Source : Webster's Revised Unabridged Dictionary (1913)

Chickweed \Chick"weed`\ (-w[=e]d`), n. (Bot.)
   The name of several caryophyllaceous weeds, especially
   {Stellaria media}, the seeds and flower buds of which are a
   favorite food of small birds.

Source : WordNet®

Stellaria media
     n : a common low-growing annual garden weed with small white
         flowers; cosmopolitan; so-called because it is eaten by
         chickens [syn: {common chickweed}]