Crows exhibit at least two dozen differnt calls, while ravens can express themselves with 20-100's of different vocalizations, depending on the expert cited. However, Kilham observed the opposite to be true at the New Hampshire farm study site, where the ravens outmaneuvered the crows in the forest and well as in the open. Kilham observed crows doing fewer jump-backs when approaching food. Ravens are more cautious or less audacious. After awhile if no crows were present, they would use their own lookouts.
Several accounts exist of ravens using crows as their sentinels.
The logical, and proper evolutionary response would seem contrary, with the discovers indignantly fending-off other prospective takers.