INDIVIDUAL AUDITORY RECOGNITION IN THE LEAST TERN (STERNA ALBIFRONS) LYNN J. MOSELEY• Department of Zoology, University of North Carolina, Chapel Hill, North Carolina 27514 USA ABST•CT.--Playback experiments performed in a colony of Least Terns near Fort Macon, North Carolina, demonstrated that adults can distinguish the call of their mate from that of a stranger. Sonagram analysis of the "Purrit-tit-tit" call, the most common vocalization used when an adult approachesits mate, revealed that both temporal and spectral characteristicsof the first note of the call varied significantly among individuals, whereas all measured features of the second note were not significantlydifferent for different birds. Presumably the first note is used to convey an individual's identity, while the secondnote indicates a tendency to approach the mate and perform certain courtship behaviors. Received 17 January 1978, accepted26 March 1978. IN his review of individual auditory recognition in birds, Beer (1970a) stated that the best evidence for the existenceof vocal recognition comeseither from colonial species, for which the omnidirectional properties of sound far exceed those of visual signals in a crowded colony, or from speciesthat inhabit thick vegetation, as foliage obstructsvision but doesnot greatly hinder sound transmission. This study examines vocal recognition between mates in the Least Tern (Sterna albifrons), a colonially- nesting larid. Individual recognition by voice has been hypothesized for several speciesof co- lonial birds and documentedfor a few. Most investigationsdeal with auditory com- munication between members of a mated pair or between parent birds and their young. According to Beer, investigators have used three approachesin the study of vocal recognition in birds: field observation, sound analysis, and playback experi- ments. Field observation has provided circumstantial evidence for the existence of auditory recognition between mates in several colonial species(e.g. Sterna hirundo, Tinbergen 193! and Palmer 194!; Larus argentatus, Tinbergen !953) and between parents and offspring (e.g.S. fuscata, Watson and Lashley 1915; L. argentatus, Goethe 1937; S. sandvicensis, Hutchinson et al. 1968). However, some method of experimentation is usually necessaryto eliminate the possibility that a bird is actually respondingto visual rather than auditory stimuli, as in most casescolonial birds can see as well as hear one another (Beer 1970a). Several investigators have performed experiments to test the ability of parent birds to recognizetheir own chicks. Although individual recognitionof chicks has been demonstratedfor several speciesof colonial nesters(e.g. Anous stolidus and S. fuscata, Lashley 1913; S. bergii, Davies and Carrick 1962; S. maxima, Buckley and Buckley 1972), such experiments usually do not reveal the relative importance of visual and auditory modalitiesfor suchrecognition. However, Miller and Emlen's (1974) study of the effect of altering a chick'svoice and appearanceon the recognition of young by their parents in the Ring-billed Gull (Larus delawarensis)revealed that vocally impaired chicks 12 to 20 days old were accepted whereas visually altered chicks were not. In some species, at least, visual rather than auditory stimuli are important for the development of chick recognition. Experiments involving the recognitionof parents' voices by chicks have met with somewhat more successdue to the ease with which chicks can be manipulated. Presentaddress: Department of Biology,Guilford College, Greensboro, North Carolina27410 USA. 31 The Auk 96: 31-39. January 1979 32 LYNN J. MOSELEY [Auk, Vol. 96 Controlled playback experiments, performed under both natural and laboratory conditions, demonstrate that in most cases chicks of colonial species clearly distin- guish between the voice of a parent and the voice of a strange adult (Uria aalge, Tschanz 1968; L. atricilla, Beer 1969, 1970b; S. hirundo, Stevenson et. al. 1970). White (1971) reported that chicks of the Gannet (Sula bassana) seem to have the ability to recognize the voice of a parent in a playback test, but do not always do so. Presumably in this speciesthere is no particular advantage for vocal recognition between parents and young since, unlike the specieslisted above, Gannet chicks are not mobile and remain on the nest until fledging. Investigators have also studied the ability of adult birds to discriminate the call of the mate from that of other adults in the colony. Hutchinson et. al. (1968) recorded "fish calls" from 40 adult Sandwich Terns (S. sandvicensis) and analyzed various temporal and spectral parameters of sonagramsof these calls. They concludedthat the general "patterning" of the call was always characteristic for each individual. However, they were unable to perform playback experiments to test whether the adults did, in fact, use such characteristicsto identify their mates by voice. They were also unable to record more than one sequence of vocalizations from each individual. Intra-individual variation is probably greater among calls from different sequencesthan among calls in the same sequence(see below). In a more conclusive investigation, playbacks showedthat adult Gannets recognizevocalizations of their mates (White 1971). Sonagram analysis of landing calls of individual Gannets sug- gested that recognition resulted from individually consistent temporal changes in amplitude (White and White 1970). To demonstrate individual auditory recognition, playback experiments must show that test birds respond selectivelyto vocalizationsfrom different individuals. Anal- ysis of physical characteristicsof the vocalization should reveal one or more sources of interindividual variation. Using this procedure, I investigated auditory recogni- tion between mates in Least Terns. On numerous occasions, I observed that an individual sitting quietly on its nest would suddenly stand and call repeatedly just as its mate, vocalizing loudly, approached the territory. Vocalizations of other in- dividuals were always ignored. Least Terns are colonial nesters that form stable pair bondsfor at leastOne, and possiblyseveral, breeding seasons. Both the male and female participate in incubating the eggs and raising the young. During the incubation period, members of a pair alternate incubation duties approximately every hour (Moseley 1976). On someoccasions, particularly during the first few days of incubation, nest relief ceremoniesinvolve a brief period of visual and vocal displays between mates near the nest. However, after about a week of incubation, membersof a pair becomevery efficient at switching places, to the extent that an incubating bird leaves the nest and flies out of the colony, vocalizing repeatedly, at what seemsto be the instant it hearsthe voice of its mate. Auditory signalsappeared to be the primary mode of communication in such interactions. I therefore performed a seriesof playback experiments designedto test the ability of adult Least Terns to recognizethe call of their mates. I have previouslyidentified 10 discretevocalizations of adult Least Terns (Moseley 1976). The vocalization that is typically associatedwith approach to the nest and courtship behavior between mates, and the call that occurredmost frequently in playback sequences,was the "Purrit-tit-tit," named by Schi3nert(1961). This vocalization is a 4- or 5-syllabled call approximately 0.5 s in duration, with the dominant frequency of most syllables between4-5 kHz (Fig. 1). It was nearly impossibleto establishby ear which physical January1979] AuditoryRecognition in LeastTerns 33 t• 4' O' 3' 2- a b c 012 0'.4 0'.6 0'.8 110 Seconds Fig. 1. Representative"Purrit-tit-tit" vocalization, divided into its componentsegments. features of the vocalizations differed among various birds. I therefore performed a quantitative analysis to determine which features of the "Purrit-tit-tit" varied suf- ficiently among birds to permit voice discrimination. PLAYBACK EXPERIMENTS METHODS This study was part of a generalinvestigation of Least Tern behaviorand communicationconducted in a nesting colony on the north side of Bogue Banks, approximately 1 km northwest of Fort Macon, Carteret County, North Carolina. The center of the colony lies about 200 m inland from Bogue Sound. Between 45 and 50 pairs of Least Terns nested on a former dredge spoil area in May 1975. During my visits to the colony, I observedthe terns from a burlap-coveredblind, similar to the one describedby Robins (1972). Least Terns were unaffected by the presenceof the blind and resumedapparently normal behavior within 5 min after I entered the blind. In order to identify individual birds, I devised a method for color-marking nesting terns with dye from a bottle placed at the nest and activated from within the blind (Moseleyand Mueller 1975). The method was easy to repeat if the dye began to fade. For the playback experiments, I first recorded sequencesof vocalizationsfrom 14 marked birds as they approached their nests, either for a courtship feeding or a nest relief. These sequencesaveraged 25 s in duration (SD = 12, N = 23) and consistedof one or more of the vocalizationtypes in the Least Tern's repertoire. Occasionally, calls of both members of a pair occurred in a sequence.Recordings were made with a Uher 4000 Report I-C tape recorder and a Dan Gibson Electronic Parabolic Microphone. The most complete sequencefrom each pair was later incorporated into a test tape for use in the playback experiment. A test tape consistedof a sequenceof vocalizationsrecorded