Ravens remember relationships they had with others

Ravens have the capability to remember another individual for three years depending on the relationship had..  I it was a friendly relationship, it would call out a more friendly call whereas an individual who is a foe gets a different call.  This is the first time that relationship valence has been seen in animals. 

 http://cdn.physorg.com/newman/gfx/news/2012/ravensrememb.jpg