Age-dependent song changes in a closed-ended vocal learner: elevation of song performance after song crystallization

摘要

Birdsong is a sexual signal that serves as an indicator of male quality. There is already abundant evidence that song elaboration reflects early life-history because early developmental stress affects neural development of song control systems, and leaves irreversible adverse effects on song phenotypes. Especially in closed-ended vocal learners, song features crystallized early in life are less subject to changes in adulthood. This is why less attention has been paid to lifelong song changes in closed-ended learners. However, in the eyes of female birds that gain benefits from choosing mates based on male songs, not only past but also current conditions encoded in songs would be meaningful, given that even crystallized songs in closed-ended learners would not be identical in the long term. In this study, we examine within-individual song changes in the Java sparrow Lonchura oryzivora, with the aim of shedding light on the relationship between song and long-term life history. Specifically, we compared song length, tempo, and song complexity measures between the point just after song crystallization and around 1 yr later, and also compared those traits between fathers and sons to clarify the effect of vocal learning. While it is not surprising that song complexity did not differ depending on age or between fathers and sons, we found that song length and tempo increased with age. Follow-up analyses have revealed that frequency bandwidth and peak frequency of song notes also elevated with age. Our results show that song performance related to motor skills can be improved even after song crystallization. We also suggest that song performance in closed-ended vocal learners gives a reliable clue for mate choice by reflecting male quality with aging.