Singing behind the stage: thrush nightingales produce more variable songs on their wintering grounds

Territorial behaviour of thrush nightingales outside the breeding season

Territoriality is a common pattern of space use in animals that has fundamental consequences for ecological processes. In the tropics, all-year resident songbirds usually hold territories throughout the year, whereas most all-year resident temperate species are territorial only during the breeding season. In long-distance migrants, however, the situation is mostly unexplored. Here, we report findings from a Palaearctic-African migrant, the thrush nightingale Luscinia luscina. We found that only a fraction of the males was territorial in their East African winter quarters and that this was related to the stage of their song development. Individuals with full song were territorial towards other full songsters, but not towards birds that sang plastic song (i.e. an earlier stage of song development). Plastic singers were not territorial towards full songsters and often settled closely to territorial males. We suggest that territoriality of thrush nightingales in the winter quarters may be a by-product of rising testosterone levels that trigger song crystallization. Collectively, our study indicates that changes in territoriality can occur rapidly, giving rise to shifting proportions of territorial and non-territorial individuals in a population, which may lead to complex dynamics in settlement patterns and resulting ecological interactions.

The singing question: re-conceptualizing birdsong

Birdsong has been the subject of broad research from a variety of sub-disciplines and has taught us much about the evolution, function, and mechanisms driving animal communication and cognition. Typically, birdsong refers to the specialized vocalizations produced by oscines. Historically, much of the research on birdsong was conducted in north temperate regions (specifically in Europe and North America) leading to multiple biases. Due to these historic biases these vocalizations are generally considered to be highly sexually dimorphic, heavily shaped by sexual selection and essential for courtship and territoriality. Song is also typically defined as a learned trait shaped by cultural evolution. Together, this framework focuses research specifically on males, particularly during the north temperate breeding season - reflecting and thereby reinforcing this framework. The physiological underpinnings of song often emphasize the role of the hypothalamic-pituitary-gonadal axis (associated with breeding changes) and the song control system (underlying vocal learning). Over the years there has been great debate over which features of song are essential to the definition of birdsong, which features apply broadly to contexts outside males in the north temperate region, and over the importance of having a definition at all. Importantly, the definitions we use can both guide and limit the progress of research. Here, we describe the history of these definitions, and how these definitions have directed and restricted research to focus on male song in sexually selected contexts. Additionally, we highlight the gaps in our scientific knowledge, especially with respect to the function and physiological mechanisms underlying song in females and in winter, as well as in non-seasonally breeding species. Furthermore, we highlight the problems with using complexity and learning as dichotomous variables to categorize songs and calls. Across species, no one characteristic of song - sexual dimorphism, seasonality, complexity, sexual selection, learning - consistently delineates song from other songbird vocal communication. We provide recommendations for next steps to build an inclusive information framework that will allow researchers to explore nuances in animal communication and promote comparative research. Specifically, we recommend that researchers should operationalize the axis of variation most relevant to their study/species by identifying their specific question and the variable(s) of focus (e.g. seasonality). Researchers should also identify the axis (axes) of variation (e.g. degree of control by testosterone) most relevant to their study and use language consistent with the question and axis (axes) of variation (e.g. control by testosterone in the seasonal vocal production of birds).

Neuroendocrine patterns underlying seasonal song and year-round territoriality in male black redstarts

BACKGROUND

The connection between testosterone and territoriality in free-living songbirds has been well studied in a reproductive context, but less so outside the breeding season. To assess the effects of seasonal androgenic action on territorial behavior, we analyzed vocal and non-vocal territorial behavior in response to simulated territorial intrusions (STIs) during three life-cycle stages in free-living male black redstarts: breeding, molt and nonbreeding. Concurrently, we measured changes in circulating testosterone levels, as well as the mRNA expression of androgen and estrogen receptors and aromatase in the preoptic, hypothalamic and song control brain areas that are associated with social and vocal behaviors.

RESULTS

Territorial behavior and estrogen receptor expression in hypothalamic areas did not differ between stages. But plasma testosterone was higher during breeding than during the other stages, similar to androgen receptor and aromatase expression in the preoptic area. The expression of androgen receptors in the song control nucleus HVC was lower during molt when birds do not sing or sing rarely, but similar between the breeding and the nonbreeding stage. Nevertheless, some song spectral features and the song repertoire differed between breeding and nonbreeding. Territorial behavior and song rate correlated with the expression of steroid receptors in hypothalamic areas, and in the song control nucleus lMAN.

CONCLUSIONS

Our results demonstrate seasonal modulation of song, circulating testosterone levels, and brain sensitivity to androgens, but a year-round persistency of territorial behavior and estrogen receptor expression in all life-cycle stages. This suggests that seasonal variations in circulating testosterone concentrations and brain sensitivity to androgens is widely uncoupled from territorial behavior and song activity but might still affect song pattern. Our study contributes to the understanding of the complex comparative neuroendocrinology of song birds in the wild.