Why do birds sing at dawn? Mechanisms and function of "dawn chorus" in songbirds.

초록

The dawn chorus (DC) is a phenomenon where numerous bird species sing at high frequency in the early morning, observed almost daily in many regions worldwide (primarily during the breeding season). Consequently, the DC has long been recognized as one of the most prominent biological indicators signaling dawn to humans. Despite this, the function and underlying mechanisms of the DC remain largely unknown, making it one of the great mysteries in zoology. To date, over ten hypotheses have been proposed to explain DC's function, yet none possess strong direct evidence and none apply universally to all bird species. A major reason for the limited progress in DC research is that most studies have focused on wild birds, making it difficult to precisely control or systematically manipulate experimental conditions. Furthermore, due to the nature of such ecological research, previous studies have focused almost exclusively on the adaptive function (i.e., the ultimate cause) of DC, with little research conducted on the physiological mechanisms (i.e., the proximate cause) of DC itself. 　Therefore, we analyzed the function and mechanisms of DC using the widely employed experimental animal model, the Japanese white-eye, under precisely controlled experimental conditions. Our results revealed the following findings: (1) Under experimental conditions where birds exhibit DC, birds become awake several hours before the morning light-on time, and this arousal is triggered by a circadian rhythm-induced decrease in blood melatonin concentration. (2) Since zebra finches sing only under light conditions, spontaneous singing is suppressed during darkness before lights-on. (3) This darkness-induced suppression increases the intrinsic motivation for singing, resulting in high-frequency singing behavior (i.e., DC) immediately after lights-on due to rebound from suppression. Our previous research indicates that goldfinches optimize and maintain song structure through vocal practice during solitary singing, and prolonged absence of singing leads to deterioration of song structure. Therefore, our current experimental results strongly suggest that birds enhance their intrinsic motivation to sing before dawn to compensate for the lack of vocal practice at night, and rapidly optimize song structure via DC as soon as day breaks. Since song in songbirds generally plays a crucial role in courtship and territorial defense, the rapid optimization of song structure via DC is thought to increase the bird's reproductive success. Therefore, the function and behavioral mechanism of DC suggested by these findings provide a clear explanation for why many bird species exhibit DC every morning during the breeding season. 　We also analyzed the neural mechanisms triggering DC. Using early-stage gene expression levels as an indicator, we identified brain regions showing specific increases in neural activity during the pre-dawn darkness when intrinsic motivation for singing rises. Our findings revealed that neural activity in the HVC, a nucleus in the tegmentum corresponding to the cerebral cortex, increases alongside the rise in intrinsic motivation for singing. As the HVC possesses the function of generating song vocal patterns, this increase in neural activity suggests a link to the rise in endogenous motivation for singing that triggers DC. These research findings shed light on aspects of the function and mechanisms of DC in songbirds, which had remained largely unknown until now. They are considered to represent a major breakthrough in DC research.