We usually associate friendship with human behavior, considering our exclusive domain. And that’s a fair assumption, as we don't often find animals boasting cooperative relationships beyond their blood relatives. The only complex "friend-like" relationship seen in vertebrates is known as…
An important point of the research done here is that the “friends” were not kin-relations. In our daily observations, can we exclude that some birds we observe being helpful to each other are not kin? In this study they apparently did that, and over 20 years (40 breeding seasons).
Why does kinship matter? Well, apparently there are some good scientific theories explaining the evolutionary benefit of altruism among kins. But altruism among non-kins (i.e. friendship) was still an open question, hence this study.
Interesting point i didn’t think about that