http://www.sciencedaily.com/releases/2014/10/141007111055.htm
Researchers (including Orca Lab's Helena Symonds and Paul Spong) produced a paper in 2010 discussing whether "call sharing [across vocal clans of killer whales] represents cases of vocal imitation...".
http://onlinelibrary.wiley.com/doi/10.1111/j.1748-7692.2010.00397.x/abstract;jsessionid=5604655065BAF05008662971660BC343.f01t03?deniedAccessCustomisedMessage&userIsAuthenticated=false