A PERFORMANCE CONSTRAINT ON THE EVOLUTION OF TRILLED VOCALIZATIONS INA SONGBIRD FAMILY (PASSERIFORMES, EMBERIZIDAE)

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 performa nce. Trilled vocalizations from 34 species of songbirds (Passeriformes : Emberizidae) were analyzed. Two acoustic variables, trill rate and f requency bandwidth, were measured for different trill types. In most s pecies, maximal values of frequency bandwidth were found to decrease w ith increasing trill rates. Further, trills with low trill rates exhib ited wide variance in frequency bandwidth, and trills with high trill rates exhibited only narrow frequency bandwidths. The bounded nature o f this pattern suggests that performance constraints have limited the evolutionary diversification of trills. In particular, I explore the r ole of constraints associated with vocal tract modulations during song production and evolution. Identification of this constraint may enhan ce our ability to explain particular patterns of trill evolution.