AGE-RELATED DIFFERENCES IN RUDDY FORAGING AND AGGRESSIVE BEHAVIOR

SARAH GROVES

ABSTRACT.--Theforaging behavior of fall migrant Ruddy Turnstoneswas studiedon the Mas- sachusettscoast on 2 different substrates,barnacle-covered rocks and sand and weed-litteredflats. Foragingrates differedsignificantly between the 2 substrates.On eachsubstrate the foragingand successrates of adults and juveniles differed significantly while the frequenciesof successwere similarfor both age-classes.The observeddifferences in foragingrates of adultsand juvenilesmay be due to the degreeof refinementof foragingtechniques. Experience in searchingfor and handling prey may be a primary factor accountingfor thesedifferences, and foragingperformance probably improves with age and experience.Alternatively, the differencesmay be due to the presenceof inefficient juveniles that do not survive to adulthood. Both adultsand juveniles in the tall-depressedposture were dominant in aggressiveinteractions much morefrequently than in the tall-levelposture. In mixedflocks of foragingadult and juvenile ,the four possibletypes of aggressiveinteractions occurred nonrandomly. Adult over juvenile interactionsoccurred more frequently than expected,and juvenile over adult interac- tions were never seen.A tentative explanationof this phenomenonmay be that juveniles misinter- pret or respondambivalently to messagesconveyed behaviorally by adultsand thusbecome espe- cially vulnerableto aggressionby adults.The transiencyof migrantsmade it unfeasibleto evaluate the persistenceof this nonrandom aggression.--ManometBird Observatory,Manomet, Mas- sachusetts02345. Present address:Department of Zoology, University of British Columbia, Van- couver, British Columbia V6T 1W5, . Accepted 13 July 1976.

THE occurrenceof higher mortality in young birds than in adults is widespreadin avian (Lack 1954). This markedly differential mortality in many species suggeststhat juveniles are somehowmore inept than adults in copingwith the rigors of survival. A variety of behavioral,social, and geneticfactors may accountfor such differences. Lack (1966) argues that food supply is the major factor in density-dependent population regulation, and Ashmole (1963) hypothesizesthat starvation is a major factor in the higher juvenile mortality and implies that young birds are lessefficient than adults in foraging. Foraginginefficiency can be attributed to a variety of behav- ioral differencesincluding acquired proficiency and skill in searchingfor and han- dling prey (feedingrate, frequencyof prey capture,size of prey selected,etc.) and thoseof a socialor psychologicalnature related to aggression,dominance, flocking, and other social factors. To what extent these aspectsof behavior interact to influ- ence foraging efficiencyis difficult to determine. Likewise the precisenature of behavioral differencesmay be ambiguousbecause of the difficulty of distinguishing between inherited, learned, and developed behaviors. The purposeof this studywas to examinedifferences, if any, in foragingand social behavior of conspecificjuveniles and adults. Ruddy Turnstones (Arenaria interpres) were studied south of the breeding ground during southboundmigration. Two as- pects of behavior, foraging efficiencyand the frequency of aggressiveinteractions between adults and juveniles were recorded in detail. Adults and juveniles were found to differ significantlyin both theseaspects.

STUDY AREA AND METHODS

RuddyTurnstones were studied on the Massachusettscoast in PlymouthCounty, a regioncharacterized by exposedocean and estuarinebeaches and extensivetidal flats. Turnstoneswere watchedforaging on 95 The Auk 95:95-103. January 1978 96 S^• GROVES [Auk, Vol. 95 different substratesincluding coarsepebbly and rocky beaches,wet firm sand closeto the water line, the high-tide wrack linel andrespeciallyon barnacle-coveredrocks and sand, mud, and weed-littered flats. The turnstonestypically appearedand started foraging as soonas the falling tide beganto exposethe tidal flats and continuedforaging until the rising tide coveredthe feedingground. Barnacle-coveredrocks rangingfrom small fragmentsto large glacial erraticswere dominant featuresof the low tide topography. Barnacles(Balanus balanoides), a major turnstonefood, were abundant on theserocks, as was rockweed (Fucus vesiculosis).Several times during the study, high seasand storm-driven waves depositedlarge quantifiesof algae (Ulva lactuca and Chondruscripus), eel grass(Zostera marina), and an associatedfauna of marine invertebratesover the exposedtidal flats, especiallyat the high tide line. Theseaccumulations of -infestedalgae and eel grasspersisted as long as a week and were frequented by foraging turnstones. Through the fall migration, mid-July to early October, data were gathered on 286 turnstones (164 adults, 122 juveniles) during nearly daily observationperiods from 5 August to 25 September1974. The data gatheringtechniques were refined from methodsused in preliminary studiesof 139 turnstonesduring the same period in 1973. (The 1973 data on turnstonepostures are included here, but the 1973 foraging data lack details on various componentsof foraging discussedhere, so they are omitted.) Daily observationperiods ranged from 30 to 180 min. Individual birds were watched for periodsof 30 to 1,500 sec with a 20 x spottingscope and timed to the nearestsecond with a stop watch. As soonas the identity of an individual became uncertain the observation was ended and a new subject selected arbitrarily. Each bird's age and plumagewere noted, the characterized,the number and ages of turnstonesaround the subject noted, and the estimated size of the area occupiedby a group of turnstones (1-25 m2) recorded.Throughout each observation the bird'sposture and posturalchanges were recorded as were the occurrence and outcome of all interactions with other birds. In ,fall migrant adult and juvenile Ruddy Turnstonesare readily distinguishedby plum- age characteristics.All adultsseen during the fall migration were in predominantlyalternate ,and most had obvious feather wear. (Of 37 adult turnstones trapped during this period, several birds had arrested molt with a few basic contour feathers in the body plumage, but none showed active molt.) Juveniles, which did not begin arriving until mid-August, were in the more subdued and uniformly coloredfresh juvenal plumage. Up to 20 different birds were watched daily. Many of the same individuals were doubtlessseen on subsequentdays, but becauseof the impossibilityof identifying individuals positivelyfrom day to day, it was necessaryto evaluate each timed observationof a bird as a separate case. About 30 turnstoneswere color-bandedwith unique combinationsto facilitate individual identification, but only one of thesebirds was ever seen more than a day after banding.

TURNSTONE FORAGING TECHNIQUES

Turnstones foraged most commonly on barnacle-covered rocks, frequently in patchesof abutting barnacles. Typically a foraging turnstone selectsa barnacle and with the mandiblesheld closetogether rapidly deliversone or two sharp blows to the upper (e.g. tergal and scutal)plates of the barnacle, which usually sufficeto open the barnacle. The flesh is then eaten by a sequenceof pecks into the open barnacle with occasionalmovements to remove piecesof broken plates. Extensive pecking and maneuvering around the barnacle with the bird's head tilted through various angles is common, presumably to clean out as much of the barnacle flesh as possible.Often a seriesof barnaclesconsumed by a turnstoneare closeor adjacent to each other, and a bird may remain within a small space(approximately 50 cm x 50 cm) on one rock for 20 min or more. On occasiona turnstonewill peck among the plates of a dead barnacle or displace another feeding turnstone from a barnacle and feed in the barnacle openedby the displacedbird. Birds also foraged on tidal flats varying from clear open sand and mud flats with scattered patchesof growing eel grassto shoresthickly covered with tide-cast eel grassand algae. Various (shrimp, crabs, amphipods)and polychaete were locally abundant on the sand flats and in the vegetation. On sand and weed-litteredflats, turnstoneslocate prey by directedforaging move- January 1978] Behavior 97

mentsincluding flicking up piecesof weed with the bill, usingthe bill to peck or dig in the sand,overturning piles of weedby lungingforward and pushingwith the bill and forehead, and flipping stoneswith an upward movement of the bill. Several manipulations of the substrateand prey may precede a prey capture. Sandand weed-litteredflats representmore heterogeneous foraging situations than barnacle-coveredrocks. Qualitative examinationof the substrateand quantitative observationsof foragingbirds showthe inconspicuousinvertebrate population to be patchilydistributed through the sand,mud, and weedlitter. Procuringfood in sucha situationis relatively more complexthan feedingon barnacles,and involvesactive searchingfor a variety of acceptableprey items, pursuing, and capturingmobile crustaceans and worms. Two typesof foragingactivities were classifiedand recordedduring observations of individualbirds. (1) Each foragingmovement involving contact of the bird'sbill with the substrateor manipulation of the substratein search of food was a directed foragingmovement (F). Thesemovements included using the bill to flick overpieces of weed, overturn rocks, and peck in the sand as well as unsuccessfulattempts to capture single prey items. Unfortunately, frequent difficulties in distinguishing clearlybetween searching activities and unsuccessfulattempts to captureprey, espe- cially on weed-litteredflats, necessitatedclassifying these two activitiestogether as directedforaging movements. (2) A singleforaging movement resulting in the inges- tion of a prey item was a prey capture(P). In the field prey captureswere readily identified, as many prey items such as barnacles,crustaceans, and worms were visibleto the observerwhen a bird (usually)lifted its head and pausedwhile making a visible swallowing movement. The following manipulationswere performedon the data to obtain measurements of foragingrates and efficiencyfor eachbird observed(t = time in sec):(1) foraging rate = (P + F)/t; (2) successrate = P/t; (3) frequencyof success= P/(P + F). Re- suits were analyzed by the Mann-Whitney U-test and Chi-square one sample test (Siegel 1956). Birds in the two foraging situations, barnacle-coveredrocks and sand and weed- littered flats, were consideredseparately as foraging rates in the two situations showed highly significant, age-independentdifferences (Table 1, P < 0.001, Mann-Whitney U-test). Adults and juvenileswere comparedin eachforaging situa- tion. The results,presented in Table 1, showstatistically significant differences in the foragingand successrates of adultsand juveniles;the time-independentfrequen- cies of successare nearly equivalent.

DISCUSSION The resultsindicate that foragingrates but not successrates vary significantlyin differenthabitats and that regardlessof habitat, juvenile Ruddy Turnstonesare not as adept as adults at procuringfood. Although approximatelyequivalent proportions of foragingmovements in both age classesresult in ingestionof prey items,the two age groupsdiffer in the rates at which they forage and captureprey. This suggests that experiencein searchingfor and mechanicallyextracting and handlingprey may be a primary factor accountingfor the observeddifferences in foragingand success rates. It hasbeen suggested that foragingability in birdsimproves with ageand experi- ence (Ashmole1963) and further, that foraginginefficiency is a major factor con- 98 SARA•IGROV•.S [Auk,Vol. 95

TABLE 1 FORAGINGRATES, SUCCESS RATES, FREQUENCIES OF SUCCESSOF TURNSTONESON Two SUBSTRATES

Barnacle-covered rocks Adults, N = 108 Juveniles, N = 78 All birds, N = 186 Foragingrate, range 0.066-0.$45/sec 0.015-0.364/sec 0.015-0.$45/sec P+F • 0.222/sec 0.188/sec 0.208/sec t SD 0.070/sec 0.069/sec 0.072/sec z • = 3.39, P < 0.001 Successrate, range 0.044-0.478/sec 0.000-0.364/sec 0.000-0.478/sec P • 0.185/sec 0.1 $ 6/sec 0.173/sec •- SD 0.069/sec 0.069/sec 0.070/sec z = 3.12, P < 0.01 Frequencyof success, range 0.273-1.000 0.000-1.000 0.000-1.000 P • 0.814 0.812 0.813 P+F SD 0.137 0.196 0.164 X• = 0.10, NS Sand and weed-littered flats Adults, N = 36 Juveniles,N = 31 All birds,N = 67 Foraging rate, range 0.049-1.652/sec 0.043-0.667/sec 0.043-1.652/sec P + F • 0.375/sec 0.246/sec 0.315/sec t SD 0.281/sec 0.140/sec 0.235/sec z = 2.34, P < 0.05 Successrate, range 0.000-1.000/sec 0.000-0.383/sec 0.000-1.000/sec P • 0.2 $8/sec 0.1 $8/sec 0.211/sec •- SD 0.203/sec 0.106/sec 0.172/sec z = 2.07, P < 0.05 Frequency of success, range 0.000-1.000 0.000-1.000 0.000-1.000 P • 0.645 0.603 0.626 P + F SD 0.253 0.230 0.242 X• = 0.93, NS : = standardnormal variate calculated from Mann-WhitneyU-test. Foragingrates of all birdson barnaclesvs. all birdson tidal flats,z = 3.98, P < 0.001.

tributingto high mortalityin youngbirds, but until recentlythere has beenlittle quantifiedevidence to confirmor refute this hypothesis.Four recentstudies have demonstratedage-related differences in foragingefficiency and successin Brown Pelicans,Pelecanus occidentalis (Orians 1969), Little Blue Herons,Florida caerulea (Recherand Recher1969), Sandwich , Sterna sandvicensis(Dunn 1972),and Royal Terns, Sterna maxima (Buckley and Buckley 1974). In thesefour species,juveniles are lessefficient foragers than conspecificadults in comparablesituations. The data availableindicate that the observeddisparities betweenadults and juvenilesmay be attributedmainly to differencesin foraging rates(rates of searching,diving, etc.) and the degreeof refinementof foragingand prey handlingtechniques (height of dives, frequencyof droppingprey, etc.) and suggestthat experiencewith prey may attenuatethese differences. One consequenceof lessefficient foraging may be that juvenilesobtain lessfood than conspecificadults in comparablesituations. In somespecies such as the Royal (Ashmoleand Tovar 1968),extended parental care may partiallycompensate for any fooddeficit. There may be instancesof independentjuveniles compensating for differencesin the quantityof preyobtained by foragingfor longerperiods of time than adultsas reportedin the RoyalTern (Buckleyand Buckley1974), but thisdoes not seemto be the casein SandwichTerns or turnstones.Dunn (1972)reported that January1978] Ruddy TurnstoneBehavior 99 although further observationsare needed,he detectedno differencesin duration of foraging periods of adult and juvenile Sandwich Terns. Similarly, foraging time available to turnstonesin this study was limited by the tides, and both adults and juveniles foraged continuouslyduring the available foraging time. Data on another intertidal feeder, the European Oystercatcher(tIaematopus os- tralegus)wintering in northernScotland (Heppleston 1971), provide evidencethat underextreme environmental conditions juveniles feeding at slightlylower rates than adults are unable to compensateadequately for any food deficit incurred. In 1965-66 first winter birds accountedfor 77% of winter mortality (N -= 29), apparently from starvation. If inexperience is the main source of the observed foraging inefficiency of juveniles, then it shouldfollow that with time and maturity juvenileswould develop greaterproficiency at locatingand handlingprey, and disparitiesin foragingefficien- ciesof adults and juveniles would diminish. Alternatively, the observeddifferences in foraging efficienciesbetween adults and juveniles may represent two classesof birds: inefficientjuveniles that do not survive to adulthoodand adults. Dunn (1972) mentionsthis possibilityin his discussionof SandwichTerns but then pointsout that newly fledgedterns are notably poor fishersand dependenton parental feedinguntil migration away from the breedinggrounds. Similarly, each year the first juvenile turnstonesto appear were decidedlyhesitant and somewhatinept in their foraging efforts, but this marked hesitancydisappeared within a few days. Probably the most inefficientjuveniles do not reach adulthood,but thosethat survive their first few monthsto adulthoodimprove in their foragingefficiency as they mature.

POSTURES AND AGGRESSIVE INTERACTIONS

Two postureswere commonly observedamong foraging turnstones.In the tail- level posture(Fig. 1A) the head is upright, the wingsare againstthe body, the tail is held in line with the body axis, and the contourfeathers lie smoothand flat. In the tail-depressedposture (Fig. lB) the tail of the foragingbird is depressedbelow the body axis with the white of the upper rectricesvisible above the dark feather tips. The degreeto which the tail is depressedvaries amongindividuals and in different situations.The tall-depressedposture is frequently associatedwith chasingand ag- gression.It is sometimesaccompanied by lowering of the head and raising of the scapulars,which givesthe bird a hunchedappearance. In someinstances the wings are slightly droopedand held away from the body. Aggressiveinteractions are most often seenamong turnstonesin typical foraging groups ranging from 2-3 up to 20 birds. Such interactions usually last only a few secondsand occurwhen the foragingpaths of two birdsintersect. The encountermay involve posturingor chasing.Physical contact involving pecking and grabbingof the opponentwith the feet occasionallyoccurs. Aggressiveinteractions end when one bird moves away from the encountersite. In all observed interactionsinvolving adults and juvenilesthe adult was always dominant, and the juvenile movedaway from the encountersite. Occasionallya turnstonewill approach and displacea feeding turnstone and then eat the prey item the displaced turnstone left, but in most cases,aggressive interactions do not appear to be centered around particular food items. Violations of individual distance or feeding territories may explain some ag- gressiveinteractions. The numbersand agesof birdsforaging together were recordedto providedata on 100 SAP.• GROVES [Auk, Vol. 95

Fig. 1. Posturesof turnstones:A, Tail-level posture; B, Tail-depressed posture. the social context in which aggressiveinteractions occur. All interactionswere iden- tified by type according to ages of the participants and outcomesof interactions: adult over adult, adult over juvenile, juvenile over adult, juvenile over juvenile. From 14 August to 12 September when both adults and juveniles were present, expectedfrequencies of eachpossible type of aggressiveinteraction were predictedon the basis of the number of adults and juveniles present in observed groups of turnstones.The model used by Ridpath (1972) to predict frequenciesof age-related interactions was applied to the data in this study. For a mixed adult and juvenile flock of any specifiedcomposition p = number of adults/(numberof adults + juveniles), q = number of juveniles/(number of adults + juveniles), and p+q=l.

Assuming random occurrenceof all types of aggressiveinteractions, p2+ 2pq + q2= 1 predicts the expectedfrequencies of aggressiveinteractions where p• representsall adult over adult interactions,q• representsall juvenile over juvenile interactions, and 2pq representsall adult over juvenile and juvenile over adult interactions.It is important to note that no juvenile over adult interactionwas ever observed. During the timed observationsof foraging birds, detailed notes on posture and aggressiveinteractions were obtainedon 286 turnstones.Among thesebirds, thosein

TABLE 2 POSTURES OF TURNSTONES AND DOMINANCE IN AGGRESSIVE INTERACTIONS

No. of birds No. of birds dominant in Posture Age observedin posture aggressiveinteraction 1973• Tail_levela Adult 32 1 Juvenile 21 2

Tail_depressed4 JuvenileAdult 4145 2315 19742 Tail_level• Adult 74 6 Juvenile 54 3

Tail_depressed6 JuvenileAdult 6890 2515 Tail-level vs. tail-depressed,X2 = 16.50, P < 0.001. Tail-level vs. tail-depressed,X = 13.80, P < 0.001. Adult rs. juvenile,X 2 = 0.91, NS. Adult rs. juvenile,X 2 = 1.03, NS. Adult rs. juvenile, X2 = 0.29, NS. Adult rs. juvenile,X 2 = 0.51, NS. January1978] Ruddy TurnstoneBehavior 101

TABLE 3 SUMMARY OF FLOCK COMPOSITION AND FREQUENCY OF TYPES OF AGGRESSIVEINTERACTIONS

Flock composition Aggressiveinteractions No. of No. of No. of Adult Adult Juvenile Juvenile adults juveniles cases over adult over juvenile over adult over juvenile

17 1 1 1 - - - 6 2 1 5 4 - 1 5 1 1 1 1 - - 4 1 2 6 3 - - 4 2 3 3 11 - 2 4 3 1 2 - - - 3 1 1 2 2 - - 3 2 1 - 2 - - 2 1 2 4 6 - - 2 2 3 - 6 - 1 2 3 1 - 1 - 1 2 4 2 - 5 - 2 1 1 1 - 1 - - 1 2 2 - 5 - 1 1 4 1 - 4 - - 1 5 1 - 2 - -

the tail-depressedposture were dominantin aggressiveinteractions much more often than those in the tail-level posture (Table 2, X2 = 13.80, p < 0.001). Likewise, during the preliminary observationsof 139 turnstones in 1973, birds in the tail- depressedposture were dominant in aggressiveinteractions much more often than birds in the tail-level posture(Table 2, X2= 16.50, p < 0.001). No age-related differencesin the frequency with which adults and juveniles in the tail-level or tail-depressedpostures were dominantin aggressiveinteractions were notedin either year (Table 2). Aggressiveinteractions occurred in 24 of 37 mixed adult and juvenile flocks ob- served. For each of theseflocks of known-agecomposition, the observedfrequencies of each type of aggressiveinteraction (Table 3) were comparedto the calculated expectedfrequencies. For all 24 flocks and each possibletype of interaction the directionsof deviationsfrom expectedfrequencies were tallied (Table 4) and evalu- ated by the sign test (Siegel 1956). During the period when adults and juveniles were present together, adult over adult interactions were seen less frequently than ex- pected (P = 0.002) as were juvenile over juvenile interactions (P = 0.022). Interac- tions involving adults and juveniles occurred more frequently than expected (P = 0.006) and were always of the adult over juvenile type.

TABLE 4 COMPARISONOF OBSERVEDAND EXPECTEDFREQUENCIES OF DOMINANCEIN AGGRESSIVE INTERACTIONS IN 24 MIXED-AGE FLOCKS OF TURNSTONES

p2 2pq q2 Adult Adult Juvenile Juvenile over adult over juvenile over adult over juvenile

Obs. > Exp. 4 19 6 Obs. = Exp. 0 0 0 Obs. < Exp. 20 5 18 P (sign test, two-tailed) 0.002 0.006 0.022 102 SARAHGROVES [Auk, Vol. 95

DISCUSSION

In mixed flocks of adult and juvenile turnstonesthe observed frequenciesof possi- ble types of aggressiveinteractions are nonrandom. The higher than expected fre- quency of adult over juvenile interactions suggeststhat dominance in aggressive interactionsis age-related. Interactions involving adults and juveniles always ended with the juvenile moving away from the encountersite. Ridpath (1972) observed similar behavior in the Tasmanian Native Hen (Tribonyx morterii), specificallythat conflictsbetween adults and young are more frequent than would be expectedif conflictsare independent of age, and they are always initiated and won by adults. The lower than expectedfrequency of juvenile over juvenile and adult over adult aggressiveinteractions raises the possibility that either juvenile turnstonesare some- how inept at avoiding adults or that adults are selectively aggressive towards juveniles. As adults stay with their young on the breeding ground during the pre- fledging period in late July (Nettleship 1973), apparent ineptnessof juveniles at avoiding adults may be a vestige of their recent period of presumably aggression-free associationwith their parents. In encounterswith adults in foraging south of their nesting grounds, juveniles may take time to learn that the context in which they interact with adults has changed. Young Adelie Penguins(Pygoscelis adeliae) are reported to respondinappropri- ately to messagesconveyed by the displays of other conspecifics,especially adults (Ainley 1975). Similarly, juvenile Ruddy Turnstones may be responding inappropri- ately to messagesconveyed behaviorally by adults. For instance, an adult turnstone maycommunicate by itsposture, especially the' tail-depressed posture, that it is likely to respondaggressively to any bird approachingwithin a certaindistance. A juvenile turnstone may misinterpret or respond ambivalently to this messageand thus ap- proach an adult closeenough to becomethe subject of aggression.If juveniles com- monly misinterpret or respondambivalently to such messages,this could accountfor the unexpectedlyhigh frequency of adult over juvenile aggressiveinteractions. Presumablyjuveniles learn to avoid aggressionby adults and behave lessambiva- lently in the presenceof adults, but the transiencyof fall migrants coupled with the lack of observations of marked individuals made assessmentof the persistenceof nonrandom aggressionin mixed-ageflocks of turnstonesunfeasible in this study. For similar reasons,the costto a juvenile of involvement in frequent aggressiveinterac- tions could not be measured in terms of lost foraging time.

ACKNOWLEDGMENTS

It is a pleasureto acknowledgethe facilities and encouragementoffered by the Manomet Bird Obser- vatory and the support, in part, of U.S. Fish and Wildlife Service contract No. 14-16-008-687. I am especiallygrateful to Brian A. Harringtonfor timely suggestionsand thoughtfuldiscussion throughout the study and to Kenneth A. Youngstromfor writing a program for sortingand analyzingthe data. JamesN. M. Smith and an anonymousreviewer made useful commentson an earlier manuscript.

LITERATURE CITED

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