The mustard seed is among the smallest seeds known to botanists. It germinates during the spring rains, rapidly growing into a waist- to shoulder-high plant. Since it is an annual, it does not continue to increase in size over the years as a tree grows.
Although the branches of the mustard plant are not strong enough to provide permanent nests for larger birds, they do provide convenient roosts for the flocks of small birds that fly from field to field in the spring. In one parable Jesus used the tiny mustard seed, insignificant when seen by itself, to illustrate the kingdom would spread rapidly from small beginnings (Matt. 13.31). In another parable he indicated that even faith as small as a grain of mustard seed held the intrinsic power to achieve notable results (Matt. 17.20).