The intent of the parable is to contrast the seemingly insignificant life and ministry of Jesus Christ, the disciples, and the kingdom of God in its present order of the "now and not yet". It speaks to the fact of the kingdom of God in the hearts of believers; that will one day be manifested as great. This is a parable of encouragement to continue and live in the hope of the Glory of God to come. The world looks at the good message and news of the Kingdom, and at the messengers, as un-important in comparison to academic degrees, world philosopies, material riches, politcal power, and whatever else secular man values.
It is a comment related to the prior parable in Mark about the man who sowed seeds and did not know how they grew, addressing the miraculous work on God. The branches and birds reflect the truth of God's government and rule reaching the nations of the world. Birds here refer to the habitation of peoples, as it is referred to in the book of Daniel. Some point out the birds as a negative aspect taking the symblolsim from the book of revelations. But seen in its context the parable refers to God's kingdom and blessing to all peoples who trust in Jesus.