Unusual individual bird song may lead to insights for song learning in Grasshopper Sparrows song
In Grasshopper Sparrows (Ammodramus savannarum), males sing two distinct song types. The first of these, (the “buzz” song) occurs throughout the breeding season and typically consists of several introductory notes followed by a rapidly modulated trill. The second type (the “warble” song) is more varied and consists of frequency modulated notes of different types. Only males that are already paired produce the warble song. The buzz song is therefore thought to serve in territory establishment and mate attraction while the specific functions of the warble song remain uncertain. In previous field studies, we observed an individual who did not produce a species typical buzz song, but sang something similar to vocalizations produced by males during the plastic song phase of song development. This individual reappeared at our field site for multiple years. While he was unpaired most years, he was able to successfully maintain a territory. During one year in which he secured a mate, he sang the species-typical warble song. In order to understand how odd song is interpreted, and the role it may have played in territoriality and mate attraction, we performed song playback experiments using species-typical buzz songs, the odd song of this male, and white noise.
During playback, we recorded six behavioral responses of territorial males: number of songs, song latency, number of flights, closest approach, time spent within 12 meters of the speaker, and mean approach distance. We used principal component analysis to reduce the dimensionality of our data. This returned two components with eigenvalues greater than one, together which explain 61% of the total variance. Neither factor was significantly different between normal and odd song playbacks (p > 0.05). Preliminary results showed no statistically significant differences between territorial male responses to the normal and odd buzz song. While additional data are required to provide sufficient statistical power for a more definitive assessment, our preliminary results suggest that both the normal song and the odd song were perceived as sufficiently species-typical for territory establishment. We envision several possible explanations for this response. If this odd song is plastic song which crystalized prematurely, it would demonstrate that two distinct song types are learned at different rates. We plan to run playback experiments using examples of the Grasshopper Sparrow’s plastic song to examine this possibility. Our results could also indicate that males and females pick up different aspects of an individual’s song for their assessments.