Smith, G., Brenowitz, E., Beecher, M., & Wingfield, J. (1997). Seasonal changes in testosterone, neural attributes of song control nuclei, and song structure in wild songbirds. The Journal of Neuroscience : The Official Journal of the Society for Neuroscience, 17(15), 6001-10. Retrieved from http://www.jneurosci.org/cgi/pmidlookup?view=long&pmid=9221796 (Original work published 1997)