Speciation in many animal taxa is catalysed by the evolutionary diversification of mating signals. According to classical theories of speciation, mating signals diversify, in part, as an incidental byproduct of adaptation by natural selection to divergent ecologies, although empirical evidence in support of this hypothesis has been limited. Here I show, in Darwin's finches of the Galápagos Islands, that diversification of beak morphology and body size has shaped patterns of vocal signal evolution, such that birds with large beaks and body sizes have evolved songs with comparatively low rates of syllable repetition and narrow frequency bandwidths. The converse is true for small birds. Patterns of correlated evolution among morphology and song are consistent with the hypothesis that beak morphology constrains vocal evolution, with different beak morphologies differentially limiting a bird's ability to modulate vocal tract configurations during song production. These data illustrate how morphological adaptation may drive signal evolution and reproductive isolation, and furthermore identify a possible cause for rapid speciation in Darwin's finches.

Abstract.-Behavioral evolution can be influenced by constraints, for example, of phylogeny and performance. In this paper I describe a pattern in the evolution of birdsongs that may reflect a constraint on vocal performance. Trilled vocalizations from 34 species of songbirds (Passeriforrnes: Emberizidae) were analyzed. Two acoustic variables, trill rate and frequency bandwidth, were measured for different trill types. In most species, maximal values of frequency bandwidth were found to decrease with increasing trill rates. Further, trills with low trill rates exhibited wide variance in frequency bandwidth, and trills with high trill rates exhibited only narrow frequency bandwidths. The bounded nature of this pattern suggests that performance constraints have limited the evolutionary diversification of trills. In particular, I explore the role of constraints associated with vocal tract modulations during song production and evolution. Identification of this constraint may enhance our ability to explain particular patterns of trill evolution.Key words.-Birdsong, constraints, Emberizidae, evolution of behavior, performance, vocalization.Received May 2, 1996. Accepted December 4, 1996.Much recent work in evolutionary biology has explored the idea that evolution can be shaped or limited by "constraints," for example, of genetic, developmental, biomechanical, or phylogenetic origin (Arnold 1992). Historically, analyses of constraints have focused on the evolution of morphological traits (e.g., Raup 1966;Gould and Lewontin 1979;Alberch et al. 1979;Thomas 1979;Maynard Smith et al. 1985;Wake and Larson 1987). It is only recently that the study of constraints has begun to be incorporated into analyses of behavioral evolution, as illustrated by studies exploring the influences of phylogeny (Ryan 1986;Prum 1994;Ryan and Rand 1995), functional morphology (Lauder 1986;Nowicki et al. 1992a), ecology (Wainwright 1988), and performance (Wells and Taigen 1986;Halliday 1987;Garland and Losos 1994). The study of constraints can complement adaptive or optimization perspectives that are more typically brought to bear on the study of behavioral evolution (McLennan et al. 1988;Stamps 1991;Wenzel 1992;Prum 1994; Klopfer and Podos, in press).In this paper I describe a pattern in the songs of songbirds (Passeriformes: Emberizidae) that may reflect a familywide constraint on song evolution. To provide a possible explanation for this pattern, I discuss a hypothesis of motor constraints on vocal performance; this hypothesis derives in part from recent advances in our understanding of how birds sing (Nowicki and Marler 1988). The goal of this study is not only to explore how constraints on performance might influence behavioral evolution across a species-rich family, but also to provide a focus for discussion of (1) the interpretation of bounded data as evidence for performance constraints on behavioral evolution and (2) the integration of adaptive and constraint-based perspectives in the study of vocal evolution.Studies of song evolution in ...

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