Why does it mention Tzipor and Dror?
Rashi: Tzipor wanders, and Dror returns to its nest. Dror is Sanunis (swallow); it is called Dror, for it is Dar (dwells) in the house like in the field.
Malbim: Tzipor nests also in the house. Even so, it wanders from place to place, and does not remain fixed in its place. Dror does not accept authority. It is not found in the house at all, only in a nest - "Gam Tzipor Matz'ah Vayis u'Dror Ken Lah" (Tehilim 84:4). It normally flies far outside the house.
What is the comparison to a curse without reason?
Rashi: It will come on the one who said it. Malbim - this is like these birds, which wander and fly, and return to their nests by themselves. This is based on the pronunciation ("Lo" with a Vov). The Kesiv ("Lo" with an Aleph) means that not only will it not remain fixed, and wander like a Tzipor - it will not enter the house at all, even temporarily, like a Dror.
