Acoustic profiling in a complexly social species, the American crow: caws encode information on caller sex, identity, and behavioural context

Abstract

Previous research on inter-individual variation in the calls of corvids has largely been restricted to single call types, such as alarm or contact calls, and has rarely considered the effects of age on call structure. This study explores structural variation in a contextually diverse set of "caw" calls of the American crow (Corvus brachyrhynchos), including alarm, foraging recruitment and territorial calls, and searches for structural features that may be associated with behavioural context and caller sex, age, and identity. Automated pitch detection algorithms are used to generate 23 pitch-related and spectral parameters for a collection of caws from 18 wild, marked crows. Using principal component analysis and mixed models, we identify independent axes of acoustic variation associated with behavioural context and with caller sex, respectively. We also have moderate success predicting caller sex and identity from call structure. However, we do not find significant acoustic variation with respect to caller age.

Keywords: PACS Code 43.80.Ka; acoustic feature analysis; call classification; caller identity; sexual dimorphism; vocal ontogeny.

Figure 1

Traditional and locally periodic representations of a crow call. The call is from AS, a yearling male. (A) Traditional spectrogram, using 512-sample Hamming-windowed frames and a 6-sample step size. (B) Locally periodic representation, using 240-sample frames and a 24-sample step size. Note that this representation assigns energy only to frequencies corresponding to harmonics of the estimated fundamental for each frame.

Figure 2

Distribution of call lengths and mean pitches among all calls, with first principal component of acoustic parameters projected onto this surface.

Figure 3

Distribution of calls with known behavioural context, on two principal components.