Bird song has intrigued and delighted humans for hundreds of years. It is well understood how and why birds sing, how and why males and females differ in the types of sounds that they make, and how and why song production increases in the spring/summer and decreases in the fall/winter. However, whether bird hearing is the same or different between sexes and whether that hearing changes seasonally is poorly understood. In this work the electrical responses of the auditory nerve and brainstem of male and female songbirds will be recorded during the spring and the fall to understand whether the auditory system changes between seasons and whether this change is different between the sexes. Fall birds will be implanted with hormones (estrogen or testosterone) to mimic a spring-like condition. Their responses will be recorded before and after hormone implantation to determine if hormones can change the response of the auditory system to sound. Finally, the inner ear of the bird will be stained to determine whether there are receptors for hormones present in the ear, which could drive seasonal changes. It is expected that females will be more sensitive to sounds than males and that all birds will be more sensitive to sounds in the spring to improve communication during critical times. It is also expected that there will be estrogen and testosterone receptors in the ear which could mediate sex and seasonal effects on auditory processing. Songbirds may be an important model for the study of human hearing because women show hormone-induced changes in hearing that men do not show. Songbirds can help us understand the physiological basis for why humans show these patterns, and this should in turn prove useful in studying the basis of a number of hearing-related diseases. This work will train a promising young graduate student to become an independent scientist. Additonally this study will add to the scientific training of multiple undergraduates who will participate in the research.
For many species, including humans, acoustic communication is very important for social interactions. The ability to correctly create and interpret communication signals (like human speech or bird songs) directly affects that ability of individuals to find social groups, avoid danger, and find mates. In this work we investigated how the ability to process communication signals (specifically bird song) differs between males and females, how it changes over time, and whether we could induce changes with hormones. We found that females were more sensitive to frequency information than males when the birds were in reproductive condition (i.e. in the spring, when finding mates). However, when they were in non-reproductive condition (in the fall and winter) there was no difference between males and females. This change may allow females to evaluate potential mates in the breeding season. We found that when we gave birds in non-reproductive condition hormones to mimic breeding conditions we could induce plasticity in the auditory system. Understanding plasticity in the auditory system, particularly in highly vocal species such as songbirds, can give us greater insight into the plasticity of human auditory systems, and may provide us with potential ways to combat auditory deficiencies. For instance, there is some evidence in humans that estrogen plays a neuro-protective role in the auditory system. Our results suggest that estrogen may indeed improve some types of auditory processing. If these mechanisms are present across a broad range of taxa, they may play an important role in creating therapies for those with communication disorders. In synergy with this project we trained undergraduate students on how to conduct research. They designed a project that was related to our interest in songbird communication. The undergraduate students investigated the effects of different kinds of habitats on the ability of communication signals to travel between the sender (speaker) and receiver (hearer). For humans, we find it more difficult to communicate in urban areas because loud noises mask our speech. We were interested in how urbanization would affect the ability of the song of the brown-headed cowbirds (a pest species) to propagate. We found that the song of brown-headed cowbird song was affected by both habitat type (open vs. closed) and level of urbanization (urban vs. rural). Surprisingly, the brown-headed cowbird song travelled furthest in open urban environments. The hard surfaces found in open urban areas (e.g. sidewalks, roads, buildings) may provide a sound channel that enhances the propagation of brown-headed cowbird song. When we analyzed the introductory and final phrase of the cowbird song separately, we found that travel of the introductory phrase decreased in open urban environments, but the travel of the final phrase increased in open urban environments. Previous work by other labs suggests that these different portions of the vocalization are designed to be broadcast to different receivers. The introductory phrase is primarily directed at female receivers, while the final phrase may be used more commonly in male-male communication. Our work suggests that the effects of habitat and the level of urbanization influence these different parts of the vocalizations differently, and may therefore differentially affect male and female receivers. Our results could have conservation implications. Acoustic signals are important for reproductive success in songbirds. The brown-headed cowbirds and house sparrows used in this study are invasive pests and have negative impacts on native bird species. We could potentially minimize the impact of invasive species by reducing sensitivity to behaviorally relevant stimuli. This could provide a non-lethal population control plan, if reduced auditory sensitivity reduces reproductive success.