Testing the function of female song in the Bachman's sparrow.
Rindy Anderson Florida Atlantic University
Our lab studies animal communication, in particular, the acoustic structure and social function of bird song. One of our ongoing projects at Johnathan Dickinson State Park is to study the structure and function of female song in the Bachman's sparrow. In mid-July, we rotated our SM4 recorder near the nests of several of the mated pairs we were monitoring, trying to capture recordings of the elusive females. Female birds do not sing in the majority or North American songbird species, but Bachman's sparrow females do sing. They don't sing with the showiness or bravado that their mates do, but yet they do produce song-like vocalizations. We want to know why.
Over the past two breeding seasons, we made several observations of females singing in the proximity of their mates, and near the nests they were building. We obtained good quality recordings from one female, which will allow us to develop a protocol for making larger scale acoustic comparisons between the songs of males and females next season. In addition, we placed the recorder on the territories of males that had been subjects in the aggression experiment we were completing. We recorded each male for 24-48 hours, two critical pieces of information: singing patterns from pre-dawn to post-dusk, and singing patterns during undisturbed, unprovoked singing. We are now comparing those singing patterns to the patterns we recorded in response to a simulated territorial intrusion by a singing rival male. In April 2018, we will obtain recordings of 8-10 females to use for acoustic analysis and for playback experiments designed to test when and why females sing.
Female birds do not sing in the majority or North American songbird species. Bachman's sparrow females do not sing with the showiness or bravado that their mates do, but they do produce song-like vocalizations. Below is a spectrogram (a visual representation of sound plotting song pitch over time, much like music is visualized) showing an example of one female's song that we recorded using our SM4 song meter. Also pictured are examples of male broadcast songs (called primary song) and an example of "warbled song," which is quite distinct from primary song. The female songs we have visualized so far bear some resemblance to male warbled song, being a non-stereotyped, seemingly jumbled series of notes. During the next field season, we will use the songs we have recorded to perform playback experiments to measure female behavioral responses to a simulated female intruder. We bought four additional SM4 units, which will allow us to rotate the meters among the territories of many more females to capture song at different stages of the nesting cycle. In addition to our study of female song, we used the SM4 meter to record many hours of male Bachman's sparrows singing at the dawn chorus, and throughout the day. From these recordings we are gaining understanding about how males use their song type repertoires in different behavioral contexts, and the degree to which males share song types. Our preliminary data suggest that neighbors share a large number of song types on average (> 50%) while non-neighbors share fewer song types (< 30%). This pattern has implications for how males use their songs to communicate with neighbors, and how song type sharing may influence where young males choose to defend a territory.
We continue our efforts to gather data on the acoustic structure and social function of female song in Bachman's Sparrow. This is a shy, elusive species. Females are tricky to find, and even more challenging to record, because they sing infrequently. In 2017 we captured a few recordings of female song. So far in 2018, we have observed females singing in the field, but are still working to capture audio recordings.
Our observations suggest that females sing when fertile, in temporal proximity to copulation. Perhaps their songs serve as an invitation to mate? Working on this hunch, we are placing our SM4 recorders on trees within 10 meters or so of nests that are being built, and as eggs are being laid. We have not yet analyzed the many hours of recordings obtained so far (over 620 hours!), but we are hopeful that we havecaptured examples of female song from several different birds.
With these recordings, we will compare the acoustic structure of female song to those of males, and quantify variation in female songs both within and between females. Male Bachman's sparrows sing over 40 types of Primary Song – do females also sing many song types? Does an individual's song vary from day to day, or does she sing a consistent song? How will females respond to songs of other females played on their territories? We look forward to digging into our data to answer these and other questions about this interesting species.