Developmental stress and birdsong: current evidence and future directions

The developmental stress hypothesis posits that the honesty of bird song is maintained by costs incurred during development, rather than at the time of adult song production. Song is learned, and the neural systems underlying song learning and production develop early in life at a time when nutritional and other stressors are prominent. Thus, adult song may reflect the early developmental conditions of a male and/or how well that male was able to cope with developmental stressors. Substantial evidence in support of the developmental stress hypothesis has accrued over the last decade, but much remains to be done. Here, we review the current evidence in support of the hypothesis and highlight future directions. With few exceptions, there is now solid evidence that a variety of developmental stressors impair development of song and the brain. We note that birdsong is potentially an example of a larger class of sexually-selected indicators of developmental conditions. Future issues include examining the effects of stress on females, resolving whether song indicates developmental stability or developmental environment, exploring the neuroendocrine mechanisms by which stress modifies song development, determining whether stressed birds are subject to phenotypic programming, and determining the effects of stressors on song in adult birds. The developmental stress hypothesis thus provides a rich framework for future integrative studies of birdsong and development.