It is commonly thought by Bible scholars, although not by all, that the Parable of the Mustard Seed (Luke 13:18) is about the spreading out of the Gospel, no doubt, because Jesus mentions the plant as a metaphor for the Kingdom of God. The problem with this understanding is context. Jesus uttered the parable in the presence of both his enemies and those who are normally impressed with what he says and does (Luke 13:14-17).
The Kingdom of God had been the subject of Jesus’ teaching throughout his journey to Jerusalem and while he was there (Luke 9:60, 62; 10:9, 11; 11:2, 20; 12:31-32). Now that he was in the synagogue (Luke 13:10), he was probably summing up the theme of his current teaching. He interrupted his teaching there to heal the woman and the results of that interruption were both favorable and unfavorable, and this is the subject of his teaching now.
In Luke 13:19 Jesus compares the Kingdom of God to a mustard seed that is planted and grows large enough to be considered a tree. Matthew seems to emphasize the size of the seed, while Luke emphasizes the size of the mature plant. Both call the mature plant a tree, and both have birds lodging in its branches, but Matthew’s emphasis seems to be the beginning, while Luke’s is at the end result of the planting.
Mustard is a condiment. It isn’t eaten as a main dish of a meal. Rather it is used to season the meal itself. Eventually, the mustard plant becomes a small tree, great in the sense that it is still merely a herb (Luke 13:19), but small, if it is compared with mature trees. One may be reminded at this point of the book of Judges where Jotham’s parable of a bramble that reigned over trees, Jotham used this parable against Abimelech and the men of Shechem, who made Abimelech king over them (Judges 9:7-15). The bramble is a thorny bush that grows about as high as the mustard tree. It bears leaves that look similar to the olive tree, but its berries are bitter. The point is neither the bramble nor the mustard tree are used for the same thing trees are used (shade, fruit that satisfies hunger or thirst, building products etc.).
The mustard herb that becomes a tree puts forth branches strong enough for birds to rest upon, and, when they do, they eat the seeds out of its pods. This seems to depict the servants of the devil to whom Jesus pointed in the Parable of the Sower (cf. Luke 8:5, 11-12). In Luke 10:17 they might be called demons, and in the context of Luke 10:18 and Revelation 12:7-9, they would seem to point to Jesus’ enemies, the Pharisees and lawyers who kept speaking evil of what Jesus said and did (cf. Luke 11:53-54). In another place Jesus mentioned that the Pharisees and lawyers not only don’t enter the Kingdom of God themselves, but they also prevent others from entering (Luke 11:52; cf. Matthew 23:13). They do this by plucking the seeds (word of God) that were planted in people’s hearts, as in the Parable of the Sower, or out of the seed pods in the Parable of the Mustard Seed. In both parables it is the seed, not the plant itself that is important to the Kingdom of God, even when a herb has turned into a tree.
In essence, if we consider the Gospel narratives and draw our conclusions from them, the enemies of Christ (the birds) were eventually successful in taking away the seed (word of God) planted in the hearts of the people or in the seed pods meant for the people (i.e. those who were impressed with what Jesus did and glorified God—cf. Luke 13:17), and we know this because even they called out for Jesus’ crucifixion (Mark 15:10-15), though earlier they had been willing to receive him as their Messiah (cf. Matthew 21:9; Luke 19:37-38). This being so, it seems to me that the birds in the branches of the mustard tree represent the eventual apostasy of Israel (cf. Matthew 24:12) and perhaps the professing Church as well.