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Respuesta territorial del Soterrey blanco (Thryothorus rufalbus) a los cantos de los vecinos/desconocidos
Territorial response of Rufous-and-white Wrens (Thryothorus rufalbus) to neighbor/stranger conspecific playbacks
I tested the Rufous-and-white Wren (T. rufalbus) for discrimination between the songs of neighbor and stranger conspecifics. I found significant differences in behavioral responses between the song playbacks of neighboring and unfamiliar conspecifics in seventeen pairs of the Rufous-and-white Wren. Birds sang for longer periods and sang more songs in response to playbacks of strangers. However, the amount that they approached the playback source did not differ between the song types. These results suggest that Rufous-and-white wrens recognize the songs of neighbors and identify the individuals as less threatening than strangers. This diminished response may be an adaptive strategy to reduce the energy spent in territory defense. Wrens do not differentially approach the song source because this behavior is not a primary territorial defense strategy.
Yo prob la habilidad del Soterrey Rufo y Blanco para discriminar entre los cantos de los soterreses vecinos y los cantos de los soterreses desconocidos. Encontre diferencias en las reacciones entre los cantos de los vecinos y los desconocidos en diecisiete parejas del Soterrey Rufo y Blanco.
Text in English.
Canto de aves
Tropical Ecology 2006
Ecologa Tropical 2006
t Monteverde Institute : Tropical Ecology
Territorial response of Rufous-and-white Wrens (Thryothorus rufalbus) to neighbor/stranger conspecific playbacks Kaitlin Dunn Department of Zoology and Environmental Studies University of Wisconsin, Madison ABSTRACT I tested the Rufous-and-white Wren ( T. rufalbus ) for discrimination between the songs of neighbor and stranger conspecifics. I found significant differences in behavioral responses between the song playbacks of neighboring and unfamiliar conspecifics in seventeen pairs of the Rufous-and-white Wren. Birds sang for longer periods and sang more songs in response to playbacks of strangers. However, the amount that they approached the playback source did not differ be tween the song types. These results suggest that Rufous-and-white wrens recognize the songs of neighbors and identify the individuals as less threatening than strangers. This diminished response may be an adaptive strategy to reduce the energy spent in territory defense. Wrens do not differentially approach the song source because this behavior is not a primary territorial defense strategy. RESUMEN Yo prob la habilidad del Soterrey Rufo y Blanco para discriminar entre los cantos de soterreyes vecinos y los cantos de soterreyes desconocidos. Encontr dife rencias en las reaccines entre los cantos de vecinos y los desconocidos en diecisiete parejas del Soterrey Rufo y Blanco. Los soterreyes cantaron por ms tiempo y cantaron ms cantos al contacto con el canto de los so terreyes desconocidos. Sin embargo, no hubo una diferencia en la cantidad de individuos que se acercaron a la fuente del canto. Los resultados sugieren que el Soterrey Rufo y Blanco reconoce los cantos de sus vecinos y saben que ellos son menos amenazantes que los desconocidos. Esta respuesta puede ser una estrategia adaptiva para reducir la energa que el soterrey necesita utilizar para defend er su territorio. Posiblemente el Soterrey Rufo y Blanco no se acerca el fuente del canto porque esta no es un a estratega defensiva para esta especie. INTRODUCTION Territorial defense is often a costly endea vor for an animal. Many territorial birds minimize this cost by singing to communicate ownership of an area rather than engaging in direct physical contact (Davies and Hous ton 1984). However, several studies have suggested that singing is stil l quite energetically costly a nd is directly and indirectly limited by energy constraints (Kroodsma and Miller 1996). A singing Carolina Wren uses five times more oxygen than it does while resting (Eberhardt 1994). Territorial birds must devote a certain amount of their available energy to singing in order to define and defend territorial boundaries. Many species of birds, such as the St riped-backed Wren a nd the Carolina Wren, have been found to distinguish between the songs of their familiar neighbors and the 1
songs of unfamiliar, potential intruders (B ard et al. 2002; Shy and Morton 1986; Wiley and Wiley 1997). Species of territorial birds not only demo nstrate the capability of recognizing individuals by their song but al so respond differently to the songs of neighboring and non-neighboring birds (Kroods ma 1976). Once neighbors identify each other and where territorial boundaries lie, they can save energy by decreasing the degree to which they defend these boundaries. This phenomenon is known as the dear enemy effect (Alcock 2005). Studies have been done to demonstrate th is dear enemy effect in many bird species. However, it cannot be sa id that vocal recognition is uni versal in territorial birds (Kroodsma and Miller 1996). The purpose of this study is to investigate whether T. rufalbus gives differential defensiv e territorial responses to the songs of neighbor and stranger conspecifics. I predict that songs from strangers will solicit a more aggressive reaction than will the songs of the more familiar neighbors. MATERIALS AND METHODS Study Species The Rufous-and-white Wren belongs to the family Troglodytidae. In Costa Rica, the Rufous-and-white Wren is a common reside nt of the North Pacific slope and the mountains of the Nicoya Peninsula (Stiles and Skutch 1989). An inhabitant of open, scrubby woodland, the birds are usually found in breeding pairs that de fend their territory year round. The male and female duet in co mplex phrases, but males sing significantly more (Ahumada 2001; Stiles and Skutch 1989). Th e song is characteristic of their genus, with easily localizable pure tones and wh istles. However, the individuals are conspicuous and difficult to see (Ahumada 2001). Study Site I conducted my study at the Ecological Farm in Cerro Plano, Costa Rica, in October and November 2006. The Ecological Farm is in Zone 2 (premontane wet forest) where Rufous-and-white Wrens are fairly common (Fogden 1993). The 30 ha reserve has four trails that run through secondary forest. Data Collection Initially, I identified seventeen pairs of wrens and the general location of their territories. I broadcasted a previous territorial recording of T. rufalbus (from Costa Rican Bird Songs CD) while walking the trails at the Ecologi cal Farm. When a subject or subjects responded to the recording, I marked that lo cation with a piece of flagging tape and assigned that territory a nu mber one through seventeen. Using an Apple iPod A1059 with a Micromemo microphone attachment, I recorded the territorial songs of each pair by soliciting their calls with the sample T. rufalbus recording. I continued to play the samp le song in order to get them to move closer and to obtain multiple song repetitions In many territories I could only get one bird to respond and this was all that I reco rded. All recordings were at least fifteen seconds long and contained at least two song repetitions. 2
I tested each pair of wrens for th eir responses to song playbacks from a neighboring pair and from a stranger pair that was at leas t two territories removed. I tested each pair in two trials with a neighbors song and two trials with a strangers song, but never on the same day. I stood in the same spot in a birds territory for each trial, which consisted of one minute of playback, th ree minutes of silence, and one minute of playback. After each minute of playback I documented three measures of territorial behaviors: the total length of song respons e, the number of song repetitions in the response, and whether the bird(s) approache d. The second trial wa s initiated twenty minutes after the end of the last response from the previous trial. All trials took place between 6:00 a.m. and 11:00 a.m. A paired t-test compared the song response length to nei ghbor songs with the song response length to stranger songs. A pa ired t-test was also used in comparing the number of defensive song repetitions sung in response to neighbors versus the number sung to strangers. I used a Chi-square test for independence to l ook for significance in whether or not a bird approached the sound source. RESULTS I found that subjects generally responded fo r a longer amount of time when the songs of strangers were played (t-tes t, t = -5.77, df = 33, p < .05, Fi gure 1). A greater number of song repetitions were sung in response to th e playbacks of stranger songs (t = -5.36, df = 33, p < .05, Figure 2). There was no significant difference in the number of approaches made to the song source (x = 1.375, df = 2). Thus, subjects did not approach during one playback type more th an during the other. DISCUSSION As predicted, the territorial behavior of T. rufalbus differed in response to the songs of neighbor and stranger conspecifi cs, with strangers eliciting a stronger response. Subjects sang more defensive songs and for a longer amount of time when the songs of strangers were played, but they did not differ in the amount that they appr oached the source of playback. This means that subjects were able to identify the song of a neighbor as different from the song of a stranger. Mennill and Vehrencamp (2005) showed that male Rufous-and-white Wrens share song types with their neighbors and that this sharing decreases with increasing distance between territories. My resu lts indicate that this song type matching may play a role in territorial interactions between neighbors because it allows individuals to identify one another Further research could indicate if the territory holder memorizes the songs of individual neighbors or if it r ecognizes that a similar song type confers neighbor status. The results of this study indicate that Rufous-and-wh ite Wrens view neighbors as less likely to invade their territory than st rangers. Because territo rial boundaries between neighbors are likely to be established, the th reat of territory i nvasion by a neighbor is lower than by a stranger looking to esta blish in a new territo ry (Kroodsma 1976). 3
Reduced aggression to neighbors may be a significant savings in energy (Kroodsma 1976). Therefore, a bird that can recognize th e songs of neighbors and give a diminished response will have more energy for other pro cesses such as reproduction. This bird will have an adaptive edge over birds without this capability (Alcock 2005). Subjects were not found to approach the s ource of playback more frequently when the songs of strangers were broadcasted. However, few approaches by subjects were observed in response to either playback type. A study with Nuttals White-crowned Sparrows found the difference between the number of approaches in response to neighbor and stranger playbacks was less significant than was the difference between the numbers of songs sung (Baker et al. 1981). If approach is a frequently used defense mechanism, I may not have obtained a large enough sa mple size to find significant results. Although Rufous-and-white Wrens live in breeding pairs and defend their territory year round, I most often observed singing by only one indivi dual per territory. This individual was more likely to be the male because males are known to sing more than females (Ahumada 2001). A study similar to mine should be conducted during the breeding season to determine whether females take a more active role in territorial defense during this time. Territorial respons es may be more aggressive during this time (Alcock 2005), and the occurrence of song source approach may differ between neighbors and strangers at this time. ACKNOWLEDGEMENTS I wish to thank Karen and Alan Masters fo r their work in organizing the independent study projects. I also am very grateful to Tania Chavarria Pizarro for all of her of guidance and help with statis tical analysis. A big tha nk-you goes to Cam Pennington and Tom McFarland for their assistance with many of the details and for their many useful comments and suggestions. Thanks to Mirella Salazar for use of the study site and to Grant Connette for taking me bird watching in the Monteverde Cloud Forest Reserve. LITERATURE CITED Ahumada, J.A. 2001. Comparison of the Reproductive Biology of Two Neotropical Wrens in an Unpredictable Environment in Northeastern Colombia. The Auk 118(1): 191-210. Alcock, J. 2005. Animal Behavior: An Evolutionary Approach Sinauer Associates, Sunderland Massachusetts. Baker, M.C., D.B. Thompson and G.L. Sherman. 1981. Neighbor/Stranger Song Discrimination in WhiteCrowned Sparrows. The Condor 83(3): 265-267. Bard, S.C., M. Hau, M. Wikelski and J.C. Wingfi eld. 2002. Vocal Distinctiveness and Response to Conspecific Playback in the S potted Antbird, a Neotropical Suboscine. The Condor 104: 387-394. Davies, N.B. and A.I. Houston. 1984. Territory Economics. In: Krebs, J.R. and N.B. Davies, (editors). Behavioural Ecology: An Evolutionary Approach Sinauer Associates, Su nderland, Massachusetts. Eberhardt, L.S. 1994. Oxygen consumption during singing by male Carolina Wrens ( Thryothorus ludovicianus ). The Auk 111:124-130. 4
Fogden, M. 1993. An Annotated Checklist of the Birds of Moveteverde and Peas Blancas. Michael Fogden, Monteverde, Costa Rica Kroodsma, D.E. 1976. The Effect of Large Song Re pertoires on Neighbor Recognition in Male Song Sparrows. The Condor 78: 97-99. Kroodsma, D.E. and H.E. Miller. 1996. Ecology and Evolution of Acoustic Communication in Birds. Cornell University Press, Ithaca and London. Mennill, D.J. and S.L. Vehrencamp. 2005. Sex Di fferences in Singing and Duetting Behavior of Neotropical Rufous-and-white Wrens ( Thryothorus rufalbus ). The Auk 122(1): 175-186. Shy, E., and E.S. Morton. 1986. The role of distance, familiarity, and time of day in Carolina Wren responses to conspecific songs. Behavioral Ecology and Sociobiology 19: 393-400. Stiles, F.G. and A.F. Skutch. 1989. Birds of Costa Rica. Cornell University Press, Ithaca, New York. Wiley, R.H. and M.S. Wiley. 1977. Recognition of neighbors duets by Stripe-backed Wrens ( Campylorhynchus nuchalis ). Behaviour 62: 10-34. FIGURES 5
Std. Dev. Std. Err. Mean 20 30 40 50 60 70 80 90 seconds len g th of res p onse ( seconds ) NS Figure 1. Length of response (in seconds) to playbacks from neighbors (N) and strangers (S). Subjects sang for a longer amount of tim e in response to the s ongs of strangers than they did to neighbors (paired ttest, t = -5.77, df = 33, p < .05). Std. Dev. Std. Err. Mean 2 4 6 8 10 12 14 16 18 # songs # of son g s NS Figure 2. Number of songs sung in response to playbacks from neighbors (N) and strangers (S). More songs were sung in res ponse to the songs of strangers than to the songs of neighbors (paired t-test t = -5.36, df = 33, p < .05). 6