Age-Related Differences in Ruddy Turnstone Foraging and Aggressive Behavior
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AGE-RELATED DIFFERENCES IN RUDDY TURNSTONE 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 birds in the tall-levelposture. In mixedflocks of foragingadult and juvenile turnstones,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, Canada. Accepted 13 July 1976. THE occurrenceof higher mortality in young birds than in adults is widespreadin avian species(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 invertebrate-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 bird became uncertain the observation was ended and a new subject selected arbitrarily. Each bird's age and plumagewere noted, the habitat 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 Massachusetts,fall migrant adult and juvenile Ruddy Turnstonesare readily distinguishedby plum- age characteristics.All adultsseen during the fall migration were in predominantlyalternate plumage,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 crustaceans(shrimp, crabs, amphipods)and polychaete worms were locally abundant on the sand flats and in the vegetation. On sand and weed-litteredflats, turnstoneslocate prey by directedforaging move- January 1978] Ruddy Turnstone 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