Graduate Thesis Or Dissertation
 

Singing behavior of the Bewick's wren : development, dialects, population structure, and geographical variation

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  • The songs of the Bewick's Wren are discrete units of one to three seconds duration with intersong intervals several times the length of the song. Individual song repertoires at the William Finley National Wildlife Refuge, Oregon, range from 13 to 20 song types, and a given song type is repeated many times before another song type is introduced. The entire song repertoire is often sung before any song type bouts are repeated. Attempts to stimulate and to reduce attendant habituation may have been a selective force in the evolution of large song repertoires. Juveniles gain independence from their parents at approximately 35 days of age. Birds disperse as far as 3.2 km, and males may establish permanent territories and begin developmental subsong by 60 days of age. Sensitivity for song learning is probably maximal between 30 and 60 days. Some song variations which are developed by young males appear to be retained from exposure to songs prior to establishing a territory, but most song variations retained in the repertoire are those which match the songs of neighboring territorial males. Thus, neighboring males have very similar song type repertoires. Juveniles may disperse across minor habitat barriers which prevent frequent contact of territorial males on opposite sides, and song learning across such barriers is therefore hindered. The spread of new song variations formed through imperfect copying of a model song, or perhaps through improvisation or "drift", may be restricted by such minor barriers, and local dialects in relatively isolated habitat patches may thus be formed. Because the sensitive period for song learning in the Bewick's Wren occurs after dispersal from the home territory, dialect boundaries do not reflect population boundaries as is hypothesized for some species. The song repertoire size of individuals at the Finley refuge is dependent upon the date of hatching; birds hatched early in the breeding season are exposed to and learn more songs or song elements than birds hatched later in the breeding season. If juvenile males assess habitat quality when searching for a territory, repertoire size in the first breeding season may be positively correlated with habitat quality, and females could possible recognize this correlation. Differences in 14 measured song parameters at nine geographical locations in Oregon, California, Arizona, and Colorado reveal marked geographical differences. The frequency range of song sphrases was correlated with latitude and presumed species richness, and the frequency range of song phrases for the depauperate fauna of Santa Cruz Island, California, was similar to the frequency range of wrens at more northern latitudes, suggesting the possibility that species-rich communities may demand more stereotyped and less variable songs for species recognition. No such patterns were evident in the other 13 measured song parameters.
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