Acoustic variation in alarm calls of Corvidae–effect of morphology, ecology and phylogeny

Bioacoustic parameters of vocalization are generally well adapted to the environment that the species inhabits and are influenced by the morphology and life history of the species. These effects are frequently studied on bird songs; however, little is known about how ecology, phylogeny and morphology affect alarm calls. We investigated these factors in the crow family (Corvidae, Passeriformes) that exhibits extensive distribution around the world and is well known for their variable, conspicuous alarm calls. Using a free database of sounds (xeno-canto) as well as own recordings, we collected alarm calls of 66 species representing 21 out of 24 genera within the family. We measured four vocal characteristics, which showed high variability among the family and low mutual correlation - peak frequency, peak frequency change, harmonicity and call duration. The effect of body mass and preferred habitat openness of the species on these four parameters was tested using phylogenetic regression. We also assessed the strength of the phylogenetic signal. Peak frequency, peak frequency change and harmonicity were influenced by phylogeny, while the alarm call duration evolved independently of phylogeny. Body mass significantly affects the mean peak frequency with bigger species producing sounds of lower frequency. Additionally, the openness of the preferred habitat of the species significantly affects the maximum change in peak frequency. Forest species show less peak frequency fluctuation within the alarm call, probably because dense vegetation does not favour good transmission of highly modulated signals. We conclude, that features of corvid alarm calls are affected by similar drivers as e.g. bird song.