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Studies of geographic variation in bird song can provide important insights into vocal evolution. An intraspecific approach, focused on a single species with a broad distribution, can be particularly helpful in understanding the diverse selective pressures on the characteristics of songs and singing behavior. Grass Wrens (Cistothorus platensis) are one such species, inhabiting tropical and temperate grasslands across the Neotropics. We studied Grass Wrens in Mendoza, Argentina, to determine the structure, organization, and delivery of their songs, song repertoire sizes, and patterns of song sharing among males in a resident, mainland south-temperate population. Over two breeding seasons, we recorded and analyzed 27,795 songs from 29 color-banded males. Songs of male Grass Wrens contained a few, low-volume introductory notes typically followed by one, but sometimes more, syllable types repeated as a trill. Males often repeated a sequence of two or three different song types (A-B-C, A-B-C, . . .) several times before switching to a different sequence (D-E-F). Syllable and song type sharing was high among males. The size of recorded syllable and song type repertoires of individual males varied with sampling effort. The introduction of new syllable or song types by males slowed, but did not reach asymptotes, with increasingly large samples of recordings. Many of these patterns are consistent with previous reports for Grass Wrens and other Cistothorus wrens, possibly representing core features of song design in this species group. Our results concerning song sharing among males in a resident population are also consistent with a previous proposed relationship between male song sharing and breeding-site fidelity. In the latter respect, Grass Wrens of Central and South America are more similar to Marsh Wrens than Sedge Wrens of North America, providing support for a recently proposed split between Grass Wrens and Sedge Wrens formerly considered conspecific.

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