Never cross a corvid. Crows have been known to hold grudges, and research says that ravens do, too. A new study in Animal Behavior spotted by The Verge finds that ravens remember the nature of their interactions with humans and can remember how the experience went for up to a month afterward.

The study, which was conducted by researchers at the University of Vienna and Lund University in Sweden, presented ravens with two types of interactions with human experimenters: some that could be considered fair and others unfair.

The nine ravens had all been hand-raised in captivity by humans. (The paper delightfully includes their names, which are: Laggie, Tom, Nobel, George, Horst, Louise, Joey, Rocky, and Paul.) The birds were trained to exchange a low-quality food (bread) for something they liked more (cheese). The ravens had to take a risk in giving the human their snack, though. They could give away the bread and receive cheese in return, but there was always the chance that they would give away their bread and not get anything back.

In the experiment, each bird was given many opportunities to exchange its bread pieces with a human trainer. Another bird looked on, but couldn't participate. In some sessions, the participating bird interacted with a person who always gave them cheese in exchange for their bread. In other sessions, they interacted with a person who took their bread but ate the cheese themselves—an unfair exchange.

Later, the researchers tested whether the birds would show a preference for the person who had always been fair in their interactions. A month later, eight of the nine birds chose to trade with the experimenter who had previously shown themselves to be fair in their interactions over an experimenter who had cheated them in the past (the ninth chose a third, neutral experimenter). However, this only applied to the birds’ first-hand experiences. The birds who observed another bird’s fair or unfair interactions as a third party did not seem to remember who could be trusted and who could not.

Considering crows’ abilities to remember the people who slight them, it’s not terribly surprising that their raven cousins would be able to, too. Nor is it their only complex skill: Corvids can also use tools and practice self-control. Both of which should worry you if you make a habit of making ravens and crows mad.

[h/t The Verge]