The formation of new species can be promoted by variation in acoustic signals that play an important role in mating behavior and thus can influence interbreeding between populations. The relationship between signals and speciation has been studied extensively in loud, long-range signals, but many species including birds, bats, and insects also produce quiet, short-range signals that are difficult to observe and record. New technology now makes it possible to obtain high quality field recordings of quiet signals through the use of newly developed radiotransmitters containing miniature microphones. Research supported by this proposal will address the function of short- and long-range song in the dark-eyed junco, by employing this new technology along with song playback experiments and laboratory preference tests to assess variation among populations. It is predicted that short-range song will function predominantly in courtship and differ more between populations than long-range song, establishing short-range song a potential contributor to the development of new species. This research will contribute to the mentorship and training of diverse undergraduates through Indiana University and the NSF-funded Research Experience for Undergraduates program at Mountain Lake Biological Station in Pembroke, Virginia. Much of the research will take place in public areas such as national forests and parks, allowing researchers to interact with youth groups and vacationers. This study will also strengthen scientific collaborations between Indiana University and the Universities of Melbourne (Australia), Illinois, Montana, and California at Davis. All recordings generated in this study will be contributed to the Macaulay Library of Natural Sounds (http://macaulaylibrary.org/index.do) to allow access to both the public and future researchers. The results of this project will be disseminated broadly by public lectures promoted by Audubon societies, publications in scholarly journals, and presentations at national and international conferences.
Intellectual Merit: Acoustic signals, or songs, represent one of the most thoroughly studied modes of communication in the animal kingdom. Previous research has focused predominantly on high amplitude (loud), long-range songs (LRS) while largely ignoring the low amplitude (quiet), short-range songs (SRS) produced during close proximity interactions that are analogous to a human whisper. Our research funded by this award focused on (1) investigating the function of SRS in a songbird, the dark-eyed junco, as well as (2) the degree of divergence in both LRS and SRS between junco populations. Furthermore we investigated (3) the possibility that any divergence in junco LRS or SRS could act as an isolating mechanism between junco populations thereby promoting speciation with the genus Junco. The function of SRS – We first observed that male juncos presented with a simulated intruder on their territory when their mates were or were not fertile responded more aggressively to an intruder singing SRS when their mates were fertile and the territory owner was at his highest risk for losing paternity to an intruding male. In contrast, males did not differ in their aggressive response to a simulated intruder singing LRS during their mate’s fertile and nonfertile periods. Additionally, we attached a novel type of radio-transmitter that contained a tiny condenser microphone (mic-transmitter) to free-living male juncos, which in turn allowed us to observe the production of both SRS and LRS during natural behavioral interactions. Based on a preliminary analysis of the transmitter data, males appear to produce SRS most often when foraging by themselves or when in close proximity to a female during courtship. Collectively, these results provide strong evidence that junco SRS functions predominantly as a courtship signal. We also tested the hormonal responses of both male and female juncos to playback of both SRS and LRS, but we found no detectable differences in the response of either sex to the two song classes. These results highlight the importance of SRS as a courtship signal, which challenges the current emphasis on LRS in studies of mating preferences and sexual selection in other species that rely on acoustic signaling. Additionally, studies of low-amplitude songs in other songbird species have focused predominantly on the aggressive function of some low-amplitude songs, so these results represent an important alternative viewpoint. The use of mic-transmitters in our research also served to develop this powerful method of data collection for use in future research. Divergence of SRS – We collected recordings of both LRS and SRS from five separate populations of dark-eyed junco, and we are still in the process of analyzing and comparing the structural differences in songs between the populations. Our preliminary analyses have found substantial overlap in the LRS of each population as well as some sharing of specific types of sounds, notably call notes, which appear in SRS. Anecdotally, the males in each population responded aggressively to playback of both LRS and SRS recorded in other junco populations, suggesting that males are unlikely to distinguish between the LRS or SRS of distant populations. SRS as an isolating mechanism – Our attempts to measure the preference of female juncos for the SRS of different populations largely resulted in negative results. We attempted to measure female preference via operant conditioning where a behavior (landing on a perch) was reinforced with a reward (playback of LRS or SRS), and although females appeared to learn the apparatus, they failed to exhibit any preferences between different songs even when given the choice between a junco song and the song of another species (song sparrow). We also attempted to measure the magnitude of a female’s hormonal response to different song types, but females did not reliably increase their hormone levels in response to playback of junco LRS or SRS. Thus, whether SRS is acting as an isolating mechanism in juncos or other species of songbird requires additional research. Broader Impacts: Research funded by this award either strengthened existing or fostered new collaborations between Indiana University and the Universities of Illinois, Montana, and California at Davis through a sharing of resources and access to field sites. Seven undergraduates were trained in various aspects of the scientific method including experimental design, data collection, and data analysis, and currently five of those students have chosen to pursue graduate degrees in science. Fieldwork was performed in many public areas including National Forests and Parks, which facilitated outreach opportunities to vacationers and persons in involved in outdoor recreation. To further disseminate the results of the research to the general public, presentations were given to student groups such as the Youth Conservation Corps and adult organizations such as the Road Scholars program and the Sassafras Audubon Society. To date, data collected here have also been reported in eight separate presentations at professional meetings and two research articles published in peer-reviewed journals.
