Ornithological Monographs 47

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Ornithological Monographs No.47 Parent-Offspring Conflict anditsResolution in the European Starling Elizabeth Litovich and Harry W.Power PARENT-OFFSPRING AND CONFLICT ITS RESOLUTION IN THE EUROPEAN STARLING ORNITHOLOGICAL MONOGRAPHS Edited by NED K JOHNSON Museum of Vertebrate Zoology & Department of Integrative Biology Life SciencesBuilding University of California Berkeley, California 94720 Ornithological Monographs, publishedby the American Ornithologists' Union, hasbeen establishedfor major paperstoo long for inclusionin the Union's journal, The Auk Publication has been made possiblethrough the generosityof the late Mrs Carll Tucker and the Marcia Brady Tucker Foundation, Inc Correspondence and manuscripts proposed for publication should be addressedto the Editor at the above address.Style and format should follow those of recent issues Copies of Ornithological Monographs may be ordered from the Assistant to the Treasurer of the AOU, Max C Thompson, Department of Biology, SouthwesternCollege, 100 College St., Winfield, KS 67156 Price of Ornithological Monographs 47: $11.70 prepaid ($10.50 to AOU members).Add percent(minimum $2.00) handlingand shippingchargein U.S., and 20 percent (minimum $3.00) for all other countries Make checks payable to American Ornithologists' Union Library of CongressCatalogueCard Number 92-72325 Printed by the Allen Press, Inc., Lawrence, Kansas 66044 Issued June 2, 1992 Copyright ¸ by the American Ornithologists'Union, 1992 ISBN: 0-935868-58-5 PARENT-OFFSPRING CONFLICT ITS RESOLUTION EUROPEAN AND IN THE STARLING BY ELIZABETH LITOVICH and HARRY W POWER Department of Biological Sciences Rutgers University P.O Box 1059 Piscataway, N.J 08855-1059 ORNITHOLOGICAL MONOGRAPHS PUBLISHED THE AMERICAN BY ORNITHOLOGISTS' WASHINGTON, 1992 NO 47 D.C UNION FOR EL's children, Jacqueline, Michelle and Jon Alexander Torres and HWP's child, Rebecca Power in the ferventhopethat they will be like goodlittle starlingsand not developways to deceive or coercetheir parents into capitulating to their demands TABLE OF CONTENTS CHAPTER BASIC CONCEPTS AND GENERAL METHODS CHAPTER COMMUNICATION BETWEEN PARENT AND BROOD 1 THE EXPERIMENTS 12 DESIGN 12 RESULTS 13 DISCUSSION 15 CHAPTER COMMUNICATION BETWEEN PARENT AND INDMDUAL OFFSPRING 19 THE EXPERIMENTS 19 DESIGN 19 RESULTS 20 DISCUSSION 23 CHAPTER THE SPECIAL CASE OF RUNTS 31 THE EXPERIMENTS 31 DESIGN 31 RESULTS 31 DISCUSSION 36 CHAPTER AN EXPERIMENTAL SIMULATION OF OFFSPRING 1VIANIPULATIoN 39 THE EXPERIMENTS 39 DESIGN 39 RESULTS 40 DISCUSSION 42 DELAYED FLEDGING AND OFFSPRING VICTORY 42 THE VARIABILITY CHAPTER OF EXTENDED CARE 44 THE RESOLUTION OF PARENT OFFSPRING CONFLICT 47 SUMMARY OF I•ELEVANT RESULTS 47 THE PARENT-OFFSPRING CONFLICT DEBATE: AN OVERVIEW THE HONESTY OF BEGGING AS A SIGNAL SIBLICIDE AND THE BEST BEGGAR RULE A SIMPLE TEST TO DETERMINE WHO WINS IN PARENT-OFFSProNG CONFLICT WHO WINs IN PARENTOFFSPRING CONFLICT IN STARLINGS? CHAPTER 58 60 61 62 GENERAL SUMMARY 65 ACKNOWLEDGMENTS LITERATURE 48 CITED 67 68 LIST Figure OF FIGURES Communication between a parent and its brood: feedings/ hour Communication between a parent and its brood: fecal sac removals/hour 15 Communication between a parent and individual offspring _ 21 The feeding successof "best beggars" 22 Sexual dimorphism in parental feeding during the nestling period 23 Feeding of runts vs feedingof their largestnestmate,per observation period 32 Feeding of runts when their nestmateswere soberor drunk 33 Weights of runts compared to their larger nestmates 34 Weights of runts compared to the lightest of their older nestmates 10 35 Projected fledging weights of runts compared to the nest averagefledgingweight 38 LIST Table 14 OF TABLES Manipulation and communication Phenotypic vs genotypicconflict Ethogram of nestlingbeggingbehavior 20 Hatching vs fledgingpattern 36 Nestling weights (g) 37 Fledgingtime vs length of extendedcare 41 Weight changesof isolated vs restricted nestlings 42 Feeding frequencies 43 CHAPTER BASIC CONCEPTS AND GENERAL METHODS A common feature of human experienceis conflict between parents and children Analoguesof human parent-offspringconflict abound in other sociallyadvanced species,particularly among birds and mammals Familiar examples include the common weaning conflictsof mammals and the parallel termination of feeding conflicts of altricial birds Trivers (1974) appearsto have been the first to have made a full-scale effort to understand intra-family conflict in evolutionary terms He singled out parents and offspringas very special actors in genetic conflict He argued that because each parent passeson only one half of its genetic endowment to each offspring, what is in the genetic interests of the parent is not necessarilyin the genetic interestsof the offspringand vice versa He also speculatedthat offspringcould be expectedto win some of the time, if they have the ability to manipulate their parents Manipulation could occur through either coercionor deception(Table 1) Manipulation of offspringthrough coercioncould commonlybe done by parentsas in the casesof geneticsex determination and hatchingorder; in both cases,offspringare presentedwith afait accompliand have no ability to changethe outcome of the parental action Manipulation of parents through coercionby offspring would seem to be difficult outside Homo sapiensbecausein most other species parents appear to control all the essentialresources(but see social insectsbelow) Manipulation through deceptioncould occur in many speciesand be done by either parentsor offspringprovidedthat they couldcommunicatewith eachother Parents have more opportunities for manipulation (3 out of possibilities in Table 1) than offspringhave (only out of possibilities)becauseparentscontrol resources Deceptionis the moreinterestingform of manipulationbecause with deception both parent and offspringcan be proactive as well as reactive parties, i.e., both can initiate action as well as respondto the actionsof others.This symmetry between parent and offspring,and between action and response,should permit a greaterrangeof strategiesand tacticsby both parties.The power of offspringto deceivetheir parents(and sogainmorethan their fair shareof parentalresources) liesat the heartof the Trivers-Alexanderdebateoverwhowinsin parent-offspring conflict.We will review that debatein somedetail in Chapter6 but only briefly summarize it now Trivers (1974) used mostly weaning conflict examplesfrom the literature as evidenceof his view that offspringcan win in at least a partial sense(e.g., a compromisebetweenparent and offspring),and he left open the possibilityof total offspringvictory on a more than rare basis Alexander (1974) took the positionthat offspringvictory is highly unlikely in an evolutionary(as opposed to a short-term phenotypic) sensebecause(1) parentscontrol essentialresources and can withhold benefitswhen offspringdo not meet parentalexpectationsand (2) today'swinning offspringis tomorrow'slosingparent The secondobjection is potentially the more damagingbecausealthoughparentscontrol resources, offspringcould conceivablymanipulateparentsinto believingthat offspringare meetingparentalexpectations Later Alexander(1979) madeit clearthat he agrees ORNITHOLOGICAL TABLE MANIPULATION MONOGRAPHS NO 47 AND COMMUNICATION Manipulation via Communication Coercion A Present Absent Deception Parents p2 P p N B Offspring Present Absent N N P N From Litovich (1982) p = Possible;N = Not possible that offspringcanwin in intergenerational time underspecialcircumstances, but •nly rarely.ThustheTrivers-Alexander debateis abouthowfrequently offspring victory can occur, not whetherit can occur.This point seemsgenerallymisunderstoodby commentatorson the subjectwho have caricaturedAlexander's(1974, 1979) positionmore than they have addressedit (seeThe Parent-Of/•pring Conflict Debate in Chapter 6) Most commentariesand modelshave sidedwith Trivers (1974) However, none of them provides a clear-cut empirical test to determine who wins in parentoffspringconflictin real speciesliving in real environments.Thus thesetreatments cannot be usedto determinethe frequencyof offspringvictory We addressthat shortcoming in Chapter6 by proposing a simpleempiricaltestandapplying it to our results We believe that it is important to understanda fact mentioned in few theoretical treatments,i.e., that the existenceof phenotypicconflictis itself not sufficientto prove the existenceof genotypicconflict of interest (Alexander 1974; Anderson 1989) Weaning conflictsare examplesof this concept.They could as easily represent a "discussion"between parent and offspringover the offspring'strue needs as they could representa condition of true hostility basedon the degreeof genetic dissimilarity betweenparent and offspring(R D Alexander, personalcommunication) Thus a distinctionbetweengenotypicand phenotypicconflictis in order Table showsall possibleformal casesof suchconflict and agreement 1) Phenotypic and genotypic conflict could simultaneouslyoccur between a parent and its offspringwhen they each have their own geneticbest interestsat heart In such a case, they might be expected to behave in a manner that would have the effectof increasingtheir own personalfitness,even fithis is at the expense of loweringthe other'spersonalfitness(Table 2, Type A) 2) Genotypic conflict and phenotypicagreement(Table 2, Type B) could occur as in the caseof altruisticoffspringmeetingparentalexpectationswithout regard for their own geneticself-interestand becauseof that losingto more selfishoffspring.This situation could be expectedto be uncommon becausetheseindividuals would be selectedagainst Another example would be the casewhere parents coercetheir offspringinto submittingto their demandseven thoughit is not in the offspring'sgeneticselfinterest to so (R D Alexander, personal communication) This might be common in nature PARENT-OFFSPRING CONFLICT IN STARLINGS TABLE PHENOTYPIC VS GENOTYPIC CO1N-FLICT Type Genotypic PhcnotYl•ic A Conflict Conflict B Conflict Agreement C Agreement Conflict D Agreement Agreement Example Offspring fails to persuadeparent to give it more food than the parent is favored for giving Offspring deceivesparent into giving more food than the parent is favored for giving Offspringbegseventhoughit is so weakthat a meal will not save it and it has healthy sibsthat would gain from the same meal Offspring is fed by a parent favored for feedingit Adapted from Alexander (1974) and Litovich (1982) 3) Phenotypicconflictcould occurbetweena parent and its offspringeven when their geneticinterestscoincide(Table 2, Type C) Failure of communicationcould be the cause of such an event as when offspring continue to beg despite their parents' efforts to silence them because a predator is near Another example would be the case where a parent feeds the wrong kind of food just to silence beggingoffspring (Tinbergen 1981) 4) Simultaneousgenotypicand phenotypicagreementcould occurwhen both parties gain from cooperation (Table 2, Type D), e.g., when young obey their parents' calls to be quiet as a predator approaches.Intuitively, this would appear to be the most common case More than anything else, Table showsthat parent-offspringconflict is not a simplistic phenomenonthat can be fully understoodby purely theoretical treatment It needs an empirical treatment to demonstrate the full array of parentoffspringinteractions and their most reasonableinterpretations In this monograph we seek to reduce the confusion surrounding the subject of parent-offspring conflict by reporting the results of a series of experiments on European Starlings(Sturnus vulgaris).This is an appropriate study speciesfor several reasons: 1) It is an outbred, sexuallyreproducingspecies.Only a few (n < 10) of the 1,000+ nestlings we have banded have returned to our study area to breed, and none of these has mated with a known relative This outbreeding createsgenetic dissimilarity betweenparent and offspring,settingthe stagefor genotypicconflict of interest 2) It has altricial young subjectto starvation during the nestlingperiod This createsfrequent opportunities for phenotypic conflict over the timing and amount of food delivered to nestlings 3) Potential conflictsof interestbetweenparentsand offspringare exacerbated by asynchronoushatching This createsa frequently lethal competition for food betweenthe last hatchednestling(hereafterthe "runt") and its older synchronously hatched nestmates(Stouffer and Power 1990, 1991; this study) It also createsa direct conflict of interest between the runt and the parent in that the parent dispensesfood by a rule that disfavorsthe runt, threateningits life (Chapter 4) One of the most powerful conflicts of interest has to be between individuals that disagreeover the value of one of the contestant'slives to the point that one allows the other to die [Although it is true that situations can exist where the parent 58 ORNITHOLOGICAL MONOGRAPHS NO 47 the termination of feeding The evidence must show that (1) parents decrease their investment and that the reductionof food given chicksis not just a reflection of lower food availability; (2) offspringattempt to increasethe level of parental investment; (3) the decreasein investment is costly to offspring;and (4) parents would incur a cost if they did not decreaseinvestment Graves et al (1991) found compellingevidencefor the first requirementsbut not the fourth, preventingthem from proving that bona fide geneticconflict exists in their population However, they clearlybelieve that suchconflict occurs.Their lack of good evidence for the fourth requirement is not troubling becauseannual adult survivorship is normally so high (0.9-0.93) that even large percentageincreasesin mortality consequentto continual high-level parental investment would be very difficult to measure Burger's(1980, 1981) resultson Herring Gulls in New Jerseywerevery different from Graves et al (1991) She found no evidence of a "weaning" conflict in her population A possibleanswer for the different results in the Scottish and New Jerseypopulations is relative food abundance.The New Jerseypopulation had exactly twice the fiedging successof the Scottish population, suggestinggreater food availability in New Jersey.If food were sufficientlymore abundantin New Jersey,then parents might not have been food stressed,allowing them to accede to offspringdemandsfor a longer period of time without damageto themselves This chronologicaloverview showsthat while models can be constructedthat allow any number of possible outcomes,with few exceptionsthey cannot specify which will occur in which speciesor how to recognize them in nature Models are often useful predecessorsfor empirical studiesbut never substitutesfor them (Stamps and Metcalf 1980) Despite the obviousnessof this conclusion,there often appearsto be an acceptance of the idea that models can decide empirical questions The literature of the parent-offspringconflictdebatesometimeshasthis flavor, e.g.,in an otherwise balancedreview, Clutton-Brock (1991:203) claims that the qualitative prediction from the various mathematicalmodels"that solicitationby offspringshouldcause parents to exceed their optimum level of investment and reduce parental fitness" is upheld in birds and exemplifiedby the feedingbehavior of Budgerigars(Stamps et al 1985, 1987; reviewed above) In fact, that prediction is not upheld because it has not been shown in Budgerigarsor any other speciesthat adults are in anyway hurt or that their feedingbehavior is controlledby offspring.We repeat:empirical questionscan only be answeredby empirical means This overview also showsthat empirical work on parent-offspringconflict in birds (outside the study of siblicide see below) is still in its infancy Too few studieshave been performed to allow any generalizationbeyond the statement that no real evidence of offspringvictory has as yet been presentedin birds No robust generalizationswill be possibleuntil a generally applicable empirical test is developed and widely adopted that enables determination of who wins in parent-offspring conflict We will propose such a test below THE HONESTY OF BEGGING AS A SIGNAL As noted in Chapter 1, offspringcan potentially win in parent-offspringconflict only by coercingor deceivingtheir parents into giving them more than is in the parent's best interest to give In altricial birds coercion during the nestling period PARENT-OFFSPRING CONFLICT IN STARLINGS 59 is impossible,leavingonly deceptionas a ploy That fact bringsup the question of the honestyof beggingas a signal.Is begginga true signalof offspringneed,a deceptive trick, or some combination of both? As noted in Chapter 2, some biologistsbelieve communicationis fundamentallydeceptiveand we accepted this view with reservations.However, we argued in Chapter that when the parent chooseswhich offspringto feed on the basisof a bestbeggarcompetition, it cannot be deceived,at least not over a seriesof best beggarcompetitions.If we are right, then honest offspring would be favored over deceptive ones because both would gain the samefood reward from the parent, but the latter would pay both a suppressioncost from the parent and a potential predation cost Honesty in this contextmeansbeggingonly when genuinelyhungryand stoppingbegging as soon as fed As noted in Chapters and 2, Trivers (1974), Dawkins (1976), and Dawkins and Krebs (1978) all believed that dishonestycould be successfulin offspring, and human experiencesuggeststhat it may be More recently, Dawkins and Guilford (1991) consideredthe meaning of "honesty" and gave five contextual definitionsfor it Generally speaking,they believe that honestywill be corrupted, i.e., that somelevel of cheatingwill coexistwith truth They call this coexistence, "conventional signalling." Dawkins and Guilford (1991) believe that deception is rife not becauselying always has high rewardsbut because"probing" for the truth by the receiverhas costsfor the receiver The higher these costs,the greater the opportunityfor lying However, Dawkins and Guilford (1991:870) admit that there can be cases(their context #2) where signalsare "necessarilyand reliably related to somethingthat is worth predicting."We believe that offspringbegging is such a casebecausethe offspring'scondition is important to both the parent and the offspring.Beggingshouldbe honestby Dawkins and Guilford's (1991) reasoningbecausethe costof"probing" shouldbe cheapfor the parent In a best beggarcompetition all the parent has to is to wait a few extra secondsin order to seewhich offspringis the bestbeggar;this shouldnot appreciablyraiseits costs in termsof"energy, time or risk" (Dawkinsand Guilford 1991:865),and therefore the parent has the advantage Not all workers in communication believe that signalling is fundamentally dishonest.Zahavi (1977, 1981, 1985, 1991) seemsto be the leader of this school He hasarguedforcefullyfor yearsthat a costlysignalis inevitablyan honestsignal His reasonis that parentswill be disfavored from respondingto communication that is not costlybecausethe offspringwill incur no significantpunishmentfor lying under that circumstance.But parents will be favored for respondingto communication that is costly for the offspring to make becausea high cost to benefit ratio automatically forcesoffspringto communicateonly what they truly needwhen they truly needit In a generalsense,Zahavi's theory of costlysignalling has been upheld by a model of Grafen (1990) Perhapsthe most convincingmathematicalanalysisof the honestyof begging is found in Godfray (1991) He modelledthe casewhere the parent hasonly one offspringper year and beggingis expensive.The resultwasa parent-winsoutcome with the parent giving the offspringonly what it neededand the offspringasking only for that Godfray (1991) recognizedthat his resultis more in agreementwith Alexander (1974) than with Alexander's critics If we acceptGodfray's(1991) resultthat beggingis honest,we are still confronted 60 ORNITHOLOGICAL MONOGRAPHS NO 47 with the problem of how the parent should distribute food among several simultaneouslybeggingoffspring.The fact that all were honest(in the sensethat none beggedunlesshungry) would not be particularly helpful becausethe parent would still have to decide how to allocate the food Allocation on the basis of comparativehonestyis clearly not done If it were, then starling runts would be fed preferentially since their need is greatest,and the largest,most aggressive nestlingswould be discriminatedagainstbecausetheir need is least.Yet we know that the starlingparentfeedson the basisof the bestbeggarrule (Relevant Result #2) This implies that honestyis important to the parent only to the extent that no beggingoffspringis without need Comparativereproductivevalue appearsto be what drives the parent's decision when confronted with a brood of honest beggarseven though this often resultsin the parent not feedingyoung according to their relative need Godfray (1991) discussedthe casewhere brood size was larger than one but did not formally model it Hence his remarks on that caseare necessarilyspeculative Our readingof Godfray (1991) is that he thinks the most adaptive parental responsewhen broods have more than one offspringwould be to feed young accordingto need only We disagree.If youngwere fed accordingto need only and there was insufficient food for all, the parent might wind up feedingequal but insufficientamountsto eachof its equallystarvingyoung.Undersuchcircumstanc esthe parentwould be better off feedingaccordingto the bestbeggarrole becausethat would guarantee that the parent investedonly in the most competitiveyoung We resolve our resultswith the theoretical literature of beggingby postulating a 2-level evolutionary process.On the first level, offspringare favored for begging honestly becauseparental hesitation to feed and predation pressuremake deception costly On the second level, the parent chooseswhich of several honestly beggingoffspringto feed on the basis of their comparative reproductive value rather than their comparative need The result is that the parent fledgesonly the offspringmost likely to give it grandoffspring althoughthat might frequentlycome at the expenseof offspring(especiallyrunt) mortality Thus by feedingaccording to reproductivevalue insteadof need, the parent often tradesquantity for quality SIBLICIDE AND THE BEST BEGGAR RULE Siblicidecan be thoughtof as the consequence of the bestbeggarrole taken to its extreme Instead of the parent choosingwhich offspring to feed on the basis of a non-lethalcompetition, the parentrigsa lethal competitionamongits offspring by hatching them asynchronouslyand abides by the result This "laissez-faire policy" resultsin parental investmentaccordingto the relative reproductivevalue of offspring(Mock 1987) Becausesomeyoungkill others,siblicideinitially seemslike a form of offspring victory in a dispute between parent and offspring over clutch size (Parker and Mock 1987) However, theoreticalanalysesfrom O'Connor (1978) on,'all conclude that siblicidein birds is not necessarilythe victory of largeroffspringagainsttheir parents and smaller siblings;it can be to the advantage of all parties, including those that die (Godfray and Harper 1990; Godfray and Parker 1991) This is becausethe inclusive fitness interests of all parties can be better served by the fledgingof a smaller number of highly competitive offspringthan by the fledging PARENT-OFFSPRING CONFLICT IN STARLINGS 61 of a largernumberof weaklingswith poor prospectsof survivaland victory in reproductive competition In comparingoffspringdeath in starlingsand siblicidalspecies,we note that the fate of starlingrunts is both differentfrom and similar to the fate of runts in truly siblicidal species.[These include a variety of relatively large, long-lived, birds with clutchesof two or three, e.g., some raptors, bee-eaters,boobiesand herons(Mock et al 1990).] A number of carefulstudiesof thesespecies(Drummond et al 1986; Drummond 1987, 1988; Mock 1987; Anderson 1990; Bryant and Tatner 1990; Mock et al 1990) have shownthat runtsdie not from starvation due to poor competitiveability (asdo starlingrunts)but ratherfrom beingevicted from their nestsor peckedto death Mock et al (1990) have summarizedthe literature of siblicideand reported four elementsnecessaryfor siblicideto be a regularfeature of nestlinglife: resourcecompetition, provision of food in small units, weaponry, and spatial confinement Starling nestlingshave no weaponry Thus it is not surprisingthat starlingrunt mortality occurswithout overt killing Nevertheless,the effect is the same though the causeis lessdramatic, and the timing isaspredictedby O'Connor(1978) for species with regularbroodreduction whetheror not they are siblicidal,i.e., early in the nestlingperiod (seeFig in Stouffer and Power 1991) A SIMPLE TEST TO DETERMINE WHO WINS IN PARENT-OFFSPRING CONFLICT We believe that there is an empirically simple way to determinewho wins in parent-offspringconflict We provide it here and apply it to starlingsin the next section The winner in parent-offspringconflict can be establishedby: 1) Discovering the rule(s) by which PI is dispensed; 2) Deciding who wins when the rule(s) is obeyed; and 3) Determining whether the party that loseswhen the rule(s) is obeyed can break the rule(s) in order to win Winning and losingare ideally measuredin terms of fitnessor at leastof the relative reproductive successof the contestantsbut that will usually be difficult and often impossible.A simplermethod is to keeptrack of resourcecontrol.This assumesthat the party that controlsthe resource(s) automaticallywins;it is difficult to imagine that controllingresourcescould result in consistentlylosing Supportfor the ideathat resourcecontrolisthe keyto victory in parent-offspring conflictcomesfrom very differentkinds of study.The first is the studyof Clark and Ydenberg (1990b) reviewed above As noted, their results show parental victory Parents control the critical resource,food, in their study species,the Dovekie Offspringhave little choicebut to fledgewhen their parentsstopfeeding them after they have attained fiedging size The secondkind of study is the investigationof parent offspringconflict in social insects.This investigationhas tested the genetic relatednesshypothesis formalized by Trivets and Hare (1976) but based on Hamilton (1967) Briefly stated,that hypothesisholds that, in haplodiploid specieswith effectivemonogamy and eusocialcolony structure(sensuWilson 1971), (1) sterileworkersshould prefer to invest times more in fertile sistersthan in fertile brothers becauser = 3/4between workers and sisters,but r = 1/4between workers and brothers, i.e., 62 ORNITHOLOGICAL MONOGRAPHS NO 47 sistersare times more valuable than brothersin geneticrelatedness,but (2) queensshouldpreferthat workersinvest equallyin their fertile brothersand sisters becausequeensare as closelyrelated to their sonsas to their daughters(r = 1/2in both cases).Thus a fundamentalgeneticconflictexistsbetweenqueensand their worker daughters.Despite seriousdeficienciesin Trivers and Hare's (1976) development of the geneticrelatednesshypothesis(Alexanderand Sherman 1977), a growingbody of evidence(reviewedin Oster and Wilson 1978; Nonacs 1986) supportsthe view that worker investmentis heavily weightedin favor of fertile sisters,suggestingat least a worker dominated compromise.Perhapsthe most direct supportfor the hypothesiscomesfrom Mueller's (199 l) experimentalmanipulation of r in the bee Augochlorellastriata He manipulatedr by removing or not removing foundressqueens(r = betweenworkersand their dependents in the former case,and r > in the latter case),and found that the sex ratio was more female-biasedwhen queenswere not removed in keepingwith the genetic relatednesshypothesis.This studyis particularlyimportant becauseit getsaround one of the fimdamental difficultiesof the geneticrelatednesshypothesisas originally stated That difficulty is that the asymmetry in worker investment was believedto rely in an exact senseon the asymmetryin r values,and this required that queensbe monogamousotherwise the asymmetry in r values would decline In fact, monogamy is not always a valid assumption in eusocial insects, e.g., Blanchetot(1991) found 11 differentpatrilinesin the brood of a singlehoneybee (Apis mellifera) Mueller's (1991) study getsaround the problem of polyandrydepreciatedr values by making asymmetry in r an absolutematter, i.e., r = or r > with closer relatedness between workers and fertile sisters than between workers and brothers in the latter case even if the difference in relatedness is < 3- fold Thus what countsis not the exactmagnitudeof the asymmetrybut the simple fact of the asymmetry Of critical importance to judging the value of our empirical test for determining who wins is the fact that workers physically control most resourcesin eusocial insects,not the queensthat are totally dependent upon them Thus the critical assumptionof our test, control of resourcesbestowsvictory, is supportedby both the parent-wins outcome in Dovekies and the offspring-dominatedoutcome in eusocial insects WHO WINS IN PARENT-OFFSPRING CONFLICT IN STARLINGS? Application of the simple test to European Starlingsleads to the conclusion that parentswin in parent-offspringconflict during the nesfiingperiod: l) The rule by whichPI is dispensed:parent starlingsdispensefood to the best beggarsand hole-blockers 2) Who wins whenthe rule is obeyed:parentswin becausedistributingfood to the best beggarsand hole-blockersassuresthat parents invest only in the most vigorousoffspringand henceonly in thosemost likely to reproducesuccessfully This always promotes the parent's interestsbut does not always promote the offspring'sinterests The parent's interestsare always promoted becauseonly the best possibleinvestmentis made at any givenfeeding.No long-termbad investmentis ever made becausesuccessful deceitby unworthyoffspringis not sustainable(RelevantResult #2) PARENT-OFFSPRING CONFLICT IN STARLINGS 63 The offspring'sinterestsare not alwayspromoted becauseit is fed only when it complies with the parent's rules even though it would benefit from being fed when-itdoesn'tcomplywith the rulesaslongasits chances of fledging wereso good that it was not favored for "suicide" (sensu O'Connor 1978) That even starlingrunts are seldom favored for "suicide" is shownby the resultsof Stouffer and Power (1991) who found that experimentally synchronizedbroods had significantly higher fledgingsuccessthan normally asynchronousbroodsof the same startingsize Synchronizedbroodsrequire more food becausethey have no runts and therefore fewer youngthat die in the first few days of the nestlingperiod and more young that consume food over the entire period 3) The loserwhenthe rule is obeyedcannot win by breakingthe rule: offspring cannot force their parents into changingthe rules of food dispensationbecause they not have the physicalmeans to so, and deceit does not work On an intrabrood basis, deceitful offspring (those pretending to be the best investment when they are not) might sometimesbe fed when it would be in the parent's best interest to feed other offspring, but it appears impossible that they could successfullysustain their deceit over the entire nestling period (Relevant Result #2) From a parental perspective, the motivation behind beggingis irrelevant (i.e., whether the offspringintends to deceive or tell the truth) becausethe only thing that countsis the investmentpotentialof the offspring.If that potential is revealed as clearly when the offspringlies as when it tells the truth, the parent cannot be deceived, and what a deceitful offspring"intends" as a lie reducesto exaggerationharmless to the parent's interests Nor can offspringattempts at deceit result in a compromisebetween parental and offspringvictory on an intrabrood basis.This is simply becausethe parent's interestsare not compromisedby attemptsat lyingby offspring,and the offspring's interestsare not promoted sinceit ultimately tells the truth anyway In theory, attempts to break the best beggarrule could result in either offspring victory or a compromise between parental and offspringvictory on an interbrood basis.This is becausethe brood asa whole couldtheoreticallydeceivetheir parents into giving them more than they were worth as a collective investment Such deceptionwouldwork throughthe broodappearingto be betterbeggarsthan they really were compared to some hypothetical other or future brood However, it is difficult to seehow this could work in practice.Even ifa particular brood were not as good an investment as some other brood might have been or some future brood might be, it is the only brood the parent has to invest in Thereforethe parent'sinterestsare not hurt by investingin the presentbrood so long as it doesnot exceedsome thresholdamount of PI that resultsin negative interbroodeffects.Despite effortsto find sucheffectsin our population,Power et al (1989) and Stouffer (1989) could not find them (see Relevant Result #1) Thereforewe concludethat the parent'sinterestsare not injured at an interbrood level by followingthe bestbeggarrule or by (possible)offspringattemptsto break that rule Nor can offspringachievea total or compromisevictory by attemptingto extract more PI from their parentsthan the latter are willing to give by refusingto fledge on schedule.We experimentally simulated suchoffspringdefection but found that parents appearedto give no more PI than they normally would have given had offspringfledgedon schedule(Relevant Result #7) 64 ORNITHOLOGICAL MONOGRAPHS NO 47 Thus there appearsto be no way by which nestlingstarlingscan successfully break the best beggarrule that generatesparental victory, or impose a new rule -oftheirown(delayed fledging) thatcouldpotentially allowthemto gainat their parent'sexpense.This leavesparentalvictory as the most parsimoniousexplanation of the pattern of parent-offspringconflict seen in starlingsduring the nestlingperiod CHAPTER GENERAL SUMMARY Parent-offspringconflict has been the subjectof controversyand investigation among biologistssincethe Trivers (1974)-Alexander (1974) debate brought it to the attentionof a largeaudience.This debatefocuseson the questionof who wins in parent-offspringconflict Unfortunately, many of thesepublicationsare purely theoretical and so provide neither legitimate evidence nor empirical tests with which to decide who wins, how often, under what circumstances,by how much, and with what consequences These questionscan only be answeredby empirical, ideally experimental, studies This monograph reports the results of a series of experiments on European Starlings nesting on the Livingston College Campus of Rutgers University in Piscataway,N.J Starlingsare suitable study subjectsfor several reasons,most importantly that there are groundsfor genuineparent-offspringconflictin them Closed-circuit television and speciallyconstructedviewing boxes were used to monitor events within nests.Experimental subjectswere individually marked Using the experimentaltreatmentsof(a) supplementalfeedingand (b) drugging nestlingsby meansof raisins soakedin alcohol, we show (1) that parental feeding visits (but not fecal sac removals) increasewhen broods are sober/hungryand decline when broods are drunk/sated, and (2) that individual nestlingsare fed more when they are sober/hungrythan when they are drunk/sated Parental feeding is stimulated by nestlingbegging.Nestling begging(as measuredby an ethogram that incorporates alertness, posture, movement, vocalization and relative effort) declineswhen nestlings(either as broods or individuals) are drunk/sated and rises when they are sober/hungry The first result demonstrates communication between parent and brood, and adds starlingsto the growinglist of specieswhere suchcommunication has been shown The seconddemonstratescommunication between parent and individual offspring.Individual communicationis a prerequisitefor offspringvictory through deception of parents Parents preferentially feed the "best beggar." This is the nestling that chirps the loudest, gapes the widest, stretchesits neck the longest, and flaps its wings the most relative to its nestmates at the time of a feeding visit By feeding the best beggar, parents invest in the most robust offspring, the ones most likely to survive and henceto reproduce,i.e., the oneswith the highestreproductivevalue Although individual communication createsthe hypothetical opportunity for offspringto usedeceptionin order to get more than is in their parents'bestinterests to give them, their parents' preferencefor best beggarsmakes successful,sustained deception almost impossible This is becausethe truly best beggarwill be obvious in time while a faker will collapsefrom exhaustion.Thus offspringinevitably tell their parents the truth about their own investment potential in a best beggar competition What may start out as offspringdeception ends up as mere exaggeration harmless to the parent's interests Intensebeggingby nestlingsis usuallyattributed to nestmatecompetition However, our discoverythat parentsprefer bestbeggarssuggests that beggingintensity is more a product of parental selectionthan offspringcompetition If so, then occasionallossof broods to predators attracted to nestsby loud, frequent begging 65 66 ORNITHOLOGICAL MONOGRAPHS NO 47 calls is the price parents pay for selectingfor best beggars,and it applies counterselectionon parental preferencefor best beggars Best beggar competition takes a new form late in the nestling period At that time, the nestlingsuccessfully blockingthe entranceand extendingits headthrough the hole is the one that usuallygetsfed Feedingthe hole-blockereffectivelymoves the operationaldecisionabout whom getsfed from the parent to the offspring Yet this has the same effect as feedingthe best beggarbecauseonly the most vigorous offspringcan succeedin either tactic We looked for evidence of sexualdimorphism in parental feeding behavior but found none Apparently male and female parentsusethe samecriteria for deciding which young to feed This eliminates the opportunity for offspringto play one parent against the other by meeting some of the criteria of each parent but not all of both Despite the greaterneedsof runts due to their poor competitive abilities, parents give them no specialtreatment Runts alsomust be the bestbeggaror hole-blocker at the time of a parental feedingvisit in order to get fed Nevertheless,more than half of all runts in this study fledged,and did so at about the sameweight as the lightestof their oldernestmates.Runtsachievedthisby delayingtheir own fledging until after their older nestmateshad fledged, and used the added time in the nest to get extra feedingsby playing the role of best beggar(or hole-blocker)without competition Yet the total time in the nest was no greater for runts than other nestlings, and so there was no reason to believe that runts got more than their fair shareof parentalresources.Runtspersistedonly by submittingto their parents' rules for the dispensation of food The fact that runts can delay fledgingby a few days in order to garner more parental feedingsled us to wonder whether offspringcould use delayedfledging as a tactic to win in parent-offspringconflict We simulated such offspringmanipulation by experimentally preventing fledging at the usual time by placing restrictors over entrance holes These allowed parents to feed nestlingsbut kept young from fledging Although parents did feed restricted young for an average of an additional 3.52 daysbeyondthe expectedcut-offpoint for nestlings,restricted young lost weight and were fed less frequently than they had been during the normal nestlingperiod Moreover, parents clearly spent less effort feeding restricted young than normal nestlings,and their period of extended care did not exceedthe normal period of post-fledgingcare for various starlingpopulationsin North America and Europe Thus there is no reason to believe that offspring would gain throughdelayedfledging.Nor is there any reasonto believethat parents would loseunlessthe delay in fledgingprecludedtheir having a successfulsecond brood Even if parentsdid losein this manner, offspringvictory would not ensue becauseoffspring would have achieved nothing more than depriving themselves of secondbrood siblings A strikingresult of the delayedfledgingexperimentwas the variability in the length of extended care (0-20 days) We speculatethat the novelty of restriction may have causedparents to choosedifferent responsesbecausethere was no pattern of responsein their evolutionary past that would have consistentlygeneratedhigherfitness.Alternatively, differentresponsesmay have reflecteddifferent cost-benefitschedulesfor different individuals Whatever the causeof variability in response,extended care itself is known in several species,suggestingthat it PARENT-OFFSPRING CONFLI• IN STARLINGS 67 may be fairly common when parents"judge" that specificoffspringor broodsare worth extra care In order to resolve the questionof who wins in parent-offspringconflict, we summarize the resultswe believe to be most relevant (Chapter 6) We then use them in a discussionof the literature of the parent-offspringdebate.We find some of this literature to be insightful but far too much of it to be without value in understandingnature This is becausemuch of it is too abstractto make detailed, testable predictions, e.g., several authors predict a compromise between the parental investment optima of parent and offspring but give no clue as to how to recognize or measure any optimum in nature Without testable predictions and empirical measuresit is not possibleto resolve any of the issuesabout parentoffspringconflict with which we open this summary chapter In order to fill the need for an empirical method to determine who wins in parent-offspringconflict, we proposea simple 3-part test: 1) Discover the rules by which parental investment is dispensed 2) Decide who wins when those rules are obeyed 3) Determine whether the party that loseswhen the rules are obeyedcan break the rules in order to win Application of this simple test to our study population leads to the conclusion that parent starlingswin in parent-offspringconflict during the nestlingperiod This is because: 1) Parentsdispensefood to best beggarsand hole-blockers 2) Parents win when this rule is followed becauseit assuresthat they invest only in the most vigorous offspring and hence those most likely to reproduce successfully.This always promotes the parent's interests but does not always promote the offspring'sinterest The parent's interestsare always promoted because parents always make the best possible investment at any given feeding They never make a long-term bad investment becausesuccessfuldeceit by unworthy offspringis not sustainable(see above) The offspring'sinterestsare not alwayspromoted when it obeysthe rule becauseit is not fed when it may profit from being fed but is physicallyunable to be the best beggaror hole blocker 3) Offspring cannot win by breaking the rule becausethey cannot force their parentsto changethe rule and theycannotlongdeceivetheir parentsinto believing that they are abiding by the rule when they are not Deception reducesto exaggeration harmless to the parent's interests In closing,we find it deeplyunfortunatethat the parent-offspring conflictdebate wastrivializedin its earlyhistoryinto a purelytheoreticalconsiderationof which of two abstractions,parentor offspring,wins in someall-encompassing sense,and who was right in the most narrow sense,Trivers (1974) or Alexander(1974) Parent-offspringconflict is a far richer and more important topic than that, one with practicalas well as theoreticalconsequences Fortunately,the current trend is towardempiricalstudies(e.g.,Anderson1990;Clark andYdenberg1990b;this study) ACKNOWLEDGMENTS We thank Richard D Alexander,David J Anderson,Michael P Lombardo, Ned K Johnson,PhilipC StoufferandRonaldC Ydenbergfor 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R M Alison 1975.$2.50 No 19 Bird Populations of Aspen Forestsin WesternNorth America J A.D Flack 1976 $4.00 No 20 SexualSize Dimorphismin Hawks and Owls of North America.N F R SnyderandJ W Wiley 1976 $6.00 No 21 Social Organization and Behavior o[ the Acorn Woodpeckerin Central Coastal California M H MacRoberts and B R MacRoberts 1976 $4.00 No 22 Maintenance Behavior and Communication in the Brown Pelican R W Schreiber 1977 $3.50 No 23 SpeciesRelationshipsin the Arian GenusAimophila L L Wolf 1977 $7.00 No 24 Land Bird Communitiesof Grand BahamaIsland: The Structureand Dynamicsof an Avifauna J T Emlen 1977 $5.00 No 25 Systematicso[ Smaller Asian Night Birds Basedon Voice.J T MarshalL1978.$4.00 No 26 Ecologyand Behavior oœthe Prairie Warbler Dendroica discolor.V Nolan, Jr 1978.$15.00 No 27 Ecologyand Evolution of Lek Mating Behaviorin the Long-tailedHermit Hummingbird F G Stiles and L L Wolf 1979 $4.50 No 28 The ForagingBehaviorof Mountain Bluebirdswith Emphasison SexualForagingDifferences.H W Power 1980 $4.50 No 29 The Molt of ScrubJaysand BlueJaysin Florida G T Bancroftand G E Woolfenden.1982.$4.00 No 30 Arian Incubation:Egg Temperature,Nest Humidity, and BehavioralThermoregulationin a Hot Environment G S Grant 1982 $5.00 No 31 The Native Forest Birds o[ Guam J M Jenkins 1983 $6.00 No 32 The Marine Ecologyof Birds in the RossSea, Antarctica.D G Ainley, E F O'Connor, and R J Boekelheide.x + 97 pp 1984 $9.00 ($8.00) No 33 SexualSelection,Lek and Arena Behavior,and SexualSize Dimorphismin Birds.R B Payne.viii + 52 pp 1984 $8.00 ($6.50) No 34 Pattern,Mechanism,and Adaptive Significanceo[ Territoriality in Herr/rig Gulls (Larusargentatus) J Burger xii + 92 pp 1984 $9.00 ($7.00) No 35 Ecogeographical Variationin SizeandProportionso[ SongSparrows(Melospizamelodia).J.W Aldrich x + 134 pp 1984 $10.50 ($8.50) No 36 NeotropicalOrnithology.P A Buckleyet al., Eds.xi + 1,041pp., colorplates.1985.$70.00 No 37 Avlan Monogamy.P A Gowatyand D W Mock, Eds.vi + 121 pp 1985.$11.00($9.00) No 38 An Analysis of Physical Physlological,and Optical Aspectso[ Arian Coloration with Emphasison Wood-Warblers.E H Burtt,Jr x + 122 pp 1986.$15.00($12.50) No 39 The Lingual Apparatuso[ the A[rican Grey Parrot, PsittacuserithacusLinn• (Aves:Psittacidae):Descriptionand TheoreticalMechanicalAnalysis.D G Hornberger.xii + 236 pp 1986.$25.00($20.00) No 40 Patterns and Evolutionary Significance o[ Geographic Variation in the SchistaceaGroup o[ the Fox Sparrow (Passerellailiaca) R M Zink viii + 119 pp 1986.$15.00($12.50) No 41 Hindllmb Myology and Evolution o[ the Old World SuboscinePassefineBirds (Acanthisittidae, Pittidae, Philepittidae, Eurylaimidae).RobertJ Raikow.viii + 81 pp 1987.$12.50($9.50) No 42 Speciationand GeographicVariation in Black-tailedGnatcatchers.JonathanL Atwood vii + 74 pp 1988 $10.00 ($8.00) No 43 A Distributional Survey o[ the Birds o[ the Mexican State o[ Oaxaca.Laurence C Binford viii + 418 pp 1989.$40.00($36.00) No 44 RecentAdvancesin the Study o[ NeogeneFossilBirds.K JeffreyBickartand RobertM Chandler.vi + 161 pp 1990 $19.75 ($17.75) Nos.45 & 46 Descriptionso[ Thirty-two New Specieso[ Birds [rom the Hawaiian Islands:Part L NonPasseriformes (S L Olsonand H F James) 88 pp.;PartIL Passeriformes (H F Jamesand S L Olson) 88 pp 1991.Boundtogether(not availableseparately)$25.00($22.50) Order from: Max C Thompson,Assistantto the TreasurerA.O.U., Departmentof Biology,Southwestern College,100CollegeStreet,Winfield, KS 67156.Ordersmustbeprepaid,in U.S.dollars;add5 percent(minimum $2.00)handlingand shippingcharge.Make checkspayableto AmericanOrnithologists'Union Pricesin parentheses are for A.O.U members ... STARLING ORNITHOLOGICAL MONOGRAPHS Edited by NED K JOHNSON Museum of Vertebrate Zoology & Department of Integrative Biology Life SciencesBuilding University of California Berkeley, California 9472 0 Ornithological. .. Department of Biology, SouthwesternCollege, 100 College St., Winfield, KS 67156 Price of Ornithological Monographs 47: $11.70 prepaid ($10.50 to AOU members).Add percent(minimum $2.00) handlingand shippingchargein... University P.O Box 1059 Piscataway, N.J 08855-1059 ORNITHOLOGICAL MONOGRAPHS PUBLISHED THE AMERICAN BY ORNITHOLOGISTS' WASHINGTON, 1992 NO 47 D.C UNION FOR EL's children, Jacqueline, Michelle
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