What is the symbolism of a dead bird at a threshold or door step?

Not all naturally occurring phenomenon are "signs". Often small creatures such as birds fly into buildings, are the victims of predatory animals (cats & dogs regularly leave "presents" for their humans), or are just victims of mischance. Unless this bird is of spiritual significance to you personally (as in a long standing totem or helper animal) I would not consider it to be of any importance in your life.