Behavioral response of wrens (Troglodytidae) to familiar and unfamiliar predator playbacks Carl D. Johnson Department of Biology, Gu stavus Adolphus College Abstract Predator-prey interactions are important for determini ng fitness, especially including a prey speciesâ€™ ability to distinguish a predator from a non-predator. In th is study, I investigated how three wren species: the Plain Wren ( Thryothorus modestus ), the House Wren ( Troglodytes musculus ) and the Rufus-and-white Wren ( Thryothorus rutilus ) respond to familiar and unfamiliar pred atory calls. The test subjects responded with higher warning call frequency to the predatory calls than the control (p<0.05). Wrens responded to the two predatory birds with similar call frequency, though there were varied responses in approach behavior. The fact that wrens respond to unkno wn predators similarly to known predators implies that a combination of both genetic inheritance and learning affects predatory birdcall recognition in wrens. Resumen Las interacciones depredador-presa son importantes para determinar la supervivencia de las especies, especialmente en la habilidad de las especies para distinguir entre organismos depredadores o no depredadores. En este estudio, investigue la habilidad de tres especies de soterrey: Thryothorus modestus, Troglodytes musculus y Thryothorus rutilus de responder a cantos conocidos y desconocidos para las especies. Encontr que estas especies responden cantando con mas frecuencia a cantos de depredadores que a los cantos de tipo control (p<0.05). Las especies de soterreys responden de manera similar a las dos especies de aves depredadoras, sin embargo tuvieron respuestas variadas en el comportamiento de acercarse al estimulo. El hecho de que estas especies respondieron de manera similar a depredadores conocidos y no conocidos puede explicarse a que tanto las caractersticas heredadas y los procesos de aprendizaje afectan el reconocimiento de los can tos de los depredadores en los soterreys. Introduction Predator-prey interactions have a major influence on species fitness. The ability of a prey species to recognize threats from potential predators and respond accordingly is essential to survival. There is a wide sp ectrum of defense mechanisms that animals employ to avoid and flee from predators. These behaviors are classified in two groups: the anti-detection, where they are able to hi de from their predator s and the anti-capture, where they use behaviors to avoid death af ter being threatened by a predator (Alcock 1984). Animals must first be able to distinguis h an actual predatory threat with various cues, in order to determine when to use de fense mechanisms appropriately. Animalsâ€™ abilities to distinguish these cues have been studied in terms of: visual cues (Waggett 2007), olfactory cues (Zhao et al. 2006), and auditory cues (Johnson et al. 2003). One study in Costa Rica showed that 32 species of birds have the abili ty to distinguish between threatening and non-threatening species â€™ auditory calls. They responded 75% of the time to raptor calls, while only 10% of the time to a non-threatening species call 1
(Elahi 2000). Another study has shown how Western American Crows can distinguish between a familiar predator, the Red-Shoulde red Hawk calls, and an unfamiliar species, the Madagascar Harrier Hawk calls, with which they have not had previous exposure (Houser and Caffrey 1994). Although, the ability to determine threat is well studied, the actual mechanisms of learning and heredity are often overlooked. I explored if there were trends in how wrens respond to familiar and unfamiliar predatory calls. I expected wrens to react more strongly to predator calls than a nonpredator call, the control, because having the ability to determine an auditory threat would imply higher fitness. I also questioned if wrens were able to distinguish between familiar and unfamiliar predator calls. Wrens were expected to respond by calling and advancing more in response to an unknown predator. Typically wrens respond to a known predator by giving a few warning calls and then hiding. It is expected that they will respond with less caution to the unfamiliar calls because they have not learned them to be actual threats. Methods Study Site and Bird Species The study took place in the San Luis, Cost a Rica. Study sites included Projecto Bella La Finca, the road to the San Luis Research Station, along La Trocha, and Xinia Arayaâ€™s farm. The three wren species in this study, Plain Wren ( Thryothorus modestus ), the House Wren (Troglodytes musculus ) and the Rufus-and-white Wren ( Thryothorus rutilus ), are commonly found foraging in this undergrowth of non-forested areas (Garrigues and Dean 2007). Given the nature of the territory preferen ces of wrens, most of the study sites were thick shrubs or secondary forest edges. The two predator calls used were: the Mottled Owl ( Ciccaba virgata ), which is fairly common in the San Luis area, and the Spectacled Owl ( Pulsatrix perspicillata ), which is not found in the area (Fogden 1993). The control species used was the Great Kiskadee ( Pitangus suophuratus ), common in San Luis, but not a direct threat to wrens (Stiles and Skutch 1989). Sampling Methods and Analysis Each wren was watched for two-minutes to determine calling behavior. Each individual was then given the same 18-minute stimuli series: the Mottled Owl for fourminutes, two-minutes of silence, the Great Kiskadee for four-minutes, two-minutes of silence, and then the Spectacled Owl for four -minutes. During each of the three stimuli periods the number of calls we re recorded. Wren calls are varied so each call, no matter the type, was counted as equivalent to one ca ll. Whether or not th e bird approached the stimuli was also recorded. An approach beha vior was if the bird obviously moved toward the stimuli in an investigative manor. The calls used for the stimuli were from the â€œBirds of Costa Ricaâ€ album and played using an iPod with portable JBL iPod speakers. The call response data were then analyzed by us ing a Friedmanâ€™s test and the number of wrens that approached were anal yzed with a goodness of fit test. 2
Results A total of 24 different wren individuals were studied. The wrens were able to differentiate between the owl predator calls and the control (f=12.72, p=0.001, df=2;Fig 1). They responded more frequently in res ponse to the owl calls, and they approached the predator stimuli more often than the c ontrol (Fig 2). The wrens approached more often towards the Mottled Owl than the ot her two stimuli (Fig 2) but had similar responses in call frequenc y averages (Fig. 1). 0 2 4 6 8 10 12 Mottled OwlGreat KiskadeeSpectacled Owl Species FIGURE 1. Average call response frequency of Troglodytidae family to three stimuli: the familiar predator (Mottled Owl), the control (Great Kiskadee), and the unfamiliar predator Spectacled Owl (f=12.72, p=0.001, df=2). 0 2 4 6 8 10 12 14 Mottled Owl Great KiskadeeSpecktacled Owl Species FIGURE 2. Number of Troglodytid ae individuals out of a tota l sample size (n=24) that advanced toward the stimuli during each of the four-minute stimuli play periods (x2=11.7886, df=2, p=0.0028). 3
Discussion Wrens are able to distinguish between predator and non-pred ator calls, which show that they have the ability to determine whether or not calls are threatening. For this reason, being able to distinguish the difference between bird calls improves bird fitness (Goth and Hauber 2004); this should also hold tr ue for the ability of a bird to identify predator calls from non-predator calls. These data also show that there is no difference in response to familiar and unfamiliar predator stimuli. This data could suggest that a combination of factors play into predator-specific recogniti on in wrens. It is possible that there is a common sound characteristic in both owl calls that wrens in terpret as threatening. One study showed that the mechanics of birdcalls of similar species are constrained to a similar vocal range (Podos 2004). The two owls, in my study, are phylogenically similar, and so they may have both inherited a call that is interpre ted similarly by wrens. Another study has found that closely related birds have similar neural anatomy that controls the calling function, which would be another reason that owls coul d sound alike to the wrens (Striedter 1993). Therefore, there may be some similar char acteristics, due to mechanics and neural functions, which identify the owls as like-pr edators. Another possibility is that the interpretation of familiar and unfamiliar owl calls as threatening is innate, given that there was no exposure time to learn the unfamiliar call. A study done on predator-nave adult Great Tits found that they were able to rec ognize predators innately (Kullberg and Lind 2002). Although, further research is needed to determine if wrens have the same innate abilities as Great Tits and if these owls have similar call characteristics, it is plausible that wrens have the innate knowledge of what a threatening predator call should sound like. Furthermore, these results could show that inheritance is important in determining threatening calls based on prey-predator ra nge overlap. The three wrens in this study have population ranges that overlap with the unfamiliar predator. If there is gene flow between each population of wren with and with out the presence of th at predator, it is possible that the ability to determine the unf amiliar predator as threatening was carried into the San Luis population from another gene pool. Therefore, though the unfamiliar owl is not present in this population, these wr ens are still able to identify the call as a potential threat. The data of the total amount of approach es toward stimuli shows that most wrens approach the familiar predator, less approach the unfamiliar predator, and even less approach the control. This trend does not support the initial predic tion, which stated that more wrens would approach the unfamiliar predator call. The explanation for this could be that the perceived threat of the unfamiliar predator call was greater than that of the familiar call. Since these wrens have not learned the consequences of approaching the unfamiliar call, they could be more cautious to approach. It is also possible wrens are willing to approach the familiar predator call more often because they have learned the associated threat of this predator. Several things may have influenced th e accuracy of the data, including the small sample size. In response to the stimulus, wr ens exhibited a wide variety of call types, each type being considered equal to the others. A future study should include call type and quantity, which would insure more precis e data. Lastly, the three wren species were 4
considered one group and interspecies variati on may have caused greater range of results. Studying wren species individually would a llow for more accurate species-specific results and behavioral cues. Further research should include a larger sample size that is based on individual species. More predator calls could also be us ed to find larger amounts of data. Predator species should be chosen from a variety of families to reveal stronger data. The unfamiliar predator species should have a non-ov erlapping range as the prey species to eliminate gene flow as a variable in the study. Acknowledgements Thanks to Tanya Chavarria for data analysis, fieldwork and editing, Pablo Allen and Teagan McMahon for everything and Xinia Araya and Bella la Finca for use of their farm land. Literature Cited Alcock, J. 2005. Animal Behavior, An Evolutio nary Approach. Sinaue r Associatges, Inc., Sunderland, Massachusetts, USA. Fogden, M. 1993. An Annotated Checklist of the Birds of Monteverde andPenas Blancas. Field guide. Self published. Goth, A. and M. Hauber. Ecological approach es to species recognition in birds through studies of model and non-model species. Ann. Zool. Fennici. 41: 823-842. Houser, M.D. and C. Caffrey. 1994. Anti-predator response to raptor calls in wild crows, Corvus Brachyrhynchos Hesperis . Animal Behaviour 48: 1469-1471. Johnson, F.R ; E.J. McNaughton, C.D J Shelley and D,T. D Blumstein .T. 2003. Mechanisms of heterospecific recognition in avian mobbing calls. Aust. J. Zool. 51(6): 577-585. Kullberg, C and J. Lind.An Experimental St udy of Predator Recognition in Great Tit Fledlings. Ethology. 108(5):429-441. Miller , J. A. 1986. Tropical Trickery: Birds sound False Alarm. Science News 129(3): 40. Podos, J., S. Huber and B. Taft. Bird Songs: the Interface of Evolution and Mechanism. Annual Review of Ecology, Evolution, and Systematics. 35:55-87. Stiles, G. and A. Skutch. 1989. A guide to the Birds of Costa Rica. Field Guide. Comstrock Publishing associates, Ithaca, New York. Striedter, G. 1993. The vocal control pathwa ys in budgerigars differ from those in songbirds. Comparative Neurology. 343(1): 35-56. 5
6 Waggett, R. J; E. J Buskey. 2007. Calanoid copepod escape behavior in response to a visual predator. Marine Biology 150(4):599-607. Zhao, X; MCO.Ferrar and D.P Chivers. 2006. Threat-sensitive learning of predator odours by a prey fish. Behaviour. 143 (9):1103-1121. 2006.
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Johnson, Carl D.
Respuesta de comportamiento del Soterrey (Troglodytidae) a las reproducciones de los depredadores conocidos y no conocidos
Behavioral response of wrens (Troglodytidae) to familiar and unfamiliar predator playbacks
Predator-prey interactions are important for determining fitness, especially including a prey species ability to distinguish a predator from a non-predator. In this study, I investigated how three wren species: the Plain Wren (Thryothorus modestus), the House Wren (Troglodytes musculus) and the Rufus-and-white Wren (Thryothorus rutilus) respond to familiar and unfamiliar predatory calls. The test subjects responded with higher warning call frequency to the predatory calls than the control (p<0.05). Wrens responded to the two predatory birds with similar call frequency, though there were varied responses in approach behavior. The fact that wrens respond to unknown predators similarly to known predators implies that a combination of both genetic inheritance and learning affects predatory birdcall recognition in wrens.
Las interacciones de depredador-presa son importantes para determinar la sobrevivencia de las especies, especialmente en la habilidad de las especies para distinguir entre los organismos depredadores y los no depredadores. En este estudio, investigue la habilidad de tres especies de Soterrey: Thryothorus modestus, Troglodytes musculus y Thryothorus rutilus de responder a los cantos conocidos y desconocidos de los depredadores.
Text in English.
Predator & prey
Depredador y presa
Tropical Ecology 2007
Ecologa Tropical 2007
Respuesta de comportamiento
t Monteverde Institute : Tropical Ecology