We recorded examples of complex, highly variable, song-like vocalizations uttered by female Chestnut-sided Warblers (Dendroica pensylvanica) in Massachusetts and New Hampshire. These songs occurred only during a brief period just after females arrived on the breeding grounds and were almost invariably associated with intense social interactions. Although the timing and context of the songs might suggest a territorial defense or other communicative function, the songs bore no resemblance to male songs, were uttered infrequently, and were used by only a small proportion of females. Therefore, we believe it is unlikely that the songs could be reliably recognized and interpreted by conspecifics. Perhaps the vocalizations are byproducts of seasonally high levels of circulating sex hormones, as suggested by the prolonged singing that we observed in a captive-reared female that had been implanted with testosterone.