"and the mother-bird lying on the fledglings": Just as fledglings are alive, so the eggs must be viable — to exclude winnowed (non-viable) eggs. And just as eggs require the mother-bird, so, fledglings (to be subsumed in the mitzvah) must require the mother-bird, to exclude those that are able to fly.
"and the mother-bird lying": when she is lying on them — to exclude her flying over them. I might think (that the mitzvah does not obtain) even if her wings (in her flying) touch them; it is, therefore, written "and the mother-bird lying on the fledglings — even if she is not (in "tight" bodily contact) with them.