Aspects of Singing Behaviour and Song perception in Two North American Forest Songbirds, Black-capped Chickadee (Poecile atricapillus) and Hermit Thrush (Catharus guttatus)
Roach, Sean Patrick
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Birdsong, used mainly by male songbirds, is centrally important to breeding behaviour as part of both territorial defense and mate attraction. Song perception is therefore important for both males and females, as they rely on song for conspecific recognition (identifying a singer as belonging to the same species) and individual recognition. Songbird species exhibit incredible variety not only with respect to song production, but also in terms of the acoustic features used for conspecific and individual recognition. Reflecting that variety, this thesis examined aspects of song production and conspecific recognition in two songbird species that differ greatly in terms of singing behaviour: the black-capped chickadee (Poecile atricapillus), a well-studied species with a single, simple song (fee bee), and the hermit thrush (Catharus guttatus), a less studied species in which individuals sing multiple versions of songs that are more structurally complex than those of the chickadee. Previous behavioural work has shown that, for conspecific recognition, female chickadees attend to two relative pitch ratios within the fee bee song: the frequency ratio from the start to the end of the fee (the glissando) and the frequency ratio from the end of the fee to the start of the bee (the inter-note interval). In contrast, fieldwork suggests that males attend only to the glissando. In the first study, I measured the neural response of males and females to songs with those pitch ratios either intact or altered. The lack of differences in brain activity within NCM and CMM, auditory regions associated with discrimination of conspecific versus heterospecific vocalizations, with respect to playback type suggests that discrimination of such relatively subtle structural differences takes place elsewhere in the auditory system. Birds vocalized more to songs with the species-typical inter-note interval than to songs without it, and, in the operant study that followed, birds learned that discrimination with ease. In contrast, birds had difficulty learning to discriminate the presence or absence of the glissando, and their vocal responses did not differ when it was altered. This suggests, contrary to previous field work, that the glissando may not be necessary for conspecific recognition. After an initial study to provide basic bioacoustics analyses of repertoire size, song structure, and song syntax in the little-studied hermit thrush, I examined geographic variation in song structure across the species’ large range. My studies describe extensive differences in frequency and temporal song characteristics, both between and within subspecies. I explored the implications of these differences in terms of conspecific recognition using a playback study where territorial males were exposed to local and foreign songs. Males exhibited a stronger territorial response to local songs, suggesting that they can discriminate local from foreign songs. These results are discussed in terms of recent findings that show extensive genetic divergence in the hermit thrush, suggestive of speciation. Taken together, these studies demonstrate the importance of acoustic features to conspecific recognition in two complimentary North American songbird species.