After seeing the total scientific accuracy of Jesus' account of the wheat and the tares, we now encounter the parable of the mustard seed which is fraught with problems. First of all, the mustard seed is small but it isn't the smallest seed of all. This doesn't bother me much since it's possible that in Jesus' time the mustard seed was viewed this way by the culture of his day. It's not meant to be a universal statement for all time. Besides, the seeds are still pretty small.
The bigger problem is that the mustard seed as we know it today doesn't grow into a tree. And even though it is capable of growing to surprising heights, it's not the sort of plant that flocks of birds would nest in. I'm sure you've seen mustard plants with their bright yellow blossoms covering an entire field like weeds.
In view of Jesus' spot-on observations about the nature of the tare (Persian darnell) in the previous passage, I can't believe he'd be this far off about the mustard plant. Maybe there is a translation problem here. Or maybe the mustard plant he is talking about is a different plant than the one we're familiar with. In any case, the plant he is talking about has a very small seed and grows into a very large tree that outstrips the growth of the plants around it, offering branches where the birds can gather and take refuge.
The kingdom of heaven is like this seed, starting with humble and seemingly insignificant beginnings. Jesus lived humbly among us and taught for only three years. He had twelve disciples and a few hundred other followers. None of these men were great by the world's standards, holding no significant positions of power in government or in the religious establishment. Jesus' popularity did not increase but diminished toward the end of his career, to the point where the mobs among whom he once ministered called for his execution. His resurrection transformed his disciples, yet there were only five hundred witnesses of even this miraculous event. Then Jesus ascended into heaven and left his disciples behind, who were armed only with the knowledge of what they had seen and heard.
It does not appear to be possible that such a man and such a ministry could change the world. Throughout history many great men have risen and gained followers, many spiritual men have claimed to perform miracles, many charismatic leaders have died as martyrs, yet their causes have died with them. Jesus' disciples only increased after he left us. Over the generations and throughout the world he continues to add disciples to his number. The kingdom of heaven is like a small seed, the smallest of all seeds, that grew up into a large tree and spread its branches over all the surrounding plants, where the creatures of the earth could go to find rest and shade.