The Condor 104:387±394 q The Cooper Ornithological Society 2002 VOCAL DISTINCTIVENESS AND RESPONSE TO CONSPECIFIC PLAYBACK IN THE SPOTTED ANTBIRD, A NEOTROPICAL SUBOSCINE SUSANNE C. BARD1,4,MICHAELA HAU2,MARTIN WIKELSKI2 AND JOHN C. WINGFIELD3 1Department of Psychology, University of Washington, Box 351525, Seattle, WA 98195 2Department of Ecology and Evolutionary Biology, Princeton University, Guyot Hall 303, Princeton, NJ 08544 3Department of Zoology, University of Washington, Box 351800, Seattle, WA 98195 Abstract. We studied individual variation in song characteristics of the Spotted Antbird (Hylophylax naevioides), a Neotropical suboscine, and tested for song discrimination using playback of male neighbors and strangers. Discriminant analysis of four frequency and temporal characteristics of the songs of 25 male and ®ve female Spotted Antbirds revealed signi®cant differences among individuals. Each song was assigned correctly to the individual that produced it with over 70% accuracy. However, during ®eld playback, male Spotted Antbirds did not discriminate between neighbor and stranger song. Our results suggest that selection has not favored the evolution of neighbor-stranger discrimination, perhaps because all conspeci®c intruders pose a threat. The temporal characteristics of Spotted Antbird songs differed signi®cantly between the sexes, but frequency characteristics did not. Females re- sponded signi®cantly less strongly to male playback than males, suggesting that each sex plays a distinct role in territorial defense. Key words: Hylophylax naevioides, neighbor-stranger discrimination, playback experi- ment, song, Spotted Antbird, tropical suboscine, vocal distinctiveness. VariacioÂn Individual del Canto y Respuesta al Playback Coespecõ®co en Hylophylax naevioides, un Suboscino Neotropical Resumen. Estudiamos la variacioÂn individual de las caracterõÂsticas del canto de Hylophy- lax naevioides, un suboscino neotropical y evaluamos si esta especie responde de forma diferente al playback de individuos vecinos o desconocidos. Un anaÂlisis discriminante de cuatro caracterõÂsticas temporales y espectrales del canto de 25 machos y cinco hembras de H. naevioides revelo que existõÂan diferencias signi®cativas entre individuos. Cada canto fue asignado correctamente al individuo que lo produjo en maÂs del 70% de los casos. Sin embargo, en un experimento de playback en el campo los machos no distiguieron entre el canto de inidividuos vecinos y desconocidos. Estos resultados sugieren que la seleccioÂn no ha favorecido la evolucioÂn de la discriminacioÂn entre vecinos y desconocidos porque todos los intrusos coespecõ®cos suponen una amenaza. Las caracterõÂsticas temporales, pero no las espectrales, del canto de los H. naevioides di®rieron signi®cativamente entre ambos sexos. Las hembras respondieron signi®cativamente menos que los machos al playback con cantos de machos, lo que sugiere que cada sexo desempenÄa un papel diferente en la defensa del territorio. INTRODUCTION inate between neighbors and strangers and be- The ability to distinguish between familiar and tween other conspeci®cs (Lambrechts and unfamiliar conspeci®cs based on vocal cues may Dhondt 1995, Stoddard 1996), this issue has not help maintain bene®cial relationships and mini- been addressed in suboscine passerines. Further- mize costly aggressive interactions between an- more, in Neotropical suboscine species where all imals (Stoddard 1996). While it has been estab- individuals of both sexes sing a similar song, it lished that many oscine passerines can discrim- is unclear how individuals of each sex distin- guish each other and what role each sex plays in territorial interactions. Manuscript received 2 April 2001; accepted 21 Jan- Individual vocal discrimination has been dem- uary 2002. onstrated in a number of vertebrates such as 4 Present address: Science and Natural History Film- making, Montana State University, P.O. Box 174120, group-living mammals and colonial birds, where Bozeman, MT 59717. E-mail: [email protected] complex social relationships require ®ne percep- [387] 388 SUSANNE C. BARD ET AL. tual abilities (Rendall et al. 1996, Sayigh et al. In order for vocal recognition or discrimina- 1998, Wanker et al. 1998, Holekamp et al. tion to be possible, song characteristics must 1999). In several social, nonpasserine birds that vary consistently between individuals. In other do not learn their vocalizations, vocal discrimi- words, interindividual variation must be high nation has also been demonstrated (Watanabe compared with intraindividual variation (Falls and Ito 1991, Lengagne et al. 2000). For in- 1982). Selection pressure for discrimination of stance, Speirs and Davis (1991) found that co- conspeci®cs could further increase interindivid- lonial AdeÂlie penguins (Pygoscelis adeliae) re- ual variability and decrease intraindividual var- spond differently to playback of neighbors, iability (Beecher 1982). We compared the songs strangers, and mates. Some territorial nonpas- of male and female Spotted Antbirds to deter- serine birds exhibit neighbor-stranger discrimi- mine if song varies consistently between indi- nation, and the ability to distinguish between fa- viduals. We also assessed what proportion of miliar neighbors and unfamiliar intruders may songs could be classi®ed to the individual that be advantageous in territorial maintenance. Ex- produced them based on a variety of song char- amples are the Blue Grouse (Dendragapus obs- acteristics, and whether the sexes differed in curus; Falls and McNicholl 1979), and the Pu- these characteristics. keko (Porphyrio porphyrio; Clapperton 1987). METHODS Suboscines are widespread in the Neotropics and many maintain stable territories and long- STUDY AREA AND SUBJECTS lasting pair bonds (Greenberg and Gradwohl Our study area was Limbo Plot, a 100-ha site 1986, Morton 1996, Morton and Derrickson along Pipeline Road in Soberania National Park, 1996). Discrimination between neighbors and in a lowland moist tropical forest near Gamboa, strangers based on vocal cues may also be im- Panama (9899N, 798459W). Limbo Plot has a portant to territorial suboscines. marked grid system, which facilitated the accu- The Spotted Antbird (Hylophylax naevioides) rate mapping of Spotted Antbird pairs, their ter- is a strongly territorial Neotropical suboscine. ritories, and territorial boundaries. North to Spotted Antbirds are sexually dimorphic, social- south, Limbo Plot is marked every 25 m, and ly monogamous, and stay in pair bonds for many east to west every 100 m. This ®eld site is de- years (Hau et al. 1999, 2000, Wikelski et al. scribed in detail by Robinson et al. (2000). A 2000). Males and females codefend large terri- map of Spotted Antbird territories on Limbo Plot tories of approximately 4.7 ha. (Willis 1972, was kindly provided by J. J. Nesbitt. Each Spot- Robinson et al. 2000). In large and densely veg- ted Antbird at Limbo Plot was color banded for etated territories, auditory cues may provide identi®cation. more information than visual cues at a distance. RECORDING EQUIPMENT AND PROCEDURES Discrimination of the familiar songs of neigh- To obtain recordings for song analysis, we elic- bors from the unfamiliar songs of strangers ited singing behavior by playing male conspe- could be favored in this species, to decrease ag- ci®c songs on Spotted Antbird territories. Indi- gressive interactions between neighbors and to viduals were recorded in the ®eld using a Senn- aid in territorial defense. heiser MZP 816 directional microphone attached We hypothesized that like many oscines, to a Sony Professional Walkman tape recorder. Spotted Antbirds would respond more aggres- We recorded at a distance of 3±4 m until 5±50 sively to strangers than to neighbors at the reg- songs were collected for each individual. All ular territorial boundary. We tested whether songs were recorded between 18 June and 20 Spotted Antbirds can make these distinctions by August 1997. presenting both sexes with song playback of male neighbors and strangers. We also compared SONG VARIATION ANALYSIS the responses of males and females, to determine Two hundred ®fty-seven song examples from 25 whether the sexes defend their territories equal- male and ®ve female Spotted Antbirds were dig- ly. In species where both sexes sing year round, itized and analyzed using SYRINX software partitioning of sex roles in territorial defense (Burt 2001). For each song example we mea- may be ef®cient (Willis 1972, Morton and Der- sured center frequency in kHz (the point half- rickson 1996, Levin 1996a). way between the highest and lowest frequency SPOTTED ANTBIRD VOCALIZATIONS 389 of the song), frequency range (difference in kHz Each male served both as a subject for play- between the highest and lowest frequency), du- back trials and as a source of a song for trials ration of the song, and song speed (number of on his neighbor. Stranger songs were recorded song syllables per sec, counting paired short and from Spotted Antbird males holding territories long elements as a single syllable). Song data at least 5 km from the study site. No individual's were recorded directly into a log ®le and con- song was used on more than one subject, to verted to SPSS 8.0 for Windows (SPSS Inc. avoid pseudoreplication (Kroodsma 1989b). 1997) for statistical analysis. We tested whether Playback trials were conducted between 6 Au- among-individual song differences exceeded gust and 12 September 1997, during the Spotted within-individual differences within each sex, Antbird breeding