This is a story that I've linked to before, but the video is new since then; it tells the story of a wild crow that befriended a motherless kitten, fed and protected it and now that the kitten (now more a cat than a kitten) has found a home with humans, the crow still is fast friends. [7:29]
Thats pretty amazing, but not surprising. Crows are damned smart birds, and if it was stimulated by the kitten, at a time when it was practising parenting behaviours, I could see that if could figure out the rest. Corvids in Australia are social animals and older sibs help raise their younger sibs until they achieve full adulthood and leave the area. Maybe that is true of North American crows, I don't know.
I actually saw a documentary just this weekend about a solitary young female lion who "adopted" an Oryx calf, although eventually the calf was attacked and killed by a male lion from a neighboring pride. What was even more bizarre, though, is that the young lion adopted at least 5 more Oryx calves, although some died and at least one was found by its actual mother. Why the lion did this was a mystery, but it's likely that her pride was killed when she was young (she wasn't a member of any of the 3 surrounding prides), although it was sad as the baby Oryx kept her from hunting and she could not provide the baby Oryx with milk.
I've seen footage of a young female leopard caring for an abandoned baby baboon as well, although the baboon died during the night from the cold. Likewise, I've seen footage of a mother lion coming across an abandoned leopard cub and seemingly contemplating caring for it (although ultimately she abandoned it, the area going through a severe drought). I've never heard of such behaviors in corvids until now, though, so thanks for sharing this!
American crows are also helping species, although this is not a widely known fact. i suspect this crow that had adopted the kitten was rather young when s/he did so.