Obviously this isn't the same issue as Sea-Eagle, Shrike-Thrush &c
I can't think of any other reduplictive onomatopoeic bird-names that are spelled as two words with a hyphen. Where there are such names they are usually spelled as one word, for example the African birds Brubru and Boubou. Other two syllable onomatopoeic names are also spelled as one word, ie Cuckoo, Hoopoe, Towhee, Veery &c
I think it's just a matter of preference with a name that is sui generis. My defence would be that in the GG's call the two syllables (where two syllables are produced) are distinct and equally stressed. Plus I think that Gang-Gang looks better on the page.
:-)
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John Leonard
Canberra
Australia
www.jleonard.net
I want to be with the 9,999 other things.