RESUMEN
In both humans and animals, biological differences between males and females has long been a topic of research. In songbirds, sexual dimorphisms can be seen in many species' plumage and heard in some species' songs. However, not all songbirds have such overt phenotypic sexual differences, leading to the question: are all vocalizations dimorphic? One of the most used and versatile vocalizations of the black-capped chickadee (Poecile atricapillus) is their namesake chick-a-dee call, that is produced by both sexes. This call is composed of four note types: A, B, C (together chick-a), and D (dee). Previous research has found that A notes contain information regarding the sex of the caller. However, chickadees do not categorize full chick-a-dee calls, or altered chick-a calls, based on the sex of the caller. Here we presented both male and female chickadees with altered chick-a calls (dee portion removed) of both sexes and measured the number of ZENK labeled cells in auditory nuclei. We found that calls produced by males and females had more ZENK labeled cells than the control condition; however, there was no significant difference in ZENK labeled cells between male and female listeners. Overall, our results suggest that black-capped chickadees do not perceive sexual differences in the production of chick-a calls.