Seasonal changes in the song control nuclei of the Rufous-bellied Thrush, Turdus rufiventris (Oscine, Passeriformes, and Turdidae)

Abstract

In vocal learning birds, memorization and song production rely on a set of telencephalic nuclei referred to as the song control system. Seasonal changes in song production are correlated with changes in the volume of the song control nuclei and are influenced by photoperiodic conditions and hormonal cues. The seasonal volume changes in the avian brain that controls singing are thought to involve regulation of neuronal replacement, which is a striking example of neuronal plasticity. The Rufous-bellied Thrush (Turdus rufiventris) is a seasonally breeding bird that actively sings during the spring and summer (breeding season) and is relatively silent in the fall, yet possible mechanisms behind the periodic changes in song production remain unknown. Here, we have examined two song control nuclei: High vocal center (HVC) and robust nucleus of arcopallium (RA) in fall males, spring males, and fall females of Rufous-bellied Thrush. The cytoarchitectonic organization was analyzed and quantified from Nissl-stained sections, and gene expression of song nuclei markers was examined by in situ hybridization during breeding and nonbreeding seasons. We observed a reduction in HVC volume and reductions in parvalbumin, and RGS4 expression in HVC and RA in males during the nonbreeding season. These findings provide evidence of seasonal changes in the song system of a representative tropical-breeding Turdidae species that does not maintain territories or mate bonding, setting the histological and molecular groundwork for future studies aimed at better understanding of song nuclei changes in seasonally breeding songbirds.

Keywords: PVALB; bird; brain; in situ hybridization; thrush; vocal learning.

Figure 1

The song control system of the Rufous-bellied Thrush, Turdus rufiventris. Schematic representation of the song control system showing the song control pathways.

Figure 2

The song control nucleus HVC of Turdus rufiventris. Nissl stained brain sections of T. rufiventris showing the HVC nuclei of female (a), and male during non-breeding (b) and breeding (c) seasons. The average volumes of the HVC were compared between non-breeding and breeding males. A single asterisk denotes a significant difference (P value of 0.0125) between designated groups. Error bars represent the standard error of the mean (d). Arrows denote the HVC nuclei. Scale bars are 0.2mm (a-c) and 0.05mm on close-up squares at the bottom left of each image.

Figure 3

Expression pattern of the HVC song nuclei markers PVALB and RGS4. HVC of female (a,d), and males in non-breeding (b,e) and breeding (c,f) seasons. Arrows highlight the HVC nuclei. Scale bars are 0.25mm (a-f) and 0.05mm on close-up squares at the bottom left of each image.

Figure 4

Expression pattern of the RA song nuclei markers PVALB and RGS4. RA nuclei within the arcopallium of female (a,d), and males in non-breeding (b,e) and breeding (a,f) seasons. Arrows indicate the RA nuclei. Scale bars are 0.2mm (a-c) and 0.05mm on close-up squares at the bottom left of each image.