Thibault Grava, Ken A. Otter and Angelique Grava

Abstract

In vocal learners, such as songbirds, the ability to maintain an internal acoustic structure between songs during a chorus seems to be positively correlated with the singer’s condition and may, therefore, represent a reliable measure of the singer’s condition. For instance, some internal ratios in the black-capped chickadee ( Poecile atricapillus) fee-beesong are more stable in the song of dominant males than in the song of subordinate males, suggesting that dominant birds are better at maintaining the internal song structure than subordinate males. Habitat quality is also known to affect the behaviour of this species. Birds settling in young forest have a lower song output and lower reproductive success than birds occupying mature forests, and it is suggested that those differences arise from differential food availability across habitats. As recent studies suggest that song performance can be altered by food limitation at the time of song learning, we explore whether habitat quality has a similar effect on the ability to maintain internal song structure as does social rank. We paired males by similar social rank, but who occupied different habitat types, and compared the consistency of male song within his dawn chorus. The ability to maintain an internal song structure of birds occupying young forests was consistently lower than birds occupying mature forests. Our results demonstrate that the same difference that exist in song structure between male differing in social rank also exist between males differing in the habitat in which they sing.

Allison H. Hahn, Lauren M. Guillette, Marisa Hoeschele, Daniel J. Mennill, Ken A. Otter, Thibault Grava, Laurene M. Ratcliffe and Christopher B. Sturdy

In songbirds, male song is an acoustic signal used to attract mates and defend territories. Typically, song is an acoustically complex signal; however, the fee-bee song of the black-capped chickadee is relatively simple. Despite this relative simplicity, two previous studies (Christie et al., 2004b; Hoeschele et al., 2010) found acoustic features within the fee-bee song that contain information regarding an individual’s dominance rank; however each of these studies reported a different dominance-related acoustic cue. Specifically, the relative amplitude of the two notes differed between the songs of dominant and subordinate males from northern British Columbia, while the interval pitch ratio differed between the songs of dominant and subordinate males from eastern Ontario. In the current study, we examined six acoustic features within songs from both of the chickadee populations (northern British Columbia and eastern Ontario) examined in these previous studies and used bioacoustic analyses and discriminant function analyses to determine whether there is a consistent dominance-related acoustic cue across both, or in each of these populations. Consistent with the previous findings, the current results indicate that relative amplitude differs based on dominance status in the songs from British Columbia; however, our results failed to reach significance with songs from Ontario. These results suggest that acoustic cues that signal a male’s dominance in this species vary with geographic location. Furthermore, examining songs from these two locations and one additional location in northern British Columbia, we found that discriminant function analyses could correctly classify songs based on geographic location. Considering the broad extent of the species’ range, black-capped chickadee song is considered relatively invariant; however, our results suggest that there is geographic variation in songs, although the differences are subtle compared to geographic song variation in other species.