Duets in Yellow-Naped Amazons: Variation in Syntax, Note Composition and Phonology at Different Levels of Social Organization Christine R. Dahlin & Timothy F. Wright Department of Biology, MSC 3AF, New Mexico State University, Las Cruces, NM, USA Introduction Geographic variation in vocal signals occurs in a wide range of taxa, including primates, songbirds, cetaceans, and parrots (Marler & Tamura 1962; Not- tebohm 1969; Wright 1996; Fischer et al. 1998; Nel- son 2000; Seibt et al. 2002; Yurk et al. 2002). Variation may occur in the form of notes (Baker 2003), within sequences of notes produced by one individual (solo song) (Baker 1975), or within sig- nals produced jointly by two or more individuals (duets or choruses) (Hall 2004). Signals composed of note sequences have the potential for several types of structural variation, including syntax (rules used to govern organization and timing of notes), note composition (occurrence and relative abundance of note types), and phonology (small scale variation in note structure) (Yule 1996). If these distinct types of signal structure differ in their modes of acquisition (innate, learned or elements of both) or in the form of selection they experience, then they may exhibit contrasting patterns of variation within and among individuals at a single site and geographically among sites. Research into geographic variation in syntax, note composition and phonology therefore has the potential to improve our understanding of how com- plex signals evolve, and provide insight into the function of those signal features. Correspondence Christine R. Dahlin, Department of Biology, MSC 3AF, New Mexico State University, Las Cruces, NM 88003 0001, USA. E-mail: crdahlin@nmsu.edu Received: January 5, 2009 Initial acceptance: February 24, 2009 Final acceptance: April 30, 2009 (L. Ebensperger) doi: 10.1111/j.1439-0310.2009.01669.x Abstract Complex vocal signals composed of multiple notes are used by many species. Such signals may vary in a number of features such as the rules that govern note order and timing (syntax), the relative number and types of different notes (note composition), and the acoustic structure of notes (phonology). Previous research examining male song in songbirds typically has shown greater conservation of syntax than phonology. Here we investigated whether these patterns of variation are also found in the duets of parrots. We examined geographic variation in the pair duets of yellow-naped amazons, Amazona auropalliata, at five sites within one vocal dialect. We also examined variation at several levels of social organization, including within pairs, among pairs and among sites, to assess where variability was greatest. Variation was highest at the within-pair level, although variation was also present at all other levels and for all of the duet factors. We hypothesize that variation at the among-site level allows duets to indicate site membership, while varia- tion at the within-pair level allows pairs to change their duet to match their current social or physical environment. Syntax and phonology are also more conserved than note composition for all levels of social orga- nization. This pattern may result because note composition is less impor- tant in duet function than syntax or phonology, or because development of syntax and phonology is more heavily influenced by genetic templates. Ethology Ethology 115 (2009) 857–871 ª 2009 Blackwell Verlag GmbH 857