SMall SOng REpERTOiRES and HigH RaTE Of SOng-TypE SHaRing ...

The Condor 115(4):874–881  The Cooper Ornithological Society 2013

Small Song Repertoires and High Rate of Song-Type Sharing among Canyon Wrens Lauryn Benedict1, A nne Rose ,

and

Nathanial Warning

School of Biological Sciences, University of Northern Colorado, 501 20th Street, Campus Box 92, Greeley, CO 80639 Abstract.  The song of the Canyon Wren (Catherpes mexicanus) is widely recognized but poorly studied. Here we present the first comprehensive description of Canyon Wren song, including data from a focused study in northern Colorado and data from songs recorded across the species’ geographic range. Both male and female Canyon Wrens produce a stereotyped contact call and sex-specific song. Females sing infrequently, using a single song type. Males typically sing five song types, all consisting of a descending cascade of notes to which they variably append up to seven broadband notes. Songs are delivered with eventual variety in bouts that include an average of 4.6 repetitions of one song type, with temporal breaks between bouts. In the population we studied most song types were widely shared and males’ repertoires overlapped by 94%. Some song types are apparently geographically restricted, while others are sung across large areas of the species’ range. Overall, our results suggest that the Canyon Wren’s song repertoires are relatively small compared with those of other wrens and that overlap of individuals’ repertoires is high. Local and rangewide stability of certain song types may reflect high fidelity of copying during learning, this stability may be favored by ecological and life-history traits including low density of territories, lack of migration, and long-term monogamy. Key words:  bird song, Catherpes mexicanus, Canyon Wren, geographic variation, Troglodytidae.

Pequeño Repertorio de Canciones y Alta Tasa de Intercambio del Tipo de Canción entre Individuos de Catherpes mexicanus Resumen.  El canto de Catherpes mexicanus es reconocido ampliamente pero está poco estudiado. Aquí presentamos la primera descripción exhaustiva del canto de C. mexicanus, incluyendo datos de un estudio focal en el norte de Colorado y datos de cantos registrados a lo largo del rango geográfico de la especie. Tanto los machos como las hembras de C. mexicanus producen un llamado de contacto estereotipado y un canto específico del sexo. Las hembras cantan de modo infrecuente, usando un tipo único de canto. Los machos típicamente cantan cinco tipos de cantos, todos consistentes en una cascada descendente de notas a los que se anexan de modo variable hasta siete notas de banda ancha. Los cantos son entregados con una variedad eventual en tandas que incluyen un promedio de 4.6 repeticiones de un tipo de canto, con cortes temporales entre tandas. En la población que estudiamos la mayoría de los tipos de cantos fueron ampliamente compartidos y los repertorios de los machos se superpusieron en un 94%. Algunos tipos de cantos están aparentemente restringidos geográficamente, mientras que otros son cantados a lo largo de grandes áreas del rango de la especie. En general, nuestros resultados sugieren que los reper­ torios de cantos de Catherpes mexicanus son relativamente pequeños comparados con aquellos de otras especies de Catherpes y que la superposición de los repertorios de los individuos es alta. La estabilidad local y a lo largo del rango de ciertos tipos de cantos puede reflejar una alta fidelidad de copiado durante el aprendizaje, esta estabilidad puede estar favorecida por rasgos ecológicos y de historia de vida incluyendo baja densidad de territorios, falta de migración y monogamia de lago plazo.

INTRODUCTION

Wren’s song. In his classic Life Histories of North American Birds Bent (1948) described it this way:

The Canyon Wren (Catherpes mexicanus) produces one of the most recognizable bird songs in western North America. The male’s song consists of a descending cascade of loud, liquid notes of modulated frequency, sometimes followed by one or more harsh syllables (Kroodsma 1977, Benedict et al. 2012). People have long acknowledged the beauty of the Canyon

“We saw or heard a long list of interesting birds, but the gem of them all was the Canyon Wren. Its wild, joyous strain of sweet, silvery notes greeted us as we passed some steep cliffs; they seemed to reverberate from one cliff to another, to fill the whole canyon with delightful melody and to add a fitting charm to the wild surroundings”

Manuscript received 12 March 2013; accepted 27 May 2013. 1 E-mail: [email protected] The Condor, Vol. 115, Number 4, pages 874–881. ISSN 0010-5422, electronic ISSN 1938-5129.  2013 by The Cooper Ornithological Society. All rights reserved. Please direct all requests for permission to photocopy or reproduce article content through the University of California Press’s Rights and Permissions website, http://www.ucpressjournals.com/ reprintInfo.asp. DOI: 10.1525/cond.2013.130025

874

features of Canyon Wren song   875

Though the Canyon Wren’s song is iconic, it has not been widely studied (Jones and Dieni 1995). Existing descriptions suggest that the species’ vocal repertoire includes contact calls, songs of both the male and female, and less frequent vocalizations such as alarm calls, begging calls, and subsong from young birds (Jones and Dieni 1995, Spencer 2012). Birds sing a wide variety of songs that have many forms and multiple functions (Gil and Gahr 2002, Benedict 2010). By describing the features of the song and repertoire of each species we can better understand how ecology and evolution shape song diversity. Here we provide a comprehensive description of the song of the Canyon Wren, focusing on a population in northern Colorado, with additional sampling from across the species’ range. The Canyon Wren is resident throughout its range and monogamous pairs occupy territories year round (Jones and Dieni 1995). Pairs are often widely spaced, and may or may not directly contact a neighboring territory (Jones and Dieni 1995). Current research suggests that males sing primarily in the spring and that their song functions in species identification and territory defense (Jones and Dieni 1995, Benedict et al 2012, Rose and Phan 2013). The goals of our study were to describe (1) the types of songs sung by male and female Canyon Wrens, (2) the size of individuals’ song repertoires, (3) the pattern of delivery of the Canyon Wren’s song, (4) the degree of song-type sharing within one population and (5) the persistence of song types across the species’ geographic range. The Canyon Wren belongs to a family (Troglodytidae) known for diverse and fascinating singing behavior (Mann et al. 2009). Morphological variation in the wrens is rather modest, and it is thought that natural selection has promoted elaboration of displays mostly through song evolution (Kroodsma 1977). Wrens sing a wide variety of song types, including songs with complex structure given by both males and females (Mann et al. 2009). Although most species of wren in which the female sings are neotropical, females of a few North American species, including the Canyon Wren, also sing (Jones and Dieni 1995, Spencer 2012). Among songbirds in general, song complexity may be reflected in both the form of the song (its acoustic structure) and size of the repertoire. Relatively small song repertoires (1–10 song types) are common among oscine passerines, but repertoire size varies dramatically, with some species (such as mimics) singing over 1000 different song types (Byers and Kroodsma 2009, Kroodsma and Parker 1977). In both the neotropics and temperate zones most species of wrens have large (>20 song types) repertoires (Kroodsma 1977, Mann et al. 2009). For example, the tropical Black-bellied Wren (Pheugopedius fasciatoventris) sings approximately 40 song types, and Rock Wren (Salpinctes obsoletus) of western North America may sing over 100 song types (Kroodsma 1975, Logue 2007). While the majority of wrens sing many song types, diversity is not universal: individual Winter Wrens (Troglodytes hiemalis) sing as few as two song types

(although these songs are strikingly complex and may include elements that get moved around between types) (Kroodsma 1980). The Canyon Wren’s repertoire is also thought to be small. Kroodsma (1977) and Spencer (2012) suggested that males’ repertoires typically include up to three song types, though they both acknowledged limited sampling. Phylogenetic constraints do not seem to shape the size of wrens’ song repertoires or their patterns of delivery. The Canyon Wren is closely related to the Rock Wren yet apparently differs greatly in song behavior (Kroodsma 1977, Mann et al. 2006). Patterns suggest that ecological and social factors may be more important than evolutionary history in shaping song behavior (Benedict 2008). For example, sexual selection may promote a large repertoire via female choice, particularly in polygynous species or in species that frequently switch mates (Catchpole 1987, but see Byers and Kroodsma 2009). Ecological factors may also shape song diversity: song repertoires are predicted to be small among species that live in low densities because at low densities, songs function primarily in species identification over long distances (Kroodsma 1977). It has also been suggested that nonmigratory birds and birds that live in arid environments tend to have small repertoires (Read and Weary 1992, Garamszegi et al. 2006). These hypotheses (which have not all been extensively supported—see Wiley 2000) all predict a small song repertoire in the Canyon Wren, despite the many closely related species with very large repertoires. In addition to their characteristic song types and repertoires, bird species differ in how those repertoires are revealed over time. Among wrens, there is evident diversity in songdelivery patterns. Marsh Wrens (Cistothorus palustris) sing with immediate variety, Rock Wrens sing an apparently random series of song types, and Carolina Wrens (Thryothorus ludovicianus) sing with eventual variety (Verner 1975, Kroodsma 1977). The Canyon Wren is also reported to sing with eventual variety, but it is not known how song bouts are structured and when song types are switched. As the sizes of individuals’ repertoires vary, so too do repertoire sizes at the population level, depending on whether the birds copy with high fidelity or improvise new song types (Cicero and Benowitz-Fredricks 2000, Beecher and Bren­owitz 2005). In many species neighbors share a high proportion of their song types, while in others neighbors share a minority of their song or syllable types (Hill et al. 1999, Molles and Vehrencamp 1999, Tracy and Baker 1999). The range of repertoire sharing in the wrens is wide: Sedge Wrens sing only 5% of their neighbors’ songs, while Banded Wrens share an average of 77% of their songs with neighbors (Kroodsma and Verner 1978, Molles and Vehrencamp 1999). No studies to date have examined song sharing of Canyon Wrens either at a local scale or at a wider geographic scale. Since songs seem to be important in species identification over long distances, we expect them to be relatively conserved in form within and potentially across populations.

876  Lauryn Benedict

et al .

METHODS Our study took place on public lands in Larimer County, Colorado. We located Canyon Wren territories in 2011 and 2012 through systematic surveys of arid, rocky habitat with cliffs along the Front Range of the Rocky Mountains. Surveys included acoustic monitoring of appropriate habitat and playback of Canyon Wren song. Once we located a territory, we monitored it through the remainder of each breeding season. Our data come from male and female Canyon Wrens in 19 territories (16 monitored in 2011, 3 in 2012). Canyon Wrens occur as monogamous pairs that occupy widely spaced territories year round (Jones and Dieni 1995, Jones et al. 2002). We did not mark individuals, but the spacing and simultaneous monitoring of neighboring territories made us confident that we sampled different individuals at each site. Most territories were not directly adjacent to another Canyon Wren territory, so many birds were unlikely to regularly interact with, or even hear, their nearest neighbors. The three territories on which we recorded males in 2012 were active in 2011, but we were unable to record those males in that year. Given the species’ sedentary habits, it is likely that the three birds recorded in 2012 were the territory holders in 2011. We recorded audio of territorial Canyon Wrens throughout the breeding season, between May and July of each year, and visited each site previously identified 1–4 times (mean number of visits ± SD = 2.21 ± 1.13) for focused observations of singing behavior. Recording sessions were typically 1–3 hr in length. During a session, we attempted to record all vocalizations but did not record continuously because the birds did not sing continuously. We typically did not record during periods of silence and began when birds started to vocalize. We used either a Sennheiser MKH 20 microphone with a Telinga 24-inch parabolic reflector or a Sennheiser MKH70 shotgun microphone connected to a Marantz PMD 670 solid-state digital recorder. Recordings were made in mono at a sampling frequency of 48 kHz and a sample depth of 16 bits. All recordings used in this study were of natural songs recorded when the birds had not been exposed to playback or other disturbances. We analyzed Canyon Wren vocalizations in the soundanalysis software Raven Pro 1.3 (Cornell Laboratory of Ornithology) and represented them visually as Hanningtype spectrograms with a discrete Fourier transform of 512 samples, a frequency resolution of 11.7 Hz, and a time resolution of 5.33 msec. A. Rose assigned all appropriate vocalizations to one of the following three types by eye: calls, female song, or male song. She then categorized all male song types by eye and built song repertoires for each male, as well as for the entire population. Song types are stereotyped and clearly distinctive within the population, making us confident in these assignments. To test whether our sampling of repertoires was exhaustive, we devised

song-type discovery curves for six birds from which we recorded over 100 songs each. We examined song-delivery patterns and song length with the subset of six males for which we had over 100 songs each. For all of these males we had recordings on multiple days and multiple exemplars of each song type. Given that Canyon Wrens sing with eventual variety, we asked how many songs of the same type they typically sing before switching to a new type. We refer to a series of songs of the same type as a “bout.” We also calculated the average duration (sec) of each song and the average duration (sec) of pauses between songs and between bouts. For each recording session Rose measured song start and end times in Raven and calculated the duration of pauses between songs. She noted when a song type switched and counted the number of songs per bout. We attempted to record all Canyon Wren vocalizations produced during each recording session but sometimes missed vocalizations when beginning a new recording track. Audio tracks were sometimes begun immediately following the previous track and were sometimes separated in time by multiple minutes if the birds were quiet. Given the pauses between audio tracks, we report two different estimates of the interval between songs within a bout (time since the previous song ended when the two songs are of the same type) and the interval between bouts (time since the previous song ended when the two songs are of different types). The first calculation method includes pauses between recording tracks and so includes the time between two songs on different tracks both within a bout and between bouts. This calculation may overestimate the duration of pauses between songs and between bouts if we failed to record songs that were produced in between our recorded tracks. The second calculation includes only pauses between songs and bouts recorded within a single track. This method surely underestimates the duration of pauses within and between bouts because we tended to turn off the recording device during periods of silence. Statistical analyses

For all statistical analyses of song form and delivery patterns we used JMP, version 9. We tested whether song types differ in length with a linear mixed model (LMM) that included song-length data from our focused study of six males, with song type as a fixed factor and the male’s identity as a random factor. We also tested for individual variation in the length of males’ songs with a linear mixed model of the same data set, including the male’s identity as a fixed factor and song type as a random factor. To see if males sing bouts of different lengths (measured as number of songs per bout), we used a linear mixed model including the male’s identity as a fixed factor and song type as a random factor. All models used the standard leastsquares method with maximum-likelihood estimation. We used t-tests (assuming unequal variances) to compare the

features of Canyon Wren song   877

average duration of pauses between songs within a bout to the average duration of pauses between bouts. We report some measurements with standard deviations (SD) and some with standard errors (SE), labeling them accordingly. To examine repertoire sharing and the distribution of Canyon Wren song types within the population we studied, we recorded GIS coordinates for all 19 locations and from these identified each male’s nearest sampled neighbor. We calculated a value of repertoire overlap for each male as 2 × (the number of shared song types)/(the number of song types used by bird X + the number of song types used by his closest sampled neighbor) (Harris and Lemon 1972). We then averaged these values to find an index of population-wide repertoire sharing. In many cases we may not have sampled the true nearest neighbor if it did not occur on public land, so this index should be interpreted as a general measure of song sharing in the population, not a perfect representation of neighbor–neighbor song sharing. We calculated the value of repertoire sharing for all birds and for the subset of birds recorded in 2011 only, to account for the possibility that the birds recorded in 2012 were not territory holders at those sites in 2011. To examine song-type usage across a wider geographic area, we acquired, from the Macaulay Library of Natural Sounds and the personal collection of Andrew Rush, 36 audio files of Canyon Wrens from across the range of the species. Evaluating spectrograms of these recordings visually, Rose categorized each song as one of the types sung in our study population or as a new type, as appropriate. The recordists’ notes specified the locality, allowing us to assess the geographic distribution of song types.

RESULTS We recorded calls from all territories in the population we studied. In many instances we heard and recorded two birds counter-calling. These individuals were not involved in aggressive interactions with each other and appeared to hold the territory jointly, which led us to believe that they were typically male–female pairs. The form of calls we recorded (Fig. 1a) was similar to that of calls described in earlier publications and to calls contained in the recordings that we acquired from across the Canyon Wren’s range (Jones and Dieni 1995, Spencer 2012). We recorded females’ song on five different territories, presumably from five different birds. All female songs were similar in form, being composed of a series of buzzy notes that ascended then descended in pitch (Fig. 1b). This song form matches previously described female songs (Jones and Dieni 1995, Spencer 2012). We saw no evidence of multiple song types among females. Females usually delivered a single song and did not combine them into bouts. We did not observe that females always sang in one particular context; rather, they seemed to sing randomly. In at least two instances, females sang while males were singing, overlapping the male’s song. In other instances they sang while males were silent. As a group, the male Canyon Wrens we studied sang seven different song types (Fig. 1c–i). All song types were composed of a descending cascade of clear notes, sometimes followed by a variable number of harsher, broadband notes. In most song types the cascade notes are all generally similar in form, but they vary in shape from one syllable to the next through the song, and syllables tend to be of longer duration later in the song. Each song

FIGURE 1.  Vocalization types of Canyon Wrens recorded on 19 territories in Larimer County, Colorado, 2011–2012. (a) Calls, produced by both sexes. (b) Female song. All male songs (c–i) include a cascade of notes that descend in frequency. Males sometimes append one or more broadband notes at the end of any song type (illustrated here with song types B, C, and G).

878  Lauryn Benedict

et al .

type was clearly recognizable in multiple individuals, having a relatively fixed note shape and characteristic duration. Song duration varied significantly by song type (LMM, controlling for individual: F5, 781 = 14.3, P < 0.001), with song type A being the longest at 3.16 ± 0.11 sec (SE), type E the shortest at 2.68 ± 0.11 sec (SE). Song duration also varied significantly by individual (LMM, controlling for song type: F5, 781 = 28.6, P < 0.001). Bird 3 sang the longest songs with a mean duration of 3.15 ± 0.08 sec (SE), and bird 5 sang the shortest songs with a mean duration of 2.45 ± 0.09 sec (SE). For six of our song types, we found examples with and without the harsh broadband notes at the end. We never saw these notes after songs of the seventh type (F), but that song type was the rarest so we may have missed them through incomplete sampling. Harsh end notes could occur singly or in groups of up to three, among the birds we studied, and up to seven in other populations. They were appended to songs by all birds with little evident pattern. Successive songs of the same type were regularly delivered by one individual with different numbers of harsh end notes. Song-type discovery curves for the six Canyon Wrens from which we sampled ≥100 songs indicated that the birds’ complete repertoires were represented in 10–45 recordings (mean = 26.83 ± 13.04 [SD]) songs. Table 1 lists the full repertoires of 15 birds (numbers 1–15). Fourteen of these sang five song types, while one sang six song types. The remaining four birds for which we recorded fewer than 45 songs (numbered 16–19 in Table 1) also sang songs of five types, so we consider those repertoires likely complete as well. TABLE 1.  Occurrence of seven different song types in the song repertoires of 19 male Canyon Wrens recorded in 2011 and 2012, Larimer County, Colorado. Song type Bird

Year

Songs sampled

A

B

C

D

E

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2011 2012 2012 2011 2011 2011 2011 2012

175 148 137 128 116 100 98 96 93 89 89 87 86 85 80 40 35 26 7

• • • • • • • • • • • • • • • • • • •

• • • • • • • • • • • • • • • • • • •

• • • • • • • • • • • • • • • • • • •

• • • • • • • • • • • • • • • • • • •

• •

F

G

• • • • • •

•

• • • • • • • • • • •

FIGURE 2.  The Canyon Wren’s usage of song types across Larimer County Colorado, 2011–2012. Dots indicate the locations where birds 1–19 (labeled as in Table 1) were recorded. In all territories, males sang song types A, B, C, and D. Filled dots represent territories where males sang song type E.

All 19 males sang song types A, B, C, and D (Table 1). Fourteen of them also sang song type E, while two sang song type F and four sang song type G (Table 1). Figure 2 shows the geographic distribution of monitored territories and songtype usage. Song type F is very similar in form to type E and may represent a variant of it. We include them as separate types here because type E includes several repetitions of a distinctive two-part syllable that type F lacks. All birds that sang either of the two rare song types (F and G) were located at the southern end of our study area, but the two birds that sang type F were not nearest neighbors (Fig. 2). Three of four birds that sang song type G had a nearest neighbor that also sang song type G, while one did not (Fig. 2). The average overlap of repertoire calculated for all males was 94%. When we excluded birds recorded in 2012, it was 91%. As expected, the males we studied sang with eventual variety, repeating a single song type multiple times before switching

features of Canyon Wren song   879

to a different song type. When we included pauses between audio tracks, the average duration of a pause between songs within a bout was 31.95 ± 113.01 sec (SD) and the average duration of a pause between bouts was 405.54 ± 435.56 sec (SD). Thus pauses between songs of different types were significantly longer than those between songs of the same type (t-test: t762 = 10.7, P < 0.001). When we did not include pauses between audio tracks, the average duration of each pause between songs within a bout was 22.38 ± 27.68 sec (SD) and the average duration of each pause between bouts was 79.05 ± 70.54 sec (SD). Again, this difference is significant (t-test: t635 = 11.7, P < 0.001). Song bouts contained anywhere from 1 to 26 repetitions of a single song type, with a mean of 4.60 ± 4.62 (SD) songs per bout. Individual birds differed in the average number of songs per bout (LMM, controlling for song type: F5, 160.5 = 8.9, P < 0.0001). Bird 1 sang the longest bouts, averaging 8.57 ± 1.00 (SE) repetitions of the same song type, while bird 4 sang the shortest bouts, averaging 2.56 ± 0.68 (SE) repetitions of the same song type. When we examined 36 audio files recorded between 1961 and 2012 from across the Canyon Wren’s range, we found that some vocalizations were conserved over time and distance. Many tracks included calls with a form typical of that in the population we studied. All types of male song recorded across the range of the species varied in form, but note shapes were similar enough for distinct types to be recognized (Fig. 3). Song types E and F were not represented outside of Colorado. There was one example of song type A in a recording from Wyoming, and another in a recording made in Larimer County, Colorado, in 1970. Song type B was represented as far away as Arizona and Mexico (Fig. 3). Song type C was recorded in California, Texas, and Mexico. We

also found numerous examples (Arizona, California, Texas) of an intermediate song type with a form that incorporated elements of both types B and C. Song type D was recorded in California, Colorado, New Mexico, Texas, and Nuevo León, Mexico (Fig. 3). As for type A, the Colorado example of type D was recorded in Larimer County. The 36 recordings encompassed 12 song types not represented in our Colorado sample. Seven of these contained repetitions of a single syllable type not represented in our population. Two were variant types that contained syllables with a form that resembled one (or two) of the song types in our population but appeared to be distinct. Three were combinations that included two syllable types present in other song types. Two files (9041 from Texas and 120251 from California) indicated that individual birds had five song types. No files suggested more than five song types per individual. DISCUSSION Our results confirm and add detail to existing descriptions of the Canyon Wren’s vocal repertoire. Both males and females produce a call note with a distinctive, recognizable form across the entire range of the species. The female’s song is consistent in form from individual to individual but is infrequent and may not be produced by all females. The function of female song is still unknown, but Spencer (2012) suggested it could be important for resource defense. In Colorado and across the species’ range, the male’s songs include a descending cascade of repeated syllables, which may or may not be followed by one or more harsh broadband notes. These songs consist of multiple types, all of

FIGURE 3.  Examples of similarity across space for two types of Canyon Wren song (B and D) recorded at six locations across the species’ range (indicated in gray).

880  Lauryn Benedict

et al .

which have the same general form but vary in syllable shape. We identified seven song types in our local population and at least 12 more across the geographic range of the species. The latter is a minimum estimate because sampling was not comprehensive. Male Canyon Wrens sing with eventual variety, structuring their song delivery into bouts that consist of a single song type repeated every 20–30 sec with extended pauses of over a minute between bouts of different song types. Individual males sing songs of different lengths and differ in how they structure the timing of song bouts, providing preliminary evidence that songs and singing behavior may be distinctive to individuals. Male Canyon Wrens sing for territorial defense, particularly prior to and during the breeding season (Benedict et al. 2012, Rose and Phan 2013). Song traits may indicate the male’s quality or motivation, and future analyses should examine further how specific song features vary among individuals. Males’ song repertoires are relatively small and remarkably fixed. Eighteen of 19 focal males sang five song types, the remaining male sang six song types. Archived recordings from the Macaulay Library of Natural Sounds suggest that across the species’ range a repertoire of five songs may be typical (but more sampling is needed). A repertoire of five songs is larger than previously reported for the Canyon Wren but is still small relative to wrens in general, including the North American wrens (Mann et al. 2009, Kroodsma 1977). In monogamous species that are permanent residents with widely spaced territories in arid areas, song repertoires are predicted to be small (Kroodsma 1977, Read and Weary 1992, Garamszegi et al. 2006). Among species that live in low densities, small song repertoires might be favored because they allow a listener to identify the species rapidly. A need for species recognition would likely also favor a song-delivery pattern of eventual variety, in which the repetition of a single song type allows that signal to be effectively transmitted and received before a new signal type is used (Kroodsma 1977). Our results support those predictions, suggesting that the size of the Canyon Wren’s song repertoire and its patterns of delivery may have been shaped by ecological and life-history factors. In some species “repertoire complexity” is difficult to define because song types and syllable usage vary widely (Gil and Gahr 2002). In contrast, in the Canyon Wren complexity of the song repertoire is clear. Song patterning is fixed, so repertoire complexity is simply a sum of all song types. The songs themselves don’t vary extensively in syllable composition, making shared song types easy to identify. In the birds we studied, males’ song repertoires varied little. All 19 sampled males sang four common song types. The only variation in song-type usage was in the presence or absence of song types E, F, and G. Most males sang song type E, but two individuals replaced E in their repertoires with F, which is similar in form to E. Song types E and F were not represented in the broader sample of recordings from elsewhere in

the Canyon Wren’s range and may represent a song structure restricted to north-central Colorado. Three other birds replaced song type E with song type G and another sang both E and G, resulting in a six-song repertoire. Song type G was somewhat clustered geographically, but song type F was not. Since so few birds sang unusual song types we have no strong evidence that neighbors sang repertoires more similar than those of non-neighbors, a pattern which has been shown in many other species at geographic scales similar to that which we sampled (Hill et al. 1999, Tracy and Baker 1999, Grießmann and Naguib 2002). In total, repertoire overlap in the Canyon Wrens we studied was strikingly high, at 91–94%. Over a similar geographic area in northern Colorado, House Finches (Haemorhous mexicanus) at different sites had syllable-similarity coefficients ranging from 0% to 27% (Tracy and Baker 1999). At the other end of the spectrum, a population of Carolina Wrens shared an average of 66% of song types, while one of Banded Wrens shared an average of 77% with the nearest neighbor (Morton 1987, Molles and Vehrencamp 1999). For the Canyon Wren this value is high partly because repertoires are small and fidelity of copying is high, but this result may also reflect a pattern of frequent song sharing among wrens as a group. It is notable that the four song types (A, B, C, D) sung by all birds in our population were also sung in distant locations, and that two of them (A, D) have persisted in Colorado for at least 43 years. Thus song form can be remarkably conservative over time and space. Such large-scale conservation of song form is found in other birds. For example, across most of continental North America Black-capped Chickadees (Poecile atricapillus) sing essentially a single song type (although, interestingly, their repertoires near our sampling sites in Larimer County, Colorado, are larger) (Kroodsma et al. 1999, Gammon and Baker 2004). In several species of birds, certain song forms have remained stable over long periods of time (Harbison et al. 1999, Byers et al. 2010). Yet in many other species song form does change through time and space (Benedict and Bowie 2009, Byers et al. 2010). Our data suggest that there are some geographically localized types of Canyon Wren song concurrent with large-scale sharing of other types. For example, we have not found song types E and F outside of our study population, while types B and D are widely distributed. Further studies of the Canyon Wren with focused sampling across the range would help to reveal wider patterns of song variation and the evolutionary mechanisms maintaining stability or promoting diversity. Within the population we studied, repertoire sizes were nearly fixed and coefficients of song-type sharing were very high. These features, scaled up, have the power to create a pattern of song sharing across broad geographic areas. Large-scale sharing of song types may be compounded by good fidelity of copying during song learning, and it remains to be seen why (and the extent to which) this occurs with only some song types (Grant and

features of Canyon Wren song   881

Grant 1996). Even when syllable shape and specific song types do vary, the general structure of Canyon Wren song is highly stereotyped, making the song a good indicator of species identity. Perhaps this is why the Canyon Wren is such an iconic symbol of the American West—from Oregon to Mexico its songs are immediately recognizable. ACKNOWLEDGMENTS We thank Andrew Rush, as well as the Macaulay Library of Natural Sounds and all of the recordists who contributed recordings of the Canyon Wren to its collection. Our research took place on land managed by the following agencies: Colorado Parks and Wildlife, the city of Fort Collins Natural Areas Program, and the Larimer County Department of Natural Resources. Funding for this research came from the University of Northern Colorado College of Natural and Health Sciences, from the National Science Foundation’s Biota program, and from a University of Northern Colorado Provost Fund grant. The manuscript was improved by comments from Dave Gammon and an anonymous reviewer.

LITERATURE CITED Beecher, M. D., and E. A. Brenowitz. 2005. Functional aspects of song learning in songbirds. Trends in Ecology and Evolution 20: 143–149. Benedict, L. 2008. Occurrence and life history correlates of vocal duetting in North American passerines. Journal of Avian Biology 39:57–65. Benedict, L. 2010. California Towhee vocal duets are multi-functional signals for multiple receivers. Behaviour 147:953–978. Benedict, L., and R. C. K. Bowie. 2009. Macrogeographical variation in the song of a widely distributed African warbler. Biology Letters 5:484–487. Benedict, L., A. Rose, and N. Warning. 2012. Canyon Wrens alter their songs in response to territorial challenges. Animal Behaviour 84:1463–1467. Bent, A. C. 1948. Life histories of North American nuthatches, wrens, thrashers, and their allies: order Passeriformes. U. S. National Museum Bulletin 195. Byers, B. E., K. L. Belinsky, and R. A. Bentley. 2010. Independent cultural evolution of two song traditions in the Chestnutsided Warbler. American Naturalist 176:476–489. Byers, B. E., and D. E. K roodsma. 2009. Female mate choice and songbird song repertoires. Animal Behaviour 77:13–22. Catchpole, C. K. 1987. Bird song, sexual selection and female choice. Trends in Ecology and Evolution 2:94–97. C icero, C., and M. Benowitz-Fredericks. 2000. Song types and variation in insular populations of Lincoln’s Sparrow (Melospiza lincolnii), and comparisons with other Melospiza. Auk 117:52–64. Gammon, D. E., and M. C. Baker. 2004. Song repertoire evolution and acoustic divergence in a population of Black-capped Chickadees Poecile atricapillus. Animal Behaviour 68: 903–913. Garamszegi, L. Z., J. Moreno, and A. P. Møller. 2006. Avian song complexity is associated with high field metabolic rate. Evolutionary Ecology Research 8:75–90. Gil , D., and M. Gahr. 2002. The honesty of bird song: multiple constraints for multiple traits. Trends in Ecology and Evolution 17:133–141. Grant, B. R., and P. R. Grant. 1996. Cultural inheritance of song and its role in the evolution of Darwin’s finches. Evolution 50: 2471–2487.

Grießmann, B., and M. Naguib. 2002. Song sharing in neighboring and non-neighboring Thrush Nightingales (Luscinia luscinia) and its implications for communication. Ethology 108:377–387. H arbison, H., D. A. Nelson, and T. P. H ahn. 1999. Long-term persistence of song dialects in the Mountain White-crowned Sparrow. Condor 101:133–148. H arris, M. A., and R. E. Lemon. 1972. Songs of song sparrows (Melospiza melodia): individual variation and dialects. Canadian Journal of Zoology 50:301–309 Hill, C. E., S. E. Campbell, J. C. Nordby, J. M. Burt, and M. D. Beecher. 1999. Song sharing in two populations of Song Sparrows (Melospiza melodia). Behavioral Ecology and Sociobiology 46:341–349. Jones, S. L., and J. S. Dieni. 1995. Canyon Wren (Catherpes mexicanus), no. 197. In A. Poole and F. Gill [eds.] The birds of North America. Academy of Natural Sciences, Philadelphia. Jones, S. L., J. S. Dieni, and A. C. A raya. 2002. Reproductive biology of Canyon Wrens in the front range of Colorado. Wilson Bulletin 114:446–449. K roodsma, D. E. 1975. Song patterning in the Rock Wren. Condor 77:294–303. K roodsma, D. E. 1977. Correlates of song organization among North American wrens. American Naturalist 111:995–1008. K roodsma, D. E. 1980. Winter Wren singing behavior: a pinnacle of song complexity. Condor 82:357–365. K roodsma, D. E., B. E. Byers, S. L. Halkin, C. Hill, D. Minis, J. R. Bolsinger, J. Dawson, E. Donelan, J. Farrington, F. B. Gill, P. Houlihan, D. Innes, G. K eller, L. M acaulay, C. M arantz, J. Ortiz, P. K. Stoddard, and K. Wilda. 1999. Geographic variation in Black-capped chickadee songs and singing behavior. Auk 116:387–402. K roodsma, D. E., and L. D. Parker. 1977. Vocal virtuosity in the brown thrasher. Auk 94:783–785. K roodsma, D. E., and J. Verner. 1978. Complex singing behaviors among Cistothorus wrens. Auk 95:703–716. Logue, D. M. 2007. How do they duet? Sexually dimorphic behavioural mechanisms structure duet songs in the Black-bellied Wren. Animal Behaviour 73:105–113. M ann, N. I., F. K. Barker, J. A. Graves, K. A. Dingess-M ann, and P. J. B. Slater. 2006. Molecular data delineate four genera of “Thryothorus” wrens. Molecular Phylogenetics and Evolution 40:750–759. M ann, N. I., K. A. Dingess, F. K. Barker, J. A. Graves, and P. J. B. Slater. 2009. A comparative study of song form and duetting in neotropical Thryothorus wrens. Behaviour 146:1–43. Molles, L. E., and S. L. Vehrencamp. 1999. Repertoire size, repertoire overlap, and singing modes in the Banded Wren (Thryothorus pleurostictus). Auk 116:677–689. Morton, E. S. 1987. The effects of distance and isolation on song-type sharing in the Carolina Wren. Wilson Bulletin 99: 601–610. R ead, A. F., and D. M. Weary. 1992. The evolution of bird song: comparative analyses. Philosophical Transactions of the Royal Society of London B 338:165–187. Rose A., and X. M. Phan [online]. 2013. Changes in Canyon Wren vocalizations in advance of the breeding season. Undergraduate Research Journal at the University of Northern Colorado 2, (2). . Spencer, A. 2012. Female song in Canyon Wren, with notes on vocal repertoire. Colorado Birds 46:268–276. Tracy, T. T., and M. C. Baker. 1999. Geographic variation in syllables of House Finch songs. Auk 116:666–676. Verner, J. 1975. Complex song repertoire of male Long-billed Marsh Wrens in eastern Washington. Living Bird 14:263–300. Wiley, R. H. 2000. A new sense of the complexities of bird song. Auk 117:861–868.