Real Dictionary

twitter
verb: When birds twitter, they make a lot of short high-pitched sounds.

twitter in American English
intransitive verb: to utter a succession of small, tremulous sounds, as a bird
noun: an act of twittering
transitive verb: to express or utter by twittering

twitter in American English 1
a light, sharp, intermittent vocal sound of a bird; chirping
to express or say in a twittering manner
to make a series of light, sharp, intermittent vocal sounds; chirp continuously or tremulously, as birds do

twitter in American English 2
a person who twits

twitter in British English
noun: a twittering sound, esp of a bird
noun: a website where people can post short messages about their current activities
verb: (esp of a bird) to utter a succession of chirping sounds
verb: to write a short message on the Twitter website