Well, I am not at my best this morning, but what does it mean that I retyped the poem so far like this? "birthday secret/wishes whistles". We'll have none of that, Madam Editor. You picks 'em as you finds 'em!
The real fourth word is fiery, from Buffy. Again, a surprising option and a nice foil to the cheerful note of "whistles," so now the birthday secret might have some heat to it. I tried out softly, wholly and lingers but couldn't seem to work with the excellent three-syllable choices, wanting to preserve the strong oh-gosh-I-can't-remember-which-foot-this-is rhythm.
A comma seems useful here, but of course Madam Editor reserves the right to change her mind about things like line breaks and punctuation as the poem shapes and reshapes itself. Boy, this is fun. Thanks, everyone!