Green’s Dictionary of Slang

budgie n.2

[SE budgerigar, a popular cage bird, which can be taught to speak; thus RAF jargon (paraffin) budgie, a helicopter, presumably the source of the Duchess of York’s storybook creation]

(UK Und.) a talkative person; esp. in police use, a minor informer; also attrib.

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[UK]D. Powis Signs of Crime 175: Budgie A talkative man, especially of small stature, also used as a nickname; can be used by police to refer to a minor informer – one who has details of only ‘gas meter jobs’ (unimportant local cases).
[UK](con. 1990s) N. ‘Razor’ Smith A Few Kind Words and a Loaded Gun 416: The Cardiff boys were mainly big, dopey bodybuilders with budgie syndrome who strutted about playing at being gangsters.
[Aus]P. Temple Truth 65: ‘Our friend Ivan is mentioned. That’s last week, six days ago.’ ‘Mentioned how?’ ‘One budgie says Ivan’s got something to sell.