the kingdom in a mustard seed

Matthew 13:31-32

The key words are: parable, kingdom of heaven, mustard seed, field, birds. Jesus contrasts the size of the mustard seed (the smallest of the domesticated seeds) to the size of the resulting plant (the largest of garden plants). Most of Jesus' parables contain a twist or surprise. In this parable it is that the mustard plant becomes a tree and the birds of the air find shelter in it.
Jesus is using imagery from the OT that would be very familiar to his audience. The OT talks about a couple of different nations that will become like large trees and provide shelter for birds. In the OT the birds represent foreign nations. Jesus is saying a couple of things in his parable, therefore: that the kingdom of heaven will be small and hidden (buried) but that mysterious and hidden things that happen in the seed and below the surface of the soil will result in the the kingdom of heaven growing supernaturally beyond any human expectations. Jesus is also saying that the kingdom of heaven exists for the foreigner. It will provide a home and a shelter for those outside the nation of Israel. Bruxy Cavey notes that the audience wouldn't have to look very far to find birds; they were surrounded by the eagle of Rome.
As a citizen of the kingdom of heaven, am I providing shelter and shade for the "foreigner", even those that are my enemies? Jesus indicates that this is a characteristic of the kingdom of heaven. As a citizen of the kingdom of the heaven, am I contributing to its growth? Jesus indicates that this will be another characteristic of the kingdom of heaven. As a citizen of the kingdom of heaven, am I content to work in obscurity and remain hidden from notice as long as I am doing the work of the King? Jesus indicates that this, too, is a characteristic of the kingdom of heaven.