yes, it is a separate parable.
but if a meaning is given one place or parable, why not assume it means the same thing in another parable?
for example, leaven is used in the parable in
Matthew 13:33-34.
leaven has a connotation of evil in
Mark 8:15.
couldn't leaven be a symbol of evil that a woman (false church) hid in the kingdom (visible church) until it was permeated by it?
Birds are used as symbols of evil/demons (
Rev. 18:2) so couldn't the birds who sat in the mustard tree (
Matthew 13:31-32) be evil spirits who have found a home in the (outward, visible) kingdom.
In my opinion, the kingdom means the reign of God, or the people who are under the kingship of Jesus. There is a kingdom within a kingdom. One kingdom would be the visible church world or Christendom. This would include all who claim to be Christians or who are considered Christians by the world in general. Within that outward kingdom is a core group of people who have been genuinely converted and have actually submitted themselves to Jesus as Lord and Sovereign (King).