An altered bird is a crow! They look like birds but they're bigger and darker and smarter!
If you can't tell just from looking at it that it's a crow and not a bird, walk up to it and caress (からす) it with your hand! If it caresses you back, it's a crow! If it freaks out and flies away or pecks your face, it's a normal bird. Crows are really smart and like affection. That's what makes them so cool!