I might think that an unclean bird lying on the eggs of a clean bird, or a clean bird lying on the eggs of an unclean bird were subject to the mitzvah; it is, therefore, written "the mother-bird lying on the fledglings — all must be of the same kind (i.e., clean birds).
"You shall not take the mother-bird together with the young": From (Vayikra 14:4) "And he (the Cohein) shall take for the one (i.e., the leper) to be cleansed two live clean birds," I might think that he should take them to cleanse a leper; it is, therefore, written "You shall not take" — even to cleanse the leper.