FORAGING-FLOCK
RECRUITMENT
AT A BLACK-BILLED
GULL COLONY:
IMPLICATIONS
FOR THE
INFORMATION
CENTER
HYPOTHESIS
ROGER M.
EVANS
Departmentof Zoology,Universityof Manitoba,Winnipeg,Manitoba,CanadaR3T 2N2
ABSTRACT.--The
extentto which a flock leaderadvertisesits departurefrom a colonyand
recruitsflockmatesis an importantissueof the InformationCenter hypothesis.At a colony
of Black-billedGulls (Larusbulleri),I found that attractivecallswere given by someleaders,
that leaderscalled more often than followers,and that calling leadersrecruited followers
moreoften than silentleaders.Playbackexperimentsdemonstratedthat these"contact"calls
were attractive.The resultsindicatethat somebenefits,mostlikely relatedto groupforaging,
result from flock membershipwhen the gulls are away from the colony.Benefitsof group
foragingawayfromthe colonyprovidea morelikely mechanismfor explainingfood-related
selectionpressuresfavoring a colonialforaging systemthan do the presumedbenefitsderived from the morecomplex,information-transfermechanismenvisagedby the Information
Center hypothesis.Received11 December
1980, accepted
I July1981.
ACCORDINGto the Information Center hypothesisdescribedby Ward and Zahavi (1973),
certain avian assemblages,such as communal
roosts and breeding colonies, evolved primarily becausethey function as information centers where
birds
that have
been
unsuccessful
function
as a means
ies of the Red-billed Dioch [Quelea] (Quelea
quelea)(Ward 1965), the Great Blue Heron (Ardea herodias)(Krebs 1974, 1978), and the Bank
Swallow (Riparia riparia) (Emlen and Demong
24
Gulls
nest in dense
inland
col-
onies in New Zealand (Stead 1932, Beer 1966,
Evans1970).They typically foragein flocksand
utilize unpredictable but often temporarily rich
food patches. Food items include fish, as well
as worms
and other small invertebrates
that are
exposed on open pasture land after rains or
turned up on fields by farm implements(Dawson 1958, pers. obs.).
SOME CONTEXTS FOR INFORMATION
TRANSFER AT COLONIES
1975).
The central concept of the Information Center hypothesisinvolves informationtransferbetween or among birds at the roost or colony.
Of particular relevanceare signalswhereby a
potential leader might identify itself to followers. Although information transfer of the sort
envisagedby the Information Center hypothesis could occur in the absenceof specialized
signals devoted to that function (Bertram 1978,
Waltz in press), the extent to which a leader
advertises its departure remains highly relevant to the hypothesis(Ward and Zahavi 1973).
In this report, I examine some of the contexts
and evolutionary implications of information
transfer as it pertains to signals that could
at a
billed Gulls (Larus bulleri).
Black-billed
in locating a good food patch can benefit by
following other, more successfulbirds to the
latters' food. The hypothesis is consideredto
apply mainly to birds that utilize food distributed in unpredictable but temporarily rich
patches. Data consistentwith the Information
Center hypothesishave been obtained in stud-
of flock recruitment
colony and describe observationsand experiments designed to assessthe occurrenceand
probable function of these signals in Black-
Parasitic relationship.--A parasitic relationship would arise if followers that did not know
where food was ("ignorant" birds) were able
to victimize
leaders
that did know
where
food
was ("knowledgeable" birds) by following
them to their food patches,even though such
information transfer were of no advantage to
the leader (cf. Krebs 1978, Waltz in press).This
situation could arise if a colony evolved and
were
maintained
for reasons
unrelated
to in-
formation transfer, for example, if the colony
provided protection against predators (Lack
1968). Possiblecoststo the leader might include those associatedwith leading potential
competitors to the leader's food source. Even
The Auk 99: 24-30. January1982
January1982]
FlockRecruitment
in Gulls
if suchcoststo the leaderswere negligible, it
is expectedthat knowledgeableleaderswould
not be selectedto expend time and/or energy
in advertisingtheir departures.The occurrence
of calls or other conspicuousrecruitment displays by leaderswould not be predicted in a
parasiticsystem.Attempts by leadersto slink
away quietly and inconspicuouslywould be
more likely (Bertram1978).
25
be selectedto render itself conspicuousor in-
conspicuousto otherswhen it leavesthe colony on a foraging trip. Whenever a bird flies
away from a colony,its actionsnecessarilyprovide information
in the form of visual cues to
any observersthat might be watching.Vocalizations, in contrast,need not be given and,
when present,canbe taken as evidencefor the
emission of ritualized signals subjectto positive selectionpressures(Smith 1977)of the sort
couldoccurat a colonyin a contextof reciprocal expectedfrom a bird selectedto make itself
altruism if a bird A leads another bird B to food
conspicuous,but not from one that is selected
on one trip, and bird B reciprocatesand leads to be inconspicuous. Gulls, including the
bird A to a food patch on somelater trip (Krebs Black-billedGull, are highly vocal,and prelim1978). As pointed out by severalinvestigators inary observationsindicatedthat loud callsare
with foragingflocks.The
(Trivers 1971,Maynard Smith 1978,Davies and sometimesassociated
Krebs 1978, Vehrencamp1979), however, sys- occurrence of calls and their function when
tems based on reciprocalaltruism are usually emitted were used to test the above predicopen to invasion by "cheaters."A cheaterin tions.
Someforagerscall when leavingthe colony.the present situation can be defined as a bird
Reciprocal altruism.--Information
that follows
others
transfer
to their food but does not
lead others to any rich patches it might find
itself. As in the parasitic situation, leaders in
a system based on reciprocal altruism could
Black-billedGullsdepartingfrombreedingcolonies and roostscommonlyemitted loud calls
asthey left. Similarcallswere heardfrom flocks
en route to or from a feeding site and again
facecostsassociatedwith leadingcompetitors upon arrival at a roost, colony, or foraging site.
to the food supply and would not be predicted Callingwas alsocommonin flocksflying over
to expend time or energy in advertising their
departureswith calls or other recruitment displays.
Cooperationbetweenselfishleadersand followers.--A system in which knowledgeable
leadersand ignorant followersall benefit is a
third
context in which information
could occur. This situation
colonies
transfer
at
could arise
whenever leaders would benefit directly from
the companyof flock membersaway from the
colonysite, for example,if flockingen route to
the foraginggroundsor while foragingwere
beneficial as an antipredator adaptation (Bertram 1978),or if groupforagingwere beneficial
(Rand 1954, Fisher 1958). When these away-
the river prior to colony establishment (see
Beer 1966 for similar observations). In all of
thesecontexts,the loud, single-notecallsappeared to function as "contact" calls (Smith
1977),broadcastingthe currentlocationof the
caller to others in the vicinity.
These
observations
suffice
to demonstrate
that at least someforagerscall as they leave a
colony.The mere existenceof loud calls,however, doesnot necessarilyprove that they function to attract and recruit others into a depart-
ing foragingflock.Three setsof data relevant
to the possiblefunctional role of these loud
contactcallsin recruitingflock mateswere obtained at a largebreedingcolonyon the Ashley
from-colonybenefitsof flockingoccur,leaders River, as outlined below.
Leaderscall morethanfollowers.--Black-billed
and followersshouldall benefit by leaving in
a flock. If leaving as a member of a flock is Gulls leaving a colonyto obtain food normally
beneficial, leaders should be selected to recruit
departin long, stragglingflocksthat commonly
followers.This predictionis oppositeto tha'• coalesceinto more dense units as they move
derived from the previoustwo situations(par- away. Intervalsbetweenflocksrangefrom several secondsto severalminutes and are usually
sufficient to permit a clear distinction between
successiveflocks. In these studies, I used only
flocksthat could be distinguishedas separate
asitism and reciprocal altruism).
TESTS OF PREDICTIONS
The main predictions derived from the
units. By sitting in a blind at the edge of the
colonydirectlyunder the currentlyprevailing
extent to which a potential flock leader should flight path takenby departingforagers,I could
above theoretical
considerations
deal with
the
26
record the number
ROGER
M. EVANS
of instances
in which
flock
[Auk, Vol. 99
lowers) into the flock. This result is entirely
consistent with the interpretation that flock
leadersderive some averagebenefit from having otherswith them when they leavethe colony, becauseit presumablywould be very easy
for them to forego calling if recruiting others
were not advantageous.The resultsprovide no
support for the alternative possibilities that
followers were parasitizing leaders that were
As indicated above, not all leaders called. I
at the colony for some other adaptive reason
made use of this fact to test the hypothesisthat or were participating in a system maintained
those leaders that did call gained followers solely by reciprocalaltruism.
more often than those that did not. Of the 65
It remains possible that a mixture of paraleaders that called, 51 (78%) gained recruits. sitism, reciprocal altruism, and selfish coopOnly 40 (41%) of the 98 that did not callgained eration occurred at the colonies. For example,
coloniesmay confersomeantipredatoror other
recruits (X2 = 20.96, P < 0.001).
Contact calls are attractive.--The
results deas yet unidentified benefits,which couldfavor
scribedabove suggestthat contactcallsemitted a degree of parasitic following of knowledgeby leadersfunctionto recruit followers.To test able leaders. This could still be compatible
this hypothesis further, I recorded a series of with the emissionof loud callsby leadersif the
contactcallsby birds departing from a colony benefits of leaving in flocks were sufficient to
and then played them back at a different col- more than offsetany coststo leadersthat might
ony. A Uher 4000 Report tape recorder, aug- otherwise derive from being parasitically folmented for playbackwith a 12-V amplifierand lowed. Benefits from flocking could also prean external, remotely placed 8-ohm speaker, sumably maintain a degree of reciprocation
was used. For the playback experiments, I between leaders and followers, because the
placed the loudspeaker approximately 50 m leaders on any one day could be followers
from the colony,in a directionaway from that another day. The resultsare thus entirely combeing used by most departing foragersat that patible with a mixture of these three evolutime. I then noted the number of birds that
tionary strategies, but only if the benefits of
flew over toward the loudspeaker during a se- flockingaway from the colonyare sufficientto
ries of 10 2-min playback intervals and com- more than offset potential costsof calling and
pared this with the number that flew in the leading, thereby maintaining the evolutionary
same direction in the immediately preceding stability of the system.
Although the resultssuggestthat flock lead2-min periods. An average of 3.3 birds per 2
min flew .toward the speaker during the play- ers obtained some benefit from flocking away
back periods, compared with 0.8 birds per 2 from the colony, they do not permit a distincmin during the immediately preceding, silent, tion between different types of potential bencontrol periods (T = 2.5, P < 0.01, Wilcoxon efits, for example those derived from protecmatched pairs test). These results do not pro- tion from predators while foraging or those
vide evidence that the contact calls tested are
derived from more effective group foraging
necessarilymore or less effective than other apart from predator effects. Consideration of
callsor displaysof Black-billedGulls, but they potential predator pressures and the foraging
do provide evidence that the calls given by methods of Black-billed Gulls are required to
departing foragersare attractiveto others and, assessthe relative merits of these two potential
hence, could act to recruit flock mates.
benefits of flocking.
Defense from predators could be a relevant
advantageof flocking in Black-billedGulls, but
DISCUSSION
such an interpretation is weakened appreciaThe main finding of this study was that bly by the historicallack of large mammalian
when flocks of Black-billed
Gulls leave a color other quadruped predatorsin New Zealand
ony, the first bird out (defined as the leader) (reviewed in Beer 1966). Large avian predators
often advertisesits departure by emitting loud are also rare (Falla et al. 1970). The predator
calls that attract and recruit other foragers (fol- defensehypothesisis alsoweakenedfor Blackleaders did or did not emit loud calls as they
passedby within hearingrange.The samedata
were, then obtained for subsequentmembers
of the departing flock (followers).Of 163 flock
leaders, 65 (40%) emitted calls as they departed, while only 75 (17%) of a total of 443 followers called (X2 = 33.5, P < 0.0001).
Leadersthat call gain followersmore often.-
January1982]
bills because
FlockRecruitment
in Gulls
of their
reduced
levels
27
of mob-
did not, a powerful mechanismwould exist for
bing or other antipredator behavior compared identifying the most successfulbirds. This
with their northern hemisphererelatives(Beer asymmetryfits well with what one would ex1966, pers. obs.). Benefit in the form of protec- pect from the leaders, because they are astion from predators cannot be ruled out but sumed to benefit by calling to recruit others
remains highly unconvincing for Black-billed into their flocksfor purposesof group foraging
Gulls.
Benefits from group foraging are a more
plausibleform of away-from-colonybenefitfor
Black-billed Gulls. Elsewhere(Evans in prep),
I describe
instances
where
Black-billed
Gulls
locatecurrentfood sitesby cueing in on other
foragers ("local enhancement," Thorpe 1963).
away from the colony.This interpretationfails,
however, to accountadequatelyfor the behavior of the silent leaders that are presumed not
to know where food is located. These presumed ignorant leadersare the very birds that
are expected to gain the most from group
searchfor a new food patch or from other ben-
Local enhancement
as a mechanism
for food
efits of group foraging;hence, they shouldbe
finding is well documentedand of widespread even more strongly selectedthan the knowloccurrencein gulls and other large, conspicu- edgeable birds to emit calls or other conspicous birds (Rand 1954; Sealy 1973; Scott 1973; uous recruitment signals when they initiate a
Krebs 1974, 1978; Kushlan 1977; Porter 1981). departure from the colony. The interpretation
Instancesin which foraging flockslanded se- that knowledgeable leaders call while ignorant
lectivelyamongbirds alreadyat a foragingsite leadersdo not is thus not a viable explanation
were observedrepeatedly on the study area. for a mixed-callingstrategyby leaders.
Local enhancementalso occurredamong flock
An alternative explanation for a mixed-callmembers already at terrestrial sites and in ing strategy by leaders is the reverse of the
flocks foraging over water (see Stead 1932, for above scenerio. Because birds that do not know
further documentation
of local enhancement in
where a current food patch is located should
Black-billed Gulls). Group foraging could also benefit the most by recruiting others into a
be a benefit when it results in a more efficient
departing flock, ignorant leaders might be
search of a localized area (Cody 1974). Other more likely to call. Knowledgeable leaders,
possible on-site benefits of group foraging who already know where food is, might leave
have been identified in birds (Rand 1954), but quietly. This interpretation,like the preceding
I was unable to detect any evidence of their one, provides a mechanism whereby knowloccurrence in Black-billed
Gulls.
edgeableand ignorant leaderscould be readily
Mixed vocal strategies.--Althoughthe inter- distinguished. In this case,selectivefollowing
pretation that selfish leaders and followers all of silent flock leaderswould be favored by rebenefit from group foraging away from the col- cruits requiring information about current
ony site appears to provide the best explana- food locations. This interpretation fails, howtion for the presenceof loud callsgiven by flock ever, becauseselectivefollowing of silent leadleaders at Black-billed Gull colonies, it is not
ers is directly contradictedby the data, which
immediatelyevident from the predictionsand show that vocal leaders were followed selecobservations considered thus far why flock tively.
leaders
should
have
exhibited
a mixed
vocal
strategy, in which some called and others did
What emerges from the above two scenarios
is the conclusionthat the only evolutionarily
not. Because fewer than half of the flock leaders
stable strategy(ESS)is for ignorant flock leadactually called, evidently some complicating ers to do exactly what knowledgeable flock
factorwas acting. A simple, unmixed strategy leadersdo, i.e. they shouldcallwith equal frewhereby all leaders benefit from recruiting quencywhen they leavethe colony.If ignorant
others into the flock predicts the occurrenceof leaders call less than knowledgeable leaders,
callsin virtually all cases.
they will suffer a potential cost as a result of
One possible explanation for the mixed-call- having fewer recruitswith them for group foring strategy of leaders that has relevance for aging. If ignorant leaderscall more frequently
the InformationCenter hypothesisis that call- than knowledgeableleaders,they immediately
ing was dependent upon foraging success.If set up a situation in which the following of
leaders that were successful on their most resilent birds is favored, and, again, the ignorant
cent foraging trip called and unsuccessfulbirds flock leaders will obtain fewer recruits than
28
RocER M. EVANS
they would if they called at the same rate as
knowledgeableleaders.Thus, differentialrates
of callingby knowledgeableand ignorantflock
leaders is not an ESS and is not a viable explanation for the observedmixed-calling strategy.
A definitive explanation for the mixed-calling strategy of flock leaders may not be possible from existing data. The following explanation is offered as one possible hypothesis.
My interpretation is that the rate of calling
upon leaving a colonyor other assemblagewill
depend on the amount of calling immediately
preceding the time of departure of the bird in
question.Consideredfunctionally,sucha negative-feedbacksystemwould mean that a bird
departing the colony soon after other calling
birds had departed would be more likely to
encounter
flock mates
en route to or at a for-
aging site than would a bird that left sometime
after others had gone. The latter bird is less
likely to encounter previously departed birds
and would, on average,gain relatively more by
recruitinga flockof its own. It would therefore
be more stronglyselectedto call. The same interpretationwould predict that recruitscall less
frequently, on average,than do flock leaders.
Recruitsinto a flock are most certain of having
others with them to forage and are also most
likely to have been exposed very recently to
calls by the leader. The fact that recruits did
call less than flock leaders thus provides some
supportfor this interpretation.Nothing in this
hypothesis prohibits variations in absolute
calling frequency. For example, if food were
very difficult to find, the absolutelevel of calling might well go up, but a differentialin calling rates between different individuals, dependenton the amountof callingto which they
were exposedin someprecedingtime interval,
could still occur. Circumstantial
evidence
that
absolutecalling rates may vary was provided
by observationsat one colony that was abandoned soon after nest initiation. Calling was
virtually incessantas birds wheeled about over
the colony prior to abandonment,and an inspection of the region for severalkilometers
adjacentto the colonyfailed to reveal any evidence of foraging gulls. This interpretation is
thus consistentwith existing data for Blackbilled Gulls but requires further testing.
Selectionpressures
for colonialnesting.--Several ecologicalconditions that could selectfor
nesting in colonieshave been advanced (Mock
1980). Protection from predators at the colony
[Auk, VoL 99
site may be one important condition for some
species(Lack 1968). In Black-billedGulls, the
predator-protection hypothesis is not convincing due to a paucity of potential predators(see
above, Beer 1966). As discussed above, the oc-
currence of calls by flock leaders is also contrary to expectationif Black-billedGull colonies
exist primarily for protectionfrom predatorsat
the colony site.
The Information Center hypothesis itself
constitutes another potential explanation for
the evolution
or maintenance
of roosts or col-
onies (Ward and Zahavi 1973, Krebs 1978).
Accordingto this hypothesis,benefitsderived
from information transfer at the colony are adequate to accountfor the occurrenceof suchan
assemblage.For this interpretation to be considered valid, other primary benefits at or
away from the colony must be excluded as
major evolutionary pressuresinvolved in the
evolution
or maintenance
of colonies.
Some
form of reciprocal altruism or a colony based
on assistanceto kin (e.g. Waltz in press)seems
essentialif information transfer at the colony
is to be the primary evolutionary explanation
for colonies. As discussed above, a system
based on reciprocalaltruism seemsentirely inadequate, becauseit is open to cheatersand
hence is not an ESS (see Waltz in press, for
similar conclusions). That kin selection is a viable mechanism whereby information transfer
of the sort envisagedby the Information Center
hypothesis could select for coloniesis a definite possibilitybut must remain in doubt until
evidence for selective following of kin is obtained.
Coloniesas assemblypointsfor group foraging.--The resultsof this study suggestthat a
much simpler mechanism for the transfer of
information about current food supplies could
be involved
in the evolution
or maintenance
of colonies than that required by the Information Center hypothesis.The finding that the
pattern of calling by flock leaders at Blackbilled Gull colonies is best interpreted as indicating that all flock members, leaders and
followers alike, benefit from leaving in a flock
suggeststhe simple alternativethat coloniesof
this speciesfunctionas assemblypointswhere
dispersedforagerscan reunite for purposesof
subsequent group feeding activities. Because
the very nature of efficient searchfor patchy
and unpredictable food clumps necessitatesa
degree of dispersalover the habitat, foragers
January1982]
FlockRecruitment
in Gulls
run the risk of becoming more and more
29
evolutionary approach (J. R. Krebs and N. B.
Davies, Eds.). Sunderland, Sinauer Assoc.
widely dispersedastheymoveaboutsearching
for goodfood patches.To ensurethat excessive Cot)Y, M. L. 1974. Optimization in ecology.Science
183: 1156-1164.
dispersal does not occur (i.e. to maintain a
population sufficiently large to ensure maxi- DAVIES, N. B., & J. R. KREBS. 1978. Introduction:
ecology,natural selectionand socialbehaviour.
mum net benefits of group foraging), I suggest
Pp. 1-18 in Behaviouralecology,an evolutionary
that the periodic return of dispersedforagers
approach (J. R. Krebs and N. B. Davies, Eds.).
to a centralassemblypoint can be favored and,
Sunderland, Sinauer Assoc.
hence, can provide a selectionpressurefavor- DAWSON,E. W. 1958. Food of young Black-billed
ing the evolution or maintenance of roosts or
Gulls (Larusbulleri)in a breedingcolony,North
colonies. In Black-billed Gulls, locating food
Canterbury. Notornis 8: 32-38.
patchesby cueingin on others(localenhance- EtaLEN,S. T., & N.J. DEMONC. 1975. Adaptive significanceof synchronizedbreedingin a colonial
ment) seemsto be a particularlylikely benefit
bird: a new hypothesis. Science188: 1029-1031.
derived from flock foraging (see above). Significantly, local enhancementhas also been EVANS,R. M. 1970. Parental recognition and the
"mew call" in Black-billed Gulls (Larus bulleri).
noted in the same speciesfor which the conAuk 87: 503-513.
cept of Information Centers has been invoked
(e.g. Ward 1965; Ward and Zahavi 1973; Krebs
FALLA, R. A., R. B. SIB$ON, & E.G. TURBOTT. 1970.
A field guide to the birds of New Zealand. Lon-
1974, 1978).
don, Collins.
It shouldbe emphasizedthat the hypothesis FISHER,J. 1958. Evolution and bird sociality. Pp.
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71-83 in Evolution as a process(J. Huxley, A. C.
Hardy, and E. B. Ford, Eds.). London, Allen and
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group foraging does not preclude the occurKREBS,
J. R. 1974. Colonial nesting and socialfeedrenceof informationtransferat the colony.The
ing
as strategiesfor exploitingfood resourcesin
presentinterpretationoffersan alternativeway
in which
food-related
the Great Blue Heron (Ardea herodias).Behav-
benefits could select for
colonies. Once formed, colonies could be ex-
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consistent with the Information Center hypothesis.This interpretationmeansthat information transfer at colonies, if it occurs (and
iour
--.
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mary function of heron coloniality. Colonial
I thank John Warham and the Department of ZoolWaterbird Group, 4th Annual Meeting, Ottawa.
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Pp. 10-11 (abstract).
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tispecies flocks in Barkley Sound, British Cofrontthe NaturalSciences
and EngineeringResearch
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neurobiology3: social behavior and communi-
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Roost microclimate, huddling behavior, and
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partitioningof parentalbehavior in Barn Swallows;
G. ThomasBancroft,Energeticsof postnatalgrowthin
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flycatcherscoexistingin an easterndeciduousforest;
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Kite (Rostrhamus
sociabilis);
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Brown, Cooperationand competitionin the Mexican
Jay;Robert N. Buchsbaum,Effectsof phenolicacids
andnutrientinputson the feedingecologyof geesein
saltmarshes;TerenceA. Burke,Evolutionarygenetics
of the House Sparrow, Passerdomesticus,
in northern
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colorand behavior of orioles(genusIcterus);William
Carmen, Juvenile dispersal, flocking behavior, and
habitat use in the California Scrub Jay (Aphelocoma
coerulescens
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Michael D. Carter, Socialorganizationand parasitic
habits of breeding Bronzed Cowbirds (Molothrus
aeneus);Martha Leah Chaiken, Starling colony on
groundsof Stroud'swater researchcenterin Avondale, Pa.; Robert L. Curry, Evolution and ecologyof
communal breeding in Galapagos Mockingbirds;
Veronique Delesalle,Mate choiceand reproductive
biology of the House Finch, Carpodacus
mexicanus,
with emphasison the role of male plumage variabil-
ity; TodRobertDeLong,Nocturnalbehaviorof nesting Long-earedOwls (Asiootus);David F. DeSante,
Stability and dynamicsof a subalpinebreedingbird
community; Jennifer Shopland Dillon, Effect of
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warblers (Parulidae);StefanDontchev, Geographical
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and egg mimicry in the Brown-headedCowbird;
LeonardA. Freed, Evolution of clutchsize in a tropical
passerine;DouglasC. Gayou, Socialbehavior of the
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BradleyM. Gottfried,Effectof eggsequence,clutch
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Gary R. Graves,
and aggression
in nectarivorous
Maul Drepanidids; House Sparrows(Passerdomesticus);
(continuedon p. 57)