more profitablelittoral areas
(Barbosa1994, 1996). The use of rice fields, the lessprofitable habitat,
       to                     of                          of
seems be a consequence the temporalunavailability feeding areas
and density-dependent    effectsrelated to starvationrisk (Barbosa1996).
However,predator densityis higher in rice fields than in littoral habitats
(Tombal and Tombal 1988). Raptor predationhasbeen shownto be an
 Currentaddress:           d        CNRS-URA                 Pierre Marie Curie,Bdt.
                                               258, Universitg     et
A, 7eetage, quai St., Bernard,Case                       05.
                                  237. F-75252 Pa•is Cedex France.

608]                            A. Barbosa                           Ornithol.
                                                                 Autumn   1997

important sourceof mortalityin winteringshorebirds        and Whit-
                                         an        opportunity
field 1994,PageandWhitacre1975).Thisallows excellent
to studyanti-predatorbehaviorsuchas flocking and scanning,   when be-
haviorsthat lessenthe risk of starvationmay entail an increasedrisk of
        (Lima Dill Weissburg
prtedation and 1990,     1986).
   It is well establishedthat birds benefit from feeding in flocks. Many
shorebirds   species such as Dunlin respondto the presenceof predators
by flocking (Buchanan1980, Caracoet al. 1980, Myers 1980). Flocking
enhances   predator avoidance   either through confusionor dilution effects
(Creswell 1994, Hamilton 1971, Kenward 1978) or an earlier detection of
the predator resulting from shared vigilance. ("many eyes" hypothesis,
Pulliam 1973). Ekman (1987) and Suhonen (1993) found larger flocks
in high-predationrisk situations                                   On
                                   comparedto low-risksituations. the
other hand, birds spend more time scanningfor predatorswhen preda-
tion risk increases  (Caracoet al. 1980, Ekman 1987). I predictedthat in
rice fields (high-risk habitat), Dunlin flockswould be larger than in lit-
toral habitats (low-riskhabitat) and that they will spend more time scan-
ning in rice fields than in littoral areas.

 The studywasconductedat the Ebro Delta Natural Park (northeastern
Spain;40ø43'N,00ø44'E),one of the main winteringareasfor Dunlin in
the Mediterranean with around 10,000 individuals (Barbosa 1992, Grim-
            1988). Observations
met andJones'                 were made November-April in 1990-
1991 and 1991-1992. The area consistedof rice fields (15,000 ha), market
garden (9,500 ha), and natural wetlands(beaches, marshes, lagoons)
                                                     was 16,000 ha
(7,500 ha). The area potentiallysuitablefor shorebirds
(Martinez-Vilalta 1985).
   In this area there are five raptor speciespresent (Circus aeroginosus
[3.21 individuals/km2], Circus cyaneus  [0.28], Buteo buteo[1.21], Falco
tinnunculus [1.10]) and Falco columbarius   ([0.07]; Tombal and Tombal
1988). Raptor speciessuch as these may have considerableimpact on
survivalratesof wintering shorebirds(Creswelland Whitfield 1994, Page
and Whitacre 1975).
   Scanningbehaviorwas recorded using the focal observationmethod
(Altmann 1974). Focalbirdswere randomlychosenduring regularsur-
veysof the studyarea, and their behaviorwas tape-recorded       during a
2-min period, on average (range -- 1-3.5 min). All observations     were
made during daylightwith 8X30 binocularsor a 40X60 spottingscope.
To avoid problemsof pseudoreplication     (Hulbert 1984), I attempted to
sampleindividuals   only once.Because                             I
                                      birdswere not color-marked, sam-
pled birds in a way to maximize the chance of observingdifferent indi-
viduals (i.e., in a flock of about 100 individuals,I sampled 5-10 birds
from the far side, 5-10 birds from the near side, 5-10 birds from the left
side, and 5-10 individualsfrom the right side of the flock). Each bird
sampledwasat leastwas20 m from the previous    bird sampled.This pro-
cedure maximized the chance that different individualswere sampled
  68, 4
Vol. No.                             Behavior Dunlin
                              Scanning     in                                           [609

T•a•t• 1. Scanningand flocking behavior of Dunlins in two habitatsat the Ebro Delta,
    Spain, 1991-1992.

                    rate          Time spent              Mean scan
                 (scan/min)       vigilant (%)           duration (s)          Flock size
  Habitats       •      SE         •          SE         •        SE       •           SE

Rice fields     1.76   0.25       4.20       1.17       0.96     0.14   293.92        23.51
Littoral        1.03   0.19       1.74       0.44       0.42     0.09   189.44        31.52

(Barbosa1995, 1996). Vigilance rate (number of scansper minute for-
aging),vigilancetime (percentage time spentscanning       while feeding),
and mean scan duration were recorded as variablescharacterizingscan-
ning behavior (Barbosa1995). A scanwasdefined as raisingthe head
from the head-downforaging position to a position with the bill line
abovethe horizontal (Barbosa  1995). All birdswere sampled while actively
foraging,thisavoided                          in
                       biasdue to differences scanning   behaviorrelated
to another behaviorsuchas restingor preening.I considered flock as
all birds (con- and hetero-specific)within 10 m of the nearestneighbor.
   Scanningbehavior is affected by flock size,with individual levelsof
vigilancedecreasing the number of individuals     increases (e.g.,Abram-
son 1979). However, did not find any significant    relationship between
scanningvariablesand flock size (scanrate r = 0.08 P • 0.05; mean scan
duration r = 0.05 P • 0.05; time spent vigilant r = 0.06 P • 0.05).
Therefore, data were not corrected for flock sizein subsequent   analyses.
  Analysis variance(ANOVA) wasusedto analyzethe differences           be-
tween scanningand flocking variableswith differential predation risk.
Percentages  were arcsin-transformed  prior to analysis.

   I observed    77 individuals   in littoral       habitats and 28 in rice fields. Scan-
ning and flockingbehaviordifferedbetweenhabitats.   Mean scanduration
                longer in rice fields (high-riskhabitat) than in littoral
was significantly
(low-riskhabitat) (Fl,104 8.97, P • 0.01; Table 1). Dunlins spentmore
time vigilantin rice fieldsthan in littoral (Fl.•04 5.78,P • 0.01;Table
1). Vigilance rate also showedsignificantdifferencesbetween habitats
(F•,•04 4.00, P • 0.05; Table 1). Flockswere larger in high-riskhabitats
than in low-risk              ----
                habitats(F•,104 3.70, P = 0.05; Table 1).

  Recent studiesfocuson predator abundanceor presenceas the prime
determinantof riskin habitatusetrade-offs.Caldwell(1986) reportedthat
foragingheronsswitch                                      are
                      from areasin whichpredatoryattacks common
to thosewhere predatorsare rare, even though the safe areasare less
energeticallyprofitable. Grubb and Greenwald (1982) found that patch
selectionin House Sparrows   reflectsa predation-energytrade-offinflu-
610]                                  A. Barbosa                                Ornithol.
                                                                             Autumn   1997

enced by the distanceto protectivecover.This is alsotrue for Willow Tits
(Hogstad1988). At Ebro Delta, more profitableareas(littoral) havespace
constraints birdsmustuseless-profitable      areas(rice fields)to minimize
the risk of starvation winter (Barbosa1996), althoughthe latter habitat
hasa high predationrisk.During winter at Ebro Delta predationpressure
by raptors (Circusaeroginosus, Circuscyaneus,             and
                                               Buteobuteo Falcotinnun-
culus)is higher in rice field than in litoral (Tombal and Tombal 1988).
Therefore, to compensate possible       costs derivedfrom the use of rice
fields,Dunlinschangetheir behavior    when feedingin littoral areas.
   Mean flock size,mean scanduration,vigilancerate and time spentscan-
ning are higher in rice field than in litoral. Flockingprobablyenhances
predator avoidance individualsmay benefit from an earlier detectionof
the predatorby sharing                                        or
                             (Pulliam1973) and from a confusion
dilution effect (Hamilton 1971, Kenward 1978). Predation seemsto be a
major selectivepressureinfluencingflockingbehaviorin foragingwinter
shorebirds (Abramson  1979, Pageand Whitacre 1971, Stinson1980), in
terms of a decreased proportion of time that each individualspendsin
antipredatorvigilance(Glfick 1987,Metcalfe1989,Redpath1988). Several
authors (Caraco et al. 1990, Carrascal and Moreno 1992, Ekman 1987,
Suhonen1993) found larger flocksand birdsspendingmore time vigilant
at high predationrisk than at low predationrisk.
   On the other hand, lengtheningscanduration achieves higher level
of vigilancewith lower frequencyof changesbetweenvigilant and non-
              thussaving time and energynecessary rise and lower
vigilantstates,           the                          to
their heads (Studd et al. 1983) and without interrupting the searchfor
hidden prey difficult to capture (Carrascalet al. 1990). Short scansdo
not allow a large visual field to be scannedwhereaslong scanswould
provide complete information (Desporteset al. 1993). Mean scandura-
tion also has been related to differencesin the complexityof the envi-
ronment that could provide protective cover (Carrascal and Moreno
1992, Metcalfe 1984). However, in my studyarea, habitatshave similar
complexity(Barbosa1994). This shows    that predation pressure seemsto
be more important that habitatcomplexity modifyingscandurationof
Dunlins at Ebro Delta. On the other hand, predator detection in the
largersurrounding  area, asinhabitedby Dunlins,shouldrequire a longer
time for a correct evaluation of the risk (Carrascal and Moreno 1992).
  Behavioralchanges           by
                     displayed Dunlin betweenhabitats   with different
predationrisk agreewith previous        By
                                 studies. increasing  variablesrelated
to scanningand flocking behavior,birds probablyincreaseefficiencyin
predator avoidanceand likely compensatethe costsderived for the use
                     habitatunder starvation
of high-predation-risk                      risk as theory predicts.

  This studyhas been carried out with the collaborationof the Agriculture and Fish De-
                            eneralitat and the Natural Park of Delta del Ebro. Financial
partment of the Catalonian C,
supportwasprovidedby a CSIC-CAMgrant and partlyby CajaMadrid. Laura provided       con-
siderable   assistance.
  68, 4
Vol. No.                                     BehaviorDunlin
                                       Scanning    in                            [611

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Received8 Jun. 1996; accepted19 Dec. 1996.

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