Translation:Is the small, green bird flying here or that red one?
My translation was, "Is the little green bird flying here or that red one?" The only difference was I put little rather than small and that means that the computer is being far too picky about what it will accept. There were no other "errors."
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Little and small are pretty much interchangeable. There are exceptions just as there are to many rules and a small boy might suggest something slightly different to a little boy. A little bird and a small bird would be the same unless there was something else somewhere in the text which suggested that one or the other might be more apt.