Testosterone treatment unveils testosterone-insensitive song in an early-branched hummingbird

Birdsong often plays a crucial role in sexual behavior, including territorial defense and mate attraction. This behavior is particularly well studied in species in which only males sing and respond to gonadal testosterone depending on the breeding season. Although hummingbirds are long known to sing in a breeding context, the sensitivity of these vocalizations to testosterone in both sexes remains unknown. Therefore, we investigated (i) which vocalizations are emitted during their breeding season, (ii) the testosterone levels in free-ranging birds, and (iii) the effect of testosterone on these vocalizations in a semi-natural environment in both sexes of the black jacobin (Florisuga fusca), an early-branched hummingbird. Females and males of black jacobin produced the same set of vocalizations (high-pitched song and chirp call) at a similar rate during the breeding season, although testosterone levels in males were significantly higher. Testosterone treatment did not affect the vocal activity or acoustic parameters of their song in both sexes. However, testosterone maintained high levels of aggression in treated birds of both sexes and hyper-developed the oviduct of females. In contrast to almost all well-studied bird species studied so far, the vocal behavior of the black jacobins in their breeding season seems to be insensitive to high testosterone levels.