We are all familiar with this because we have heard so many comments on its meaning for us today. But how about what the Jews of Jesus' time would have thought when they heard it? Barclay elucidates.
There are in this parable two pictures which every Jew would readily recognize.
First, in Palestine a grain of mustard seed stood proverbially for the smallest thing possible. For instance, "faith as a grain of mustard seed," means "the smallest conceivable amount of faith." This mustard seed did in fact grow into something very like a tree. A traveler in Palestine speaks of seeing a mustard plant, which, in its height, overtopped a horse and its rider. The birds were very fond of the little black seeds of the tree and a cloud of birds over a mustard plant was a common sight.
Second, in the Old Testament one of the commonest ways to describe a great empire was to describe it as a tree, and the tributary nations within it were said to be like birds finding shelter within the shadow of its branches (Ezekiel 17:22ff; 31:1ff; Daniel 4:10, 21). The figure of a tree with birds in the branches therefore stands for a great empire and the nations who form part of it.
The Gospel of Mark(The Daily Bible Series, rev. ed.)