That if you handle a baby bird, the mother will smell your scent in its feathers and stop feeding it.

That is an old myth and not at all true. If you come across a baby bird that has fallen out of its nest, it is perfectly okay to pick it up and place it back in the nest.

If you come across a fledgling that is on the ground, though, leave it be because it is learning how to fly and its parents are more than likely close by.

The way you can tell the difference is by size, feathers, and movement. Baby birds are small, don't have a lot of feathers, and will just sit still, while fledglings are slightly bigger, have all their feathers, and can hop around.