Singing by female Chestnut-sided Warblers

We recorded examples of complex, highly variable, song-like vocalizations u ttered by female Chestnut-sided Warblers (Dendroica pensylvanica) in Massac husetts and New Hampshire. These songs occurred only during a brief period just after females arrived on the breeding grounds and were almost invariab ly associated with intense social interactions. Although the timing and con text of the songs might suggest a territorial defense or other communicativ e function, the songs bore no resemblance to male songs, were uttered infre quently, and were used by only a small proportion of females. Therefore, we believe it is unlikely that the songs could be reliably recognized and int erpreted by conspecifics. Perhaps the vocalizations are byproducts of seaso nally high levels of circulating sex hormones, as suggested by the prolonge d singing that we observed in a captive-reared female that had been implant ed with testosterone.