Another parable put he forth unto them, saying, The kingdom of heaven is like to a grain of mustard seed, which a man took, and sowed in his field:
Note 1 at Mt 13:31: This parable is continuing the theme of growth in the kingdom of heaven (this verse), or the kingdom of God (Mr 4:30, see last paragraph of note 30 at Mt 6:10). The mustard seed's smallness contrasted with the mustard plant's largeness (up to ten feet in height) shows this to be the greatest of all herbs.
The church was started with only a handful of the people whom Jesus had ministered to during His earthly ministry. But within a short period of time, a miraculous growth began with the addition of 3,000 souls on the Day of Pentecost (Ac 2:41) and 5,000 men (Ac 4:4) after the healing of the lame man in Ac 3. That growth still continues today and will continue in the future (Isa 9:7).
The fowls of the air can be interpreted in two ways. First, according to the symbolism used in the parable of the sower, the fowls could be Satan and his forces (Mr 4:4 with 15). This would certainly be consistent with other parables in this narrative (Mt 13:36-43 and 47-50). It is a fact that many professors (of salvation) but not possessors of eternal life have infiltrated the established church and are causing much damage. The Lord will divide these "goats" from His "sheep" at the final judgment (Mt 25:31-46).
Second, these fowls of the air could also be interpreted in a positive way to represent those who are pressing into the kingdom of God (Lu 16:16). There are Old Testament examples where fowls of the air lodging in the shade of a tree were used primarily to symbolize people (Eze 31:5-6 and Da 4:21). If interpreted in this way, this parable would be teaching that people (good and bad) were coming into the kingdom. This would be compatible with the theme of growth dealt with in each of these parables, and it would also lend itself to the interpretation of children of the wicked one being sown into the kingdom (Mt 13:38).