Broken Littorina littorea shells: predation by birds?

2007·1.3 MB·English

by  Cadée, G C

BASTERIA,71:101-104,2007
BrokenLittorinalittoreashells: predation by birds?
G.C. Cadée
RoyalNetherlands InstituteforSeaResearch, P.O.Box59,NL1790ABDen Burg,The Netherlands;
[email protected]
& H.W.Nijhuis
Damastroos 19,NL3068BWRotterdam,The Netherlands;[email protected]
Littorinalittorea shells foundbrokenonacycle-track next towhere theylived,arethoughttobe
broken by shell-droppingbirds. Shells broken experimentallyby droppingthem from 6m
heightonaconcretefloor showed verysimilar damage:ahole inthe lastwhorl,whereasthe
aperturerimremained intact.Examples from the literature indicate this shell damagetobe
characteristicfor shell-droppingby birds. We suspect carrion crowsasthe culprit, but field
observationshavetoprove this.
Keywords: Gastropoda,Littorina,birds,predators,shell-dropping.
INTRODUCTION
Some brokenshells of the periwinkle Littorinalittorea(L., 1758)were found onthe
cycle-track next to the Canal through Zuid Beveland (province of Zeeland, The
Netherlands) (fig. 1).How didthey arrive onthe road,onlandnext to where they live,
and howwere theybroken?
RESULTS
Although oneneedsto seetheculprit tobe quitesure, themost probable solutionis
thatbirds dropped theperiwinkles ontheroad to crushthem.TotestwhetherL. littorea
specimens areeasilybrokenbyshelldroppingthefirstauthordropped6 livingspecimens
collectedonTexelfromabout6 m height onaconcrete floorathishome ashe did earlier
withPacificoysters (Cadee, 2001).Fourof theshellswerebroken duringthefirst trial, the
other two werebrokenwiththesecondtrial. In fiveofthesix aholewas madeinthelast
whorl, whereasthe shellopening remainedintact (fig.2). This compareswell withthe
specimens foundinZeelandreported here(fig. 1),aswellaswiththosebrokenby birds
(gulls or hoodedcrows) ontherocky shore of theIslandofMull, Scotland(fig.3). Also
othergastropod shellsdroppedbyhoodedcrows(whelks Buccinum undatumL., 1758,and
dogwhelks Nucella lapillus [L., 1758])showed acomparable damage of aholeinthelast
whorl, whereastheaperturerim remainedintact(Cadee, 2006).
DISCUSSION
Avianpreydropping is already known foralongtimeandwellreviewed (Cristol&
102 BASTERIA,Vol. 71, No. 4-6, 1007
Fig. 1ThreebrokenLittorina littoreashellsfound onacycle-track inZuid Beveland
Fig.2Experimentallybroken Littorinalittoreashellsbydroppingthem from6monaconcretefloor.
Cadee& Nijhuis: BrokenLittorinalittoreashells:predation bybirds? 103
Fig. 3Broken Littorinalittoreashells found alongthe rocky coastofhe IslandofMull,Scotland,broken
byshell-droppingbirds (diameterofcoin2cm,picturemadebyNielsCadée).
Switzer, 1999;Switzer&Cristol, 1999;Lebebvreetal.,2002). IntheNetherlandssuchshell
dropping is now mainly performedby Herring gulls(Cadee, 1995,2001 and references
therein). However,crows alsoknow the trick. Hoodedcrows (Corvus corone cornix) have
become rare in our country, but maybe observed dropping shells along the coast in
Norway, IrelandandtheU.K(Berrow, 1991;Berrowetal.1991,1992;Cadée,2006). Broken
L.littoreashellscantherebefoundregularly alongrockycoasts(see fig.2).Informeryears
hoodedcrowswere stillfrequent in winterinthe Netherlands.Tinbergen (1950) reports
on shell dropping hooded crows in 1917in theprovince of Zeeland,and Hulscher (in
Hulscher&Driessen, 2005)remembershaving seenthemsomefiftyyearsagoshelldrop-
ping on the Frisian Island Schiermonnikoog. This habit seems to be taken over in the
Netherlandsnowby carrion crows(Corvus corone corone)).InrecentyearsIobserved car-
rion crowsdroppingshells alongthe Wadden SeaontheIsland ofTexel(Cadée,2006,2007
a,b), whichIhadnot seenthemdoing hereearlier. Hulscher& Driessen (2005)observed
oncea carrion crow dropping a freshwatermussel Anodonta sp. Earlierrecords of this
behaviour were published by Whitely etal. (1990), who studied the strategies of shell
dropping incarrioncrows andCoombs (1978),whoreported dropping shellfishonrocks
bothforhoodedand carrioncrows.
Zach (1978) studied 90 freshly broken shellsof the whelk Thais lamellosa(Gmelin,
1791) dropped by northwesterncrows (Corvus caurinus) in Mandarte Island (British
Columbia,Canada) andobservedahole in onewhorlor including thenext onein24%of
the shells, the majority (76%) had the spire brokenoff. He suggests that the holes are
104 BASTERIA, Vol. 71, No. 4-6, 2007
formedfirst and that crows continue dropping till thespire isbrokenoff, to enable the
crows to extracttheentire animal.Alsoin hisbrokenwhelks the aperturerim remained
intact. Thismay be significant in distinguishing shells brokenby shell-dropping birds
fromthosebrokenindifferentways,eitherbyotherpredators orphysical factors. Crabs
e.g. oftenstartcrushing gastropod shellsatthe aperturerim (Vermeij, 1987;Cadeeetal.,
1997).
CONCLUSION
In conclusion, we think,that theperiwinkleswere dropped bybirds, mostprobably
carrion crows, toenablethemto eattheanimals.However,only theobservationofcrows
intheactofdropping periwinkles willgive theconclusiveanswer. Theintactaperturerim
seemscharacteristicforgastropod shellscrushedby shell-dropping birds.
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