While it is well known that both sexes of many bird species vocalize, the importance and prevalence of female vocalizations is often underappreciated. In part, this is due to the paucity of field studies documenting their occurrence. Our knowledge of female vocalizations is particularly limited for the primarily monomorphic suboscines, for which obtaining vocalizations of known sex presents additional methodological hurdles relative to sexually dimorphic species. One solution is to collect a voucher specimen of an individual for which focal recordings have been made, with the sex of the specimen determined during its preparation. Using this approach, we documented a sex-specific vocal repertoire for the Dusky-capped Flycatcher (Myiarchus tuberculifer). We found that not only do females and males have distinct daytime vocal repertoires, but that females had a more diverse repertoire than males, vocalized more frequently, and that the male-specific vocalization was dependent on female vocalization in the context of a female-initiated stimulus–response duet. Although duetting has been anecdotally described for many species of tyrant flycatcher (Tyrannidae), this is only the second formal analysis of female song and duetting in this family and 1 of only 6 published examples for the New World suboscines. These findings deepen our understanding of female vocalizations in these groups and suggest that the female's leading role in territory defense through duetting should be investigated in other species. The “museum approach” employed in this study will be a useful tool to help document many of the potential cases of sex-specific vocal repertoires in other monomorphic species for which sexed and banded populations may be difficult to obtain.