No, this is not a parable about the mustard seed. Rather, it is a parable about Kingdom of Heaven.
In this sense, this simply tells us that the so-called Kingdom of Heaven is not -- repeat, is not -- a place. It's just like a small mustard seed that humanity can (indeed, does) overlook. But this same seed has the potential to get nourished until it grows into a full-grown tree.
It is that little "spark" of divinity that has been planted by God (the sower) in man. It is little in the sense that it cannot manifest itself easily since our material existence often overshadows it, as we oftentimes neglect it in favor of materialism and all that this world has to offer.
But it can grow (The choice is ours). If we select the righteous way of life and let that divine spark reign supreme in our life, we actually let it grow. And when it grows into a tree, when it has grown so large and tall enough as to "dwarf" the wordly desires of our flesh, then the birds will start lodging upon its branches -- the spiritual words of God will have all the chances to come freely to us, as we can now more deeply appreciate them.