But how do they know if they remember the call, or just responding in a predictable way to the sound of the call at that particular moment? - DG

Ravens Remember Relationships They Had With Others

In daily life we remember faces and voices of several known individuals. Similarly, mammals have been shown to remember calls and faces of known individuals after a number of years. Markus Boeckle and Thomas Bugnyar from the Department of Cognitive Biology of the University of Vienna show in their recent article, published in Current Biology, that ravens differentiate individuals based on familiarity.

Additionally, they discovered that ravens memorize the closeness of a relationship and affiliation.

Until now it was unknown whether ravens could remember the closeness of a relationship based on former positive or negative interactions. In response to calls of formerly known individuals ravens not only increase the number of calls but also change call characteristics dependent on whether they hear former "friends" or "foes." This suggests that ravens remember specific individuals at least for three years. [continue]