Jesus compares the kingdom of God to two seeds. The first seed is any kind of seed; Jesus did not say which plant it corresponds to. The second seed is a mustard seed. The parable of the seed that grows by itself can only be found in the Gospel of Saint Mark. After the sower sows this seed, it begins its natural cycle of germinating and growing and giving spikes without us having to worry about its natural development. The point is not the work of the farmer but the work of the seed, which obtains its growth from a mysterious source and grows so slowly that we cannot see it growing. Only when we go away and return after a day or a week or a month can we see the seed’s progress and wonder at it. So also it is with the kingdom of God. We do our part by proclaiming the Gospel, but it is by the power of God that the kingdom comes into being. The kingdom of God is like this slow but steady growth. God is present! Watch out! Get on board!
The parable of the mustard seed is also found in Luke and in Matthew. The point is the great contrast between the very small size of the seed and the large size of the tree that comes from it. We would expect Christ to come as a mighty warrior, but he comes instead as an infant. We would expect him to select the brightest and best to be his disciples, but instead he chooses ordinary people—fishermen—even a tax collector. We would expect him to compare the kingdom of God to an oak tree or a cedar, but he compares it instead to a mustard seed—the smallest of all seeds.
Nesting birds serve to illustrate the large size of the mustard plant(tree), but might serve another purpose as well—hinting at the inclusion of Gentiles in the kingdom.