The mustard seed was the smallest seed, but it grew into a huge plant. This is Mark’s third parable of growth.
In this parable, Jesus teaches that although the Kingdom of God started small, with Jesus and the disciples, it would grow and spread across the world to unlimited numbers of followers.
The birds represent Gentile people. Jesus is saying that the Kingdom of God will include other nations and not just God’s chosen people, the Jews.
Mark’s Gentile readers would have taken great comfort and hope from this parable.