Hastings' Dictionary of the Bible (1898)
MUS'TARD. Matt 13:31-32; Matt 17:20; Luke 17:6. There can no longer be any question that this plant is the black mustard (Sinapis nigra), which often grows wild in our own country. In the fertile and warm soil of Palestine, especially when cultivated, this herb must have reached considerable size. " The Bible does not say, as is often supposed, that the birds build nests in the mustard, but only that they lodge there, as they often do in much smaller plants. Flocks of goldfinches and linnets are accustomed to settle in these plants and eat the seed, of which they are very fond.
"Small as a grain of mustard-seed" was a proverbial expression of which Christ made use. Divested of the Orientalisms of the language, which our Saviour used in Mustard. (Sinapis nigra.