Loss of complex female song but not duetting in the ancestors of Carolina wrens

Female singing and coordinated male–female duetting are often but not always found in the same species. Both behaviors are more common in tropical than temperate songbirds, but few studies have differentiated between the factors selecting for each. Here we investigate the evolution of female vocal complexity and male–female vocal coordination in Carolina wrens (Thryothorus ludovicianus), one of the few non‐tropical members of a songbird family (Troglodytidae) that is well known for producing coordinated male–female duets. Female Carolina wrens are not known to sing; rather, they produce relatively simple, sex‐specific chatters, often during territorial encounters. We analyzed field recordings to show that females coordinate these chatters with male songs at rates similar to those observed in some tropical duetting wren species. We then used phylogenetic comparative methods to show that the evolutionary ancestors of Carolina wrens had female songs that were more acoustically complex than the vocalizations of current females, suggesting past selection against female vocal complexity. Levels of vocal coordination with males, in contrast, have changed relatively little from those of tropical ancestors. Our results suggest that these two aspects of female behavior, acoustic complexity and vocal coordination with males, have evolved independently and have different functions in communication. Female singing behavior is often thought to be a precursor for the evolution of coordinated male–female duets. Carolina wrens (Thryothorus ludovicianus) provide an example of the reverse pattern: complex female song was lost in evolutionary ancestors, replaced by a simple chatter, while vocal coordination with male song (i.e., duetting) was maintained. Our results suggest that these two aspects of female behavior—acoustic complexity and vocal coordination with males—have evolved independently and, presumably, have different functions in communication.