Green’s Dictionary of Slang

(orig. Aus./N.Z.) a state of great happiness; usu. constr. with be a, feel like a, etc.

1940		Eve. Post (Wellington) 8/7: I have lately seen an actual ‘Box of Birds’. The phrase I have always heard applied to a feeling of well-beiong, pep and happiness.
1941		F. Sargeson ‘That Summer’ in Coll. Stories (1965) 164: Hello Terry, he said, how’s things? A box of birds, Terry said.
1951		D. Stivens Jimmy Brockett 141: Afterwards I felt like a box of birds.
1959		G. Slatter Gun in My Hand 144: ‘How’s yaself?’ ‘Boxa birds,’ he tells me with a wink.
1972		G.W. Turner Eng. Lang. in Aus. and N.Z. 117: Conversation in the evening consisted only of the words, ‘I feel a bit crook’. In the morning, asked how she was, the stranger replied, ‘Oh, box of birds, now.’.
1988		McGill Dict. of Kiwi Sl. 20/1: box of birds fighting fit, NZ Services, WWII; fit and well in civilian life. Sometimes additions and all singing, all feathers and shit; eg ‘G’day, how are ya?’ ‘Orrh, champion, mate. Boxa birds.’ [Ibid.] box of fluffies/box of fluffy ducks engaging variants of preceding entry.
1990		Tampa Trib. (FL) 9 Sept. 107/4: [In New Zealand] [a] person doing well might [...] remark, ‘I’m a box of fluffy ducks today’.
2002		(ref. 1940s) B. Morrison Things My Mother Never Told Me 87: Expressions of happiness come in several forms: full of beans, box o’ birds and happy as a sandboy.
2003		McGill Reed Dict. of N.Z. Sl. [as cit. 1988].