Crows are very clever; laboratory bird testers, not so much. Here, a simple case of correlation is presented as inference of a hidden causal agent.
Crows have been acing correlation tests since the day God created them. (One indication of how intelligent they are: I have never seen a crow reading Discover magazine.) They can trace causal agents on a scale so microscopic as to be non-existent; and they can infer things so subtle, that people who realize scream and run away.
But ravens, hooo! … Knock the smartest crow into a cocked hat. … They conduct empirical experiments on all the other species, and hide the journals in secret places where they will never be found.