BEHAVIORAL AND NEUROENDOCRINE MECHANISMS UNDERLYING SONG DEVELOPMENT IN GRASSHOPPER SPARROWS

Abstract

As song is a learned behavior in songbirds, it is a popular model system for studying the neuroendocrine mechanisms regulating its development. However, relatively fewer studies have explored the mechanisms of song development during the early sensitive phase in juveniles, with virtually none exploring this in a wild-caught songbird species. Bird song development typically occurs in two sensitive phases: a sensory phase for song memorization and a sensorimotor phase for vocal production and refinement. In some species which do not change their song in adulthood, song development ends with crystallization when song becomes stereotyped, which is preceded by a rise in circulating testosterone. Testosterone has also been used to study song behavior in adult females of some species who normally do not produce song, where it induces singing. I studied song development in grasshopper sparrows (Ammodramus savannarum) which sing two distinct song types in males only, one of which may be more difficult to perform. The “buzz” song contains a rapid trill which far surpasses the typical performance constraint on trill rate in other songbird species. The “warble” song is more typical of birdsong with varying acoustic elements and likely is less difficult to perform. As the warble song has received little analysis relative to the buzz song, I created a library of syllable categories to compare across seven populations of grasshopper sparrows. Many categories were shared between different populations, while the sequences of categories within a song were often unique to a single population. To study sensorimotor learning in grasshopper sparrows, I implanted a subset of males with testosterone early during the sensorimotor phase and analyzed their recorded vocalizations during this period to determine each individual’s crystallization date. As female grasshopper sparrows do not typically sing, I also implanted a subset of females with testosterone and induced male-like singing behavior to varying degrees in individual females. Additionally, I collected brain tissue to compare the volumes of the song control nuclei between males and females both with and without exogenous testosterone. From both qualitative and quantitative analyses of song recordings, I determined that males treated with testosterone completed the crystallization process 8-10 weeks earlier than control birds. When comparing brain regions, males in each treatment group had significantly larger volumes than females of the same treatment group. Further, testosterone-treated females had significantly larger volumes than control females. To my knowledge, this is the first study to extensively analyze the effect of testosterone on the development of both song types of the grasshopper sparrow or to compare the neuroanatomy of males and females in this species.