Please, people! Keep the Gospel simple.
We are not exactly sure what is meant botanically by the mustard which somehow grows into a tree-sized plant. If you study botany, you find that the mustards are the Family Brassicaceae (or Cruciferae, cross-bearers!), a very large family of plants, including many of our common vegtables, but hardly any tree-size species. We should also remember that names in the Bible are frequently hard to match with modern usages. All this is fun to work on, but quite inconsequential for the meaning of the message.
It is very clear from a plain reading that Jesus is using the mustard seed as an illustration of something very small which grows into something sizeable and impressive, which can in turn support other life. The birds just happen to nest in trees, and one might wonder if they also have a significance as a sign of spirit, but once again that is just speculation. Certainly there is no connection here with Satan or evil in either mustard seeds or birds. All this talk about the parable's predicting something about the bad behavior of the church is also totally ridiculous.
I will repeat, the message is simple: it is that something small, and easy to overlook, will grow and flourish. That is the Kingdom.
Why did Jesus bother to say this? Think simple! It is because he was correcting a misconception that the Kingdom would come with dramatic, big events; or from high places. Many people today somehow expect that such things must come with claps of thunder and earthquakes. That is NOT the picture that Jesus is giving in his teaching in this parable; and this parable accords very well with other things he says.
The more I think of these matters, the more my admiration grows for the Gospel: not that I am to sit in judgment, but in that my soul responds as before something of beauty. And here part of the beauty inheres in simplicity and clarity.
My fellow Christians, there are many things we will not understand, and which we yet must reverence; but let us not also miss the chance to see what Jesus made very clear for us.