I think of all the things to teach a bird, talking on command is the hardest. 
 First off, birds only say what they want to say.  I use a food reward in my 
training.  I do not starve my birds before training, only train them about 5 
to 10 minutes before their regular feeding time.  First they must learn a 
word or phrase.  Then repeat it around them.  With luck, they will start 
talking and may even say that line or word.  when I hear the word or phrase, 
the bird is rewarded with a half a sunflower seed.  Since these seeds are 
only used for treats, they eagerly try to please me to get the seed.  At 
first it will be hit or miss, but after much practice, they will soon say the 
word or phrase when you cue them to get that seed. 
--- InterEcho 1.08
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