Mark 4:31-35

Mark 4:31 (KJB)
It is like a grain of mustard seed, which, when it is sown in the earth, is less than all the seeds that be in the earth:

What Jesus was probably speaking of was the seed of the black mustard which is the size of a head of a pin and it was grown because it was used as a condiment. In fact, the seeds themselves were the actual condiment, so they ate some and they planted some. It is a very tiny seed and would not need much of a hole for it to take root. The people would have been familiar with this scenario since many were agricultural in their vocations.

Mark 4:32 (KJB)
But when it is sown, it groweth up, and becometh greater than all herbs, and shooteth out great branches; so that the fowls of the air may lodge under the shadow of it.

The mustard plant (Sinapsis Nigra) yields many seeds when it is full grown. It grows to a height of 23.5 inches (60 cm). The head of a pin may be 1 mm which means this seed grows to 600 times the size of the seed itself. It is interesting to note that 600 is easily divided by 30, 60, and 100. Now when the Lord speaks of it becoming a tree, He is not speaking of a comparison to an Elm tree or a Maple tree but He states that it is greatest among the herbs. The majority of herbs grow only a few inches from the ground so the mustard plant would tower over it. Now when it comes to the birds, the seeds attract them because they enjoy the taste of them. In Psalm 104, we have two passages of Scripture which give us a little insight into who the birds can be identified with. (Psa 104:16-17 KJV) The trees of the LORD are full of sap; the cedars of Lebanon, which he hath planted; {17} Where the birds make their nests: as for the stork, the fir trees are her house. The sap is what fills the branches and gives the plants life. It keeps them green all year long. The Holy Spirit fills the believer and keeps them in the power of God permanently. He is like the sap in the trees, He fills the believer and gives them strength to withstand the storms of life. The birds are like the believers whom God calls to be in His kingdom. If you notice, the birds nest in the branches but they do not leave. (Psa 121:8 KJV) The LORD shall preserve thy going out and thy coming in from this time forth, and even for evermore. Once the believer is in the branches, they never leave because the Kingdom of God is the place of permanent residence for the true believer.

This is a little tidbit on Mustard
The Romans are the first to be credited with making mustard in the way we know it today. They mixed the ground seeds with grape juice; the word "mustard" derives from mustum, "grape must," and ardens, "burning," mustum ardens- hence "must ard".

Mark 4:33 (KJB)
And with many such parables spake he the word unto them, as they were able to hear it.

Jesus spoke all these things to the crowds in parables. Whenever He spoke to the multitudes it was always in parabolic form. This was to teach the truths of the Kingdom of Heaven but it was in a parabolic form and those who became saved, would have the understanding made plain. Once again the word “hear” means to hear and understand. This is an important point because the natural man may hear the parable but to gain understanding of it, they must have their spiritual ears opened and that is why Jesus asked, “he who has ears to hear, let him hear.”

Mark 4:34 (KJB)
But without a parable spake he not unto them: and when they were alone, he expounded all things to his disciples.

Jesus used the parables to explain the aspects of the kingdom of God to the crowds and those who would be the called may want to investigate the meaning of them further. It would show that the person is being called because those who are not called would just consider them nice stories without any further meaning. When the disciples were alone with Jesus, He would expound the meaning of the parables to them and explain to them the meanings plainly. They may have had some understanding but they would have fuller understanding from the day of Pentecost on when the Holy Spirit would indwell them.

Mark 4:35 (KJB)
And the same day, when the even was come, he saith unto them, Let us pass over unto the other side.

Jesus had now only preached on the western shore of the sea of Galilee and had healed many and delivered many from devil possession. It was now time to move on and go to a different place to proclaim the Gospel. They would be heading over to the area of Gergesa for a period of rest from the weary day of teaching. Gergesa was located on the eastern shore of the Sea of Galilee.