Miller Lab alumnus Jonas Rose compares cognitive capacity across species.  Note that cognitive capacity correlates with intelligence but it is not the same thing.

Balakhonov, D., & Rose, J. (2017). Crows Rival Monkeys in Cognitive Capacity. Scientific reports, 7(1), 8809.

About the Author

The Miller Lab uses experimental and theoretical approaches to study the neural basis of the high-level cognitive functions that underlie complex goal-directed behavior.