Developmental stress and birdsong: integrating signal function and development

Early corticosterone increases vocal complexity in a wild parrot: An organizational role of the hypothalamic-pituitary-adrenal axis in vocal learning?

The neuroendocrinology of vocal learning is exceptionally well known in passerine songbirds. Despite huge life history, genetic and ecological variation across passerines, song learning tends to occur as a result of rises in gonadal and non-gonadal sex steroids that shape telencephalic vocal control circuits and song. Parrots are closely related but independently evolved different cerebral circuits for vocal repertoire acquisition in both sexes that serve a broader suite of social functions and do not appear to be shaped by early androgens or estrogens; instead, parrots begin a plastic phase in vocal development at an earlier life history stage that favors the growth, maturation, and survival functions of corticosteroids. As evidence, corticosterone (CORT) supplements given to wild green-rumped parrotlets (Forpus passerinus) during the first week of vocal babbling resulted in larger vocal repertoires in both sexes in the remaining days before fledging. Here, we replicate this experiment but began treatment 1 week before in development, analyzing both experiments in one model and a stronger test of the organizational effects of CORT on repertoire acquisition. Early CORT treatment resulted in significantly larger repertoires compared to late treatment. Both treatment groups showed weak negative effects on the early, reduplicated stage of babbling and strong, positive effects of CORT on the later, variegated stage. Results are consistent with more formative effects of corticosteroids at earlier developmental stages and a role of the hypothalamic-pituitary-adrenal axis (HPA) in vocal repertoire acquisition. Given the early emergence of speech in human ontogeny, parrots are a promising model for understanding the putative role of the HPA axis in the construction of neural circuits that support language acquisition.

Variable ambient temperature promotes song learning and production in zebra finches

Current climate change is leading to increasingly unpredictable environmental conditions and is imposing new challenges to wildlife. For example, ambient conditions fluctuating during critical developmental periods could potentially impair the development of cognitive systems and may therefore have a long-term influence on an individual's life. We studied the impact of temperature variability on zebra finch cognition, focusing on song learning and song quality (N = 76 males). We used a 2 × 2 factorial experiment with two temperature conditions (stable and variable). Half of the juveniles were cross-fostered at hatching to create a mismatch between pre- and posthatching conditions, the latter matching this species' critical period for song learning. We found that temperature variability did not affect repertoire size, syllable consistency, or the proportion of syllables copied from a tutor. However, birds that experienced variable temperatures in their posthatching environment were more likely to sing during recordings. In addition, birds that experienced variable prenatal conditions had higher learning accuracy than birds in stable prenatal environments. These findings are the first documented evidence that variable ambient temperatures can influence song learning in zebra finches. Moreover, they indicate that temperature variability can act as a form of environmental enrichment with net positive effects on cognition.

Losing a parent in early-life impairs flock size discrimination and lowers oxytocin receptor abundance in a medial amygdala homologue of adult zebra finches

Experiencing inadequate parental care during early-life diminishes adult social competencies. For example, low parental care impairs adult socio-cognitive abilities (e.g., recognizing familiar conspecifics) and affiliation (e.g., close social proximity); outcomes attributed to diminished medial amygdala nonapeptide functioning in rodents. Whether parental care has effects beyond familiarity, and if siblings have similar effects to parents, is unclear. Here, zebra finches were used to explore if parent and/or sibling number shape adult recognition and preference of small versus large flocks and nonapeptide (oxytocin, vasotocin) receptor expression in an avian homologue of the mammalian medial amygdala. Chicks were raised by single mothers or fathers in small broods or paired parents in small or large broods matched to single parents for chicks per nest or per parent, respectively. Pair-raised birds had preferred flock sizes as adults, but birds raised by single parents had equal preference for either size. Oxytocin receptor expression was lower in birds raised by single parents versus paired parents, but vasotocin receptor levels were unaffected. Such results highlight parents as formative antecedents of their offspring's social competencies related to group size preference and their nonapeptide mechanisms, outcomes that influence an animal's ability to live in social groups.

Song Preference in Female and Juvenile Songbirds: Proximate and Ultimate Questions

Birdsong has long been a subject of extensive research in the fields of ethology as well as neuroscience. Neural and behavioral mechanisms underlying song acquisition and production in male songbirds are particularly well studied, mainly because birdsong shares some important features with human speech such as critical dependence on vocal learning. However, birdsong, like human speech, primarily functions as communication signals. The mechanisms of song perception and recognition should also be investigated to attain a deeper understanding of the nature of complex vocal signals. Although relatively less attention has been paid to song receivers compared to signalers, recent studies on female songbirds have begun to reveal the neural basis of song preference. Moreover, there are other studies of song preference in juvenile birds which suggest possible functions of preference in social context including the sensory phase of song learning. Understanding the behavioral and neural mechanisms underlying the formation, maintenance, expression, and alteration of such song preference in birds will potentially give insight into the mechanisms of speech communication in humans. To pursue this line of research, however, it is necessary to understand current methodological challenges in defining and measuring song preference. In addition, consideration of ultimate questions can also be important for laboratory researchers in designing experiments and interpreting results. Here we summarize the current understanding of song preference in female and juvenile songbirds in the context of Tinbergen's four questions, incorporating results ranging from ethological field research to the latest neuroscience findings. We also discuss problems and remaining questions in this field and suggest some possible solutions and future directions.

Learning strategies and the social brain: Missing elements in the link between developmental stress, song and cognition?

Bird songs may advertise aspects of cognition because song learning and learning speed in cognitive tasks are both affected by early-life environments. However, such relationships remain ambiguous in the literature. Here, I discuss 2 lines of research that may help to demystify links between song learning and cognition. First, learning strategies should be considered when assessing performance to ensure that individual differences in learning ability are not masked by individual differences in learning strategies. Second, song characteristics should be associated with social behavior because songs have a social purpose and, consequently, should be strongly related at functional and neural levels. Finally, if song learning and cognitive abilities are correlated because they develop concurrently and/or share or compete for the same resources, I discuss ways glucocorticoids may link early-life stress, song learning and cognitive ability, focusing particularly on oxidative stress as a potential mechanism.

Early-life immune activation increases song complexity and alters phenotypic associations between sexual ornaments

Early-life adversity can have long-lasting effects on physiological, behavioural, cognitive, and somatic processes. Consequently, these effects may alter an organism's life-history strategy and reproductive tactics.In response to early-life immune activation, we quantified levels of the acute phase protein haptoglobin (Hp) during development in male zebra finches (Taeniopygia guttata). Then, we examined the long-term impacts of early-life immune activation on an important static sexual signal, song complexity, as well as effects of early-life immune activation on the relationship between song complexity and a dynamic sexual signal, beak colouration. Finally, we performed mate-choice trials to determine if male early-life experience impacted female preference.Challenge with keyhole limpet hemocyanin (KLH) resulted in increased song complexity compared to lipopolysaccharide (LPS) treatment or the control. Hp levels were inversely correlated with song complexity. Moreover, KLH-treatment resulted in negative associations between the two sexual signals (beak colouration and song complexity). Females demonstrated some preference for KLH-treated males over controls and for control males over LPS-treated males in mate choice trials.Developmental immune activation has variable effects on the expression of secondary sexual traits in adulthood, including enhancing the expression of some traits. Because developmental levels of Hp and adult song complexity were correlated, future studies should explore a potential role for exposure to inflammation during development on song learning.Early-life adversity may differentially impact static versus dynamic signals. The use of phenotypic correlations can be a powerful tool for examining the impact of early-life experience on the associations among different traits, including sexual signals.