Are they birds or plants? Doesn't matter, they're really nice and helpful.
What looks like a plant at first comes out with a happily squawked "Ma!" when you jump on them. Marking your path. Lending you a point of continuation. Or to put it bluntly, if you lose a life, you'll get brought back where your last Ma Bird pal was.
How do they do it to bring you back? Hm, a thing to ponder about. Where to find them? In every level, of course! Ma!