Whenever a raven emits its "haa" sound to call others for help in dangerous situations, the sound tells the bird's sex and age. Depending on the sound, it can provide the necessary information to other ravens, which ultimately helps them make the decision on whether to assist the calling raven or not.
The study, which was published in the Frontiers of Zoology journal, was conducted by researchers from the University of Vienna and the University of Cambridge. They analyzed wild populations of ravens that gathered when wild boars were fed at the Cumberland Wildpark Grünau, Austria, between the summer of 2009 and the winter of 2010. The boars themselves represented threats which the ravens had to be careful of.
Based on the study, the frequency, duration, and amplitude of a call is different in ravens. Generally, female ravens called more often than male ravens, since the females are lower in the social rank and tend to not get in as many fights whenever there is food competition. It's also a safe bet for males to join females for easy food sources.
The "haa" sound is a way to recruit more ravens to ward off threats, especially when searching for food.