Introduction

Vocal behavior of bird species has been a common theme in much of the research in recent history. The relationship between vocal behavior and sexual selection in terms of mate attraction has contributed to the development of the vocabulary of songbirds (Hughes, 1998). For Song Sparrows, a broad repertoire of song types is important in sexual selection as well as in territorial displays. It is therefore interesting to study the evolution of this vocal behavior and classify the aspects of song learning that help in the formation of a Song Sparrows vocabulary.

Song Sparrows are found from southern Alaska and Newfoundland south to North Caroline, Georgia, Missouri, North Dakota, and Central Mexico (Farrand, 1983). The behavioral patters of Song Sparrows have been well documented on both the east and west coasts of the United States but little is known about Song Sparrows in the interior of the country. Studies of East Coast Song Sparrow’s show that they are primarily migratory, in contrast, the West Coast populations are residential (Hughes, 1998). Studies done along the coasts of Washington and British Columbia have found that residential populations of Song Sparrows share entire song types (approximately 40% measured between any two neighbors) (Hughes, 1998). This variation from east to west could be due to the amount of time allowed for young males to spend with their tutoring neighbors, residential males having much more time than migratory ones (Beecher, 1994).

In the Northern Rockies, some Song Sparrow populations are migratory while others are residential. Because of this it would be reasonable to predict that there is an intermediated amount of song sharing between neighboring males. We hypothesize that in the Missoula Valley some neighbors share entire song types and some share syllables, depending on if the population is migratory or residential.

Methods

Species and Site Description:

The Song Sparrow (Melospiza meloda) is a common bird 14.5-18 cm long found in thickets, woodland edges, gardens, and along ponds and rivers throughout North America. This small brown and black streaked bird is residential year round to many areas and seasonal in others. Male Song Sparrows are strongly territorial, some retaining dominance over the same territory for years. Breeding begins in March and continues through spring (Farrand, 1983). Because it is the breading season of these birds the capture of recorded song should be made simple by the frequency of their singing. To do this we sampled two study sites approximately ten miles apart withing Missoula valley along the Clark Fork and Bitteroot Rivers.

Sampling Methods:

We identified six neighboring Song Sparrow territories at each site and recorded each territorial male’s song. We played back a recorded male Song Sparrow song (recorded from the University of Montana IBS-CORE sound file) in order to locate males and stimulate territorial song displays, using a tape recorder (Sony TCM-5000). We used a directional microphone to record approximately ten minutes of each bird singing on four different days (April 11, 18, 20, 23, 2000).

Analysis:

At the two study sites we assigned ID numbers to males in adjacent territories. We graphed sound frequency versus time (from this point on referred to as spectrograms) of the recorded Song Sparrow repertoires using Avisoft SAS Lab acoustics program. These spectrograms, each varying between 2 and 4 seconds in length and 0 and 10 kHz in frequency, were copied onto a word file. We analyzed the spectrograms by categorizing shared syllable types into a matrix and inferring degrees of song sharing withing and between the study populations.

Results

The repertoire size of each of the 12 males we recorded depended on the recording time. The longer the length of recording time, the more change in song type for each bird.

Among the 12 males we recorded, we found no complete song matching. However, we did find that all the birds shared general song structure and syllable type. The song structure consisted of two distinct patters (Fig 1). Patter 1 consists of a series of flat introduction notes, a main body slow trill, pre-or post- main body accents, a complex syllable, and a terminal buzz. Both the introductory notes and the terminal buzz are frequently dropped. Pattern 2 was far less common, occurring in only two of the birds. It consisted of increasing tempo introduction notes, a complex syllable, internal buzz, and a terminal slow trill.

Fig 1 Shared Song Structures for Song Sparrows.

Each of the males recorded also shared individual syllables (Table 1). The most common to least common shared syllable types were: main body slow trill, pre/post main body accents, terminal buzzes, and introductory flat notes (Table 2)

Table 1

Table 2

The Bitterroot and the Clark Fork populations each shared an average of three syllables with other members of their population but were not restricted to sharing with adjacent neighbors. We found no evidence that syllable sharing was greater within each of the populations than between them (Table 3). One exception to this rule was males 5 and 6 of the Clark Fork population who shared over half of their song type with one another.

Table 3

Discussion

While we expected to find an intermediate amount of total song matching between neighboring males, we found no total song matching between neighboring males, which more closely resemble song-sharing levels found in eastern populations. Although this could be due partly to sampling error in the form of incomplete repertoire recordings, this geographic variation in degree of total song sharing implies the function of total song sharing is not consistent in all song sparrow populations. Similar to the populations studied in the east, we found that Rocky Mountain populations showed no more significant increase in syllable sharing between neighboring males than between the two river populations. This implies a consistency in syllable sharing in Song Sparrow populations across the Missoula Valley as a whole.

Why is there such variation in song sharing behavior across the county? There is no evidence that whole song matching is advantageous to young male Song Sparrow yet, in residential west coast populations, young male preferentially select song types that are shared by their tutor males and avoid using unshared songs (Hughes, 1998, Beecher 1994). The extended tutoring time in West Coast populations may allow very specific song type learning. Populations consisting primarily of migratory song sparrows such as those populations found on the East Coast and in the Rocky Mountains appear to use different song learning strategies than residential birds. It appears that these migratory populations learn songs by copying parts of tutor male songs and recombining them to form their own songs (Hughes, 1998). Because there is no significant increase in song sharing between neighboring males than between non-neighboring males, it does not appear that it is advantageous to learn song parts strictly from neighboring males. Further research is needed to determine why this may be the case.

One possibility for decreasing total song sharing in migratory populations is that young male birds do not settle with their tutor males after migrating. This may be especially true for populations that do not migrate together with some members of a population being resident and other being migratory. In migratory populations, the young males would have shortened tutoring period restricted to summer months prior to winter migration. Possibly, the acquisition of territories upon return in the summer is more competitive because birds do not retain their territories year round. Because the territories must be reclaimed each spring, the distribution of young males could be more diffuse accounting for the consistency of an average number of syllables shared across populations. Further research would be needed to determine the extent of migration and residency in Rocky Mountain populations. It would also be important to know the migratory patterns of Song Sparrows. Questions could be asked such as the following: do song sparrows migrate as whole populations, as sub-populations or as individual birds? From this information, a pattern of song sharing is mixed migratory and non-migratory populations could be formed.

Further studies need to be done to determine territorial acquisition by young males in the Rocky Mountains and the role that the size and complexity of an individual male’s song repertoire plays. Another idea for study is to determine the relationship of male birds to that of repertoire size and to subsequently determine if larger repertoire sizes yield higher mating success. In order to understand repertoire size, we would need to alter our methods to record birds for a longer period of time, insuring that complete repertoires are sampled. Also, we would need to make sure our definitions of “whole song sharing” and “syllable sharing” were consistent with other researchers so we could make accurate comparisons between results.

Although we have focused on migration and residency patters as the main reason for total song sharing, there may be other factors that contribute to this learning behavior. The geographic variation in Song Sparrow vocabulary and song learning opens the door to a vast array of unanswered questions. Further research into the learning functions of Song Sparrows and the importance of the song repertoire could improve our understanding of vocalization or song development in birds.

Works Cited

Dr. Erick Greene

Beecher, M.D., S.E. Campbell and J.M Burth. 1994. “Song Perception in Song Sparrow: Birds Classify Song Types But Not Singer.” Animal Behavior. 45 1343-1351.

Beecher, M.D., S.E. Campbell, and P.K. Stoddard. 1996. “Correlation of Song Learning and Territory Establishment Strategies in the Song Sparrow.” Proceeding of the National Academy of Sciences of the united State of America. 91: 1450-1454.

Elekonich, M.M. 1998. “Song Sparrow Males Use Female-Typical Vocalizations in the Fall.” The Condor. 100: 145-148.

Farrand, John ed. Audobon Society’s Master Guide to Birding. Volume 3 Warblers to Sparrows. New York: Alfred Knopf, 1983.

Hiebert, S.M. and P.K. Stoddard. “Repertoire Size, Territory Acquisition and Reproductive Success in the Song Sparrow.” Animal Behavior. 37: 266-273.

Hughes, Melissa and Stephen Nowicki. 1998. “Song-Type Sharing in Song Sparrows: Implications for Repertoire Function and Song Learning.” 437-446.

Searcy, W.A., S. Nowicki, and M. Hughes. “The Response of Male and Female Song Sparrows to Geographic Variation in Song.” 1997. The Condor. 99: 654-657.