Aspects of vocal communication in the carrion crow, Corvus corone corone.

Carrion crows (Corvus corone corone, L.) are highly intelligent birds in whose behaviour vocal communication plays a conspicuous part. Observation of crows reveals that their vocal repertoire is rich and varied. This study investigates the functions of the various acoustic signals produced by crows. Firstly, the signal repertoire of crows in the study area is described quantitatively with respect to call structure; previous studies have produced only variable, subjective definitions of crow calls. Next, the functions of the range of vocalizations are investigated using quantitative analyses with respect to context of the occurrence of call types in nature, and of the variation in call morphology. The use of vocalizations in selected naturally occurring contexts is then examined further by experiment. Decoys were used in controlled contexts to present some stimuli of territorial and anti-predator behaviour to a number of territorial pairs. Several discrete call types were identified, most of which were not exclusively associated with particular contexts. The situations in which call types were used did, however, allow suggestions to be made as to the functions of the signals. Calls rendered onomatopoeically as "waahs" and "kra-was" appeared to be signals to the caller's mate, functioning perhaps in the coordination of pair behaviour. Two signals always produced at low volume, "urks" and "oos", were probably used mostly in short-range aggression. Two other call types were encountered only very rarely, but were linked to specific situations. Rattle calls may signal alarm at predators and may be specific to an aerial threat. Low grunts are quiet calls used only while sitting on nests and which may be aimed at nestlings. All these discrete, definable call types were relatively rare in crow repertoires. The majority of calls produced in both natural and experimental contexts were drawn from a highly variable continuum of loud and amplitude-modulated sounds. Call morphology was especially distinctive in anti-predator contexts and it also varied considerably between calls at territorial neighbours and those at conspecific intruders into territories. The decoy experiments revealed further context-related variations in the particular situations studied, but call morphology varied more strongly with respect to behaviour. From the results it was concluded that calls potentially encode information about motivation and gender, and could also encode individual identity and information about call referents. The results are discussed with reference to the literature on the information content of animal signals, and also with consideration of previous work investigating the relationship between structure and function.