932 SHORT COMMUNICATIONS The Condor89~932-935 0 The CooperOrnithological Society 1987 DOMINANCE-SPECIFIC VIGILANCE IN THE TUFTED TITMOUSE: EFFECTS OF SOCIAL CONTEXT ’ THOMAS A. WAITE Department of Zoology, The Ohio State University,Columbus, OH 43210 Key words: Tufted Titmouse; Parus bicolor; vigi- METHODS lance; dominance;social foraging. Four pairs of wild-caught Tufted Titmice were housed The hypothetical benefits accruingto animals partici- in a large indoor aviary 4.8 m x 4.2 m x 2.7 m high pating in foraginggroups are of two classes,improved between 19 December 1984 and 5 March 1985. To predator avoidance and enhanced foraging efficiency permit instant recognition of individuals, each bird’s (reviewedby Krebs and Davies 1987).These two classes cheek patcheswere painted a unique color with water- of benefitsoften are not independent;for example,there proof felt tip markers. As Tufted Titmice are sexually is a growing literature demonstrating that the size of monomorphic and difficult to sex reliably on the basis the foraging group influences how individuals’ time of external features,and as the birds in this study were budgetsare apportioned among such antagonisticac- neither sacrificed nor laparotomized, the sex of the tivities as vigilance for predators and foraging (e.g., birds was inferred on the basis of wing cord length, Pulliam 1973,Powell 1974,Caraco 1979, Barnard 1980, body mass, and whether singing occurred.The domi- Bertram 1980, Caraco et al. 1980a, Elgar and Catterall nant individual in three replicateswas a putative male, 1981, Lendrem 1983, Studd et al. 1983, Elgar et al. and in the fourth replicate a putative female. The sub- 1984). Several studies on how birds make this trade- ordinate individual in two replicates was a putative off between foragingefficiently and avoiding predators female. The sex of the subordinate bird in the other have demonstratedthat as flock size increasesthe pro- two replicatescould not be determined usingthe above portion of time allocated by individuals to foraging criteria. To ensure that the titmice had been familiar activities is increased,while the flock’s aggregatevig- with each other in the wild, the birds of eachdvad were ilance level is maintained or even increased(e.g., Pow- captured contemporaneouslyat a single trapping sta- ell 1974, Siegfried and Underhill 1975, Caraco 1979, tion. The birds were held at 18.6 +- 1.X (X + SE) on Jenningsand Evans 1980, Sullivan 1984a). natural photoperiod, and were maintained on an ad Most previous studies have failed to consider that libitum diet of sunflower seeds (Heiianthus sp.) and individuals differing in dominance statusprobably ex- mealworms (Tenebrio sp.). In order to be able to cal- perience different benefits and costs associatedwith culate rates of feeding in kcal hr-I, the energeticcon- flock membershin (but seeMoore 1972. cited in Caraco tents of sunflower seeds(0.227 kcal seed-l) and meal- 1979; Caraco 1979; Ekman and Askenmo 1984; Ek- worms (0.086 kcal mealworm-l) were determined man 1987). In particular, the time available for (Lancaster Labs., Lancaster, Pennsylvania; details in foraging should be more constrainedfor subordinates Waite 1986). Each dvad of titmice had been used in if higher-ranking conspecificsinterfere with their for- other experiments (Waite 1986), and had been part of aging. The present study addressesexperimentally the a captive mixed-speciesflock that was comprised of a possibility that subordinates are more vigilant than male and a female Downy Woodpecker (Picoidespu- dominants becausethey must keep higher-rankingcon- bescens),a male and a female White-breastedNuthatch specificsunder surveillance to avoid aggressiveinter- (Sitta carolinensis),two Carolina Chickadees (Parus actions while also remaining vigilant for predators(cf. carolinensis),and a third Tufted Titmouse. Each tit- Robinson 1981, Waite 1986). Using captive Tufted mouse had been exposedto four playbacksof a Tufted Titmice (Parus bicolor),I examined the nonexclusive Titmouse alarm call on a single day within the 4-day hypothesesthat socially foraging animals are vigilant period prior to this study (Waite 1986). In addition, for (1) predatorsand (2) other foraginggroup members while held as a member of a captive mixed-species and, thus, vigilance is subjectto control by at least two flock, each titmouse had been exposedto an overflight proximate factors, group size and dominance status, of a one-half life-size model of a Sharn-shinnedHawk respectively.One predictioncan be generatedfrom each (Accipiterstriatus, unpubl. results).Thus, it is assumed of these hypotheses,respectively: (1) a dominant tit- that the titmice perceived some risk of predation dur- mouse shouldbe more vigilant when foragingsolitarily ing this study. Details concerning the aviary and the than when foraging as a member of a dyad, and (2) a conditionsunder which the birds were held are in Waite subordinate titmouse should be more vigilant than a (1986). dominant titmouse when they forage together. After the three Tufted Titmice had spent at least 3 days acclimatingto the aviary as members of a mixed- species flock, dominance relationships among them ’ Received 9 January 1987. Final acceptance8 April were determined. This was accomplishedby recording, 1987. during 20 15-min observationsessions, all interactions SHORT COMMUNICATIONS 933 in which one bird used a supplanting attack or aerial chaseagainst another titmouse. 51 The general experimental procedurewas as follows. (0.57) On at least the 10th day of captivity for each mixed- 1 speciesflock, the Downv Woodneckers.White-breast- r * ed Nuthatches, Carolina Chickadees, and the Tufted 4 0.011 Titmice intermediate in dominance status were re- moved from the aviary, leaving only the dominant and subordinatetitmice. I then observed the titmice each i r --I I (0.6 of the next 3 days beginningbetween 09:45 and 11:25 in one of the following contexts: subordinate solitary, 3- dominant solitary, and foraging socially (subordinate with dominant presentand dominant with subordinate present). For each dyad of titmice, the order of these three contextswas generatedfrom a random-numbers table. Observationsof birds of each dominance status foraging socially were made on the same day (order determined by a coin toss). Irrespective of dominance statusand social context, trials lasted 72.8 min (SD = 17.6 min) on average.All supplantingattacks and chas- 1 es occurringduring these trials were recorded. During the observationsof solitary individuals, the birds were in both visual and acousticisolation from one another. In each trial, the focal bird was observedas it arrived 0 at a 0.6 m x 0.6 m feedingtray positioned 1.5 m above sot SOL sot SOL the aviary floor. The time lag between the beginning of a trial and the focal bird’s first visit to the feeder DOMINANTS SUBORDINATES averaged6.6 min (SD = 5.0) and was statistically un- FIGURE 1. Time spent scanning at the feeder by related to either dominance status or social context dominant and subordinateTufted Titmice in two con- (paired t-tests, all Bonferroni’s Ps > 0.77; Snedecor texts, social (i.e., as a member of a dyad) and solitary. and Cochran 1967). No food was consumed during The bars represent the mean of means for four repli- this interim; however, the titmice presumably were cates, and vertical lines indicate f 1 SE. P-values are relatively well-fed at the onset of all trials as they had for paired t-tests. Parenthesesindicate two-tailed tests; been housed in the test arena and allowed continuous others are one-tailed. Asterisksindicate statistical sig- free accessto the feeder since dawn. As an index of nificanceafter applying Bonferroni’s probabilitiestech- vigilance, I recorded the time spent scanning at the nique for multiple comparisonsto achieve a 0.05 ex- feeder (Lendrem 1983). After arriving at the feeding perimentwise error rate (Snedecorand Cochran 1967). table, birds often would remain immobile for 1 to 5 set, and sometimes for as long as 16 sec. During this brief “freeze,” they habitually assumed a very erect any clear-cut pattern. Thus, the assumption that 1112 posture while moving the head from side to side, ap- perceived risk of attack by a predator or dominant parently scanning the environment. For each of 15 conspecific was constant throughout the experiment consecutivevisits to the feeder by the focal bird that appearsjustified. resulted in the consumption of a food item (always Multiple comparisons among results from the two fewer than 19 and 23 total visits by dominants and socialcontexts and two dominance ranks were accom- subordinates,respectively), I recordedthe elapsedtime, plished by repeated paired t-tests (ns = 4) using Bon- to the nearest second,between alighting on the feeder ferroni’s probabilitieswith an experimentwiseerror rate and either the first perch change or the first lowering of 0.05 (Snedecorand Cochran 1967). Asymmetries in of the head suchthat the axis of the bill dropped below dominancewere testedby the binomial probability test the horizontal. Either of these behaviors was followed (Snedecorand Cochran 1967). almost invariably by the graspingof a food item with the bill. Any subsequentscanning during a single visit RESULTS to the feeder was not recorded. Scanningat the feeder The dominance relationship between the two birds of could be measuredunambiguously, and appearsa rea- each replicate was unequivocal as the dominant won sonable index of the extent to which individuals are every dominance interaction both when the titmice sensitive to the risk of imminent attack by a predator were housedtogether during observationsof vigilance (Lendrem 1983) or supplantingattack by a conspecific. behavior (X = 9.5 interactions hrrl, SD = 6.6) and In only one of the 16 observation sessionswas there a when they were members of the mixed-species flocks significantcorrelation between time spent scanningat (X = 3.2 interactions hrl, SE = 1.6; one-tailed bino- the feeder and the order of visits (solitary subordinate mial Ps < 0.032 for the probability of suchan extreme in replicate 4; I = -0.608, P = 0.016, two-tailed).