The Collaborative International Dictionary of English v.0.48:

featherbrained \feath"er*brained\, feather-brained
\feath"er-brained\, a.
   Giddy; frivolous; lacking seriousness; given to frivolity;
   feather-headed. [Colloq.]

   Syn: airheaded, dizzy, empty-headed, giddy, lightheaded,
        light-headed, silly.
        [1913 Webster + WordNet 1.5]

WordNet (r) 3.0 (2006):

featherbrained
    adj 1: lacking seriousness; given to frivolity; "a dizzy
           blonde"; "light-headed teenagers"; "silly giggles" [syn:
           airheaded, dizzy, empty-headed, featherbrained,
           giddy, light-headed, lightheaded, silly]

Moby Thesaurus II by Grady Ward, 1.0:

42 Moby Thesaurus words for "featherbrained":
   birdbrained, birdwitted, brainless, capricious, dizzy,
   empty-headed, featherheaded, fickle, flighty, fluffy, fluttery,
   frivolous, frothy, gaga, giddy, giddy-brained, giddy-headed,
   giddy-pated, giddy-witted, harebrain, harebrained, impulsive,
   light, rattlebrained, rattleheaded, rattlepated, scatterbrained,
   scramblebrained, shallow, shallow-headed, shallow-minded,
   shallow-pated, shallow-witted, shatterbrained, silly, skittish,
   superficial, thoughtless, unprofound, volatile, whimsical,
   witless