Food calling in ravens: are yells referential signals?

Abstract

Ravens, Corvus corax, yell when they approach rich but defended food sources. As in other species, such food-associated calls attract conspecifics. These calls may provide information about the sender, such as its behaviour or motivation, and about the type of stimuli to which the caller is responding, such as the location, quality or quantity of a food source. We investigated whether yells convey information about different types and amounts of food. We experimentally exposed a group of free-ranging ravens foraging in a game park near Grünau, Austria to six feeding situations. Food of one of three types (meat, kitchen leftovers, wild boar chow) and either of two quantities (one or three buckets) was shown to the ravens 10min before they could gain access to it during the feeding of wild boars, Sus scrofa. Ravens responded to the sight of food with one type of yell, the long haa call. Haa calling rates varied with the type but not with the amount of food, and decreased during feeding. Although juveniles produced long yells (chii calls) in response to food they changed the context of calling with increasing independence from their parents. Ravens gave short ‘who’ yells when approaching the food. Who calling was thus not affected by the sight of food but by the feeding situation in general. This is comparable to other calls given in a foraging context, such as appeasement and intimidation calls. The different use of long and short yells relative to food availability suggests that who calls provide information about the caller, such as its behaviour at food, whereas haa calls may also provide information about the food itself. Our data are consistent with the idea that haa calls are functionally referential.