When you go outside, do the birds sound happy or angry when they see you? New research has found that at least one group of birds, ravens, remembers prior interactions with people and varies calls based on those earlier experiences.

So it's not too far-fetched to think that if you bothered a bird some time ago, the bird might unleash the avian version of swearing the next time you approach.

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The research, published in Current Biology, adds to the growing body of evidence that birds remember the appearance and voices of individuals, along with their prior encounters with them. Last year we told you how crows don't forget faces, for example.

We take such skills for granted in humans. In daily life, it's a given that we remember the faces and voices of multiple known individuals. Other studies show that different mammals can do the same thing.

If you want a lifelong buddy, you might consider getting a horse, since they remember their human friends and act accordingly. We humans can be pretty nasty and complicated with each other at times, but give a horse a carrot and a head pat for a while and you'll receive near-guaranteed kindness in return.

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Markus Boeckle and Thomas Bugnyar from the department of cognitive biology at the University of Vienna demonstrate in their new study that ravens differentiate individuals based on familiarity. They also discovered that ravens memorize relationship bonds and affiliations.

The findings revealed that ravens change their call characteristics depending on whether they hear former "friends" or "foes." The study only covered up to three years, but bird memory may extend beyond that time.

So what does an angry bird sound like? When listening to a foe, a raven responds with a call that's lower than normal in tone and starts to include "rougher characteristics." The switch from the bird version of "Hey! How are you?" to "Buzz off!" is similar to how we communicate such differences in our speech.

Strangers get an even rougher response from ravens. This is the equivalent of a person yelling, "Who are you?," if a stranger bangs on the door. There's an interesting scientific phenomenon behind having a louder, lower and rougher-sounding response.

All of those qualities make the individual sound bigger in size. You may not consciously be doing that when you yell, but the effect is the same. Ravens similarly try to sound tough and large around strangers.