Marcel comments on my previous post that he thinks the birds in the mustard tree are gentiles. This suggested to me a thought on the interpretation of the parables (take this for what it is, I do not, by any stretch of the imagination, claim to be an expert on parables).
My thought was that I try to think of a justification for viewing the birds as gentiles, and it goes something like this: "well, the tree represents the kingom of God and there are only good things in the Church so the birds must be something good that are sort of added after the main church --hey, the gentiles were the latecomers, so..."
But that seems rather a backwards way to interpret a parable. Recall that Jesus had a specific purpose in speaking in parables --he wanted to confuse people. He want people to hear without hearing. If you can figure out the whole meaning of a parable just by logic, it seems that Jesus wasn't doing a very good job of hiding things.
It seems that to understand what Jesus was saying, you have to have access to the secret key. Part of the key is his surely the private explanation that he gave to his disciples of how to interpret the parable of the sower. That parable also had seeds which grew into plants, which represented the kingdom of God. In that parable, the birds represented Satan which ate the seed, snatching the word of God away before it can bear fruit. What do you think birds do in mustard trees? I'll bet they are eating the new mustard seeds.
Bearing fruit is another thing. In other parables, plants that represent the kingdom of God or believers show that they are in God's will by bearing fruit. There is nothing in this parable that says the mustard tree is bearing fruit, just that it is sheltering these mysterious birds.
Is there any other parable where sheltering birds represents something good? Well, there's the metaphor of a chicken sheltering her chicks under her wings, but that's not a parable, the meaning is manifest, and it's really exploiting the relationship of motherhood, not the mere fact of sheltering.
Finally, there is the fact that in a number of other parables, Jesus clearly is talking about bad influences in the Church, such as the parable of the wheat and the tares and the parable of drawing in the net. This seems to have been a point that he wanted to stress: just because someone is among the believers does not mean that he is a believer.
The parable of the wheat and the tares seems to teach that it is not our responsibility to sift out the unbelievers --that is a task for God. I propose that the point of the mustard seed was slightly different --that the entire Church as a body would grow into something unnaturally large that sheltered evil.