In Jesus' parable, what does the tiny mustard seed grow into?

Answer: A tree where birds nest

Though the mustard seed is the smallest of all seeds, it grows into the largest of garden plants and becomes a tree where birds nest.

“He put before them another parable: “The kingdom of heaven is like a mustard seed that a man took and planted in his field. Although it is the smallest of all seeds, yet it grows into the largest of garden plants and becomes a tree, so that the birds of the air come and nest in its branches.””— Matthew 13:31-32 (BSB)