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Title is kind of clickbait. One trainer was giving cheese in exchange for bread, and another was giving nothing.

The raven preferred the trainer that gave something. Not really cheating or holding a grudge.

Yeah. Given the facts of the experiment, the journalist who picked the title's calling this a "grudge" is completely outlandish.

When you have to mischaracterize the facts to make your point, the problem isn't with the article.