Relative Frequency Parameters and Song Recognition in Black-Capped Chickadees

Abstract

Marler (1960) proposed that those features of bird song which are least variable should be most important for species recognition (termed the invariant features hypothesis). This hypothesis has been tested on several species of song birds, with mixed results. For example, Field Sparrows (Spizella pusilla) and Chipping Sparrows (S. passerina) do not use least variable features to classify conspecific song (Nelson 1989). On other hand, Veeries (Catharus fuscescens) and White-throated Sparrows (Zonotrichia albicollis) do use features with very little variation to recognize conspecific song. Altering normally stereotypic between-note frequency ratios in these species' songs reduces amount of aggressive response to playback (Weary et al. 1991, Hurly et al. 1990). Similarly, Chiffchaffs (Phylloscopus collybita) and Firecrests (Regulus ignicapillus) respond less strongly to playback of songs where within-note frequency sweeps have been altered (Becker 1982). Black-capped Chickadee (Parus atricapillus) song consists of two pure tonal notes: (see NORM in Fig. 1). Frequency descends during fee note (termed glissando), and there is a drop in frequency between fee and bee (termed pitch interval; Weisman et al. 1990). The frequency ratio of glissando (fee-start/fee-end), and ratio of pitch interval (fee-end/bee) are highly stereotypic, varying by less than two percent among and within birds in populations from Alberta, Wisconsin and Ontario (Weisman et al. 1990). The frequency at any given point within a song is highly predictable from any other point, that is, highly correlated to all other points. Although chickadees shift their songs over a wide range of absolute frequencies (Shackleton 1991), glissando and pitch interval remain constant. Playback studies conducted in laboratory showed that adult male Blackcapped Chickadees respond less aggressively to songs where pitch interval (fee-end/bee) had been altered, compared to normal songs (Weisman and Ratcliffe 1989). Therefore, to examine importance of glissando (fee-start/fee-end) to song recognition in this species, we played back songs to wild males in which we altered both of these relative frequency parameters. METHODS