website has a good selection from Arthur Cleveland Bent's classic multi-volume Life Histories of North American Birds--

"Allen (1919) has called our attention to the time rhythm, which he attributes to a well-developed esthetic sense of the crow. He has noted that the caw notes are not only in triplets but at times they give four caws in groups of two (2-2); again he noted that the bird cawed 2-1 a large number of times in succession and on other occasions 2-1-1. The time was so regular that he could detect no variations. The length of the several notes and their pitch and quality were uniform, the rhythm being all that differentiated the phrase from other performances of the crow.

Allen does not believe the series of combination of calls represents a code of signals, nor does he believe them to be purely mechanical and involuntary, but he thinks the crow takes delight in the rhythm and variety of his utterances. He asks the question, "Is he not, in a limited way, a true artist, a composer as well as a performer ?""