A 1969 issue of _Spin_ quotes part of the song. I am only seeing it in snippet view.

[...]
What did de blackbird say to the crow?
    Mahund how you shwing yo' tail:
If yo' don' get sunshine yo' sure to get snow,
    Mahund how yo' shwing yo' tail:
Hilo below, hilo below,
    Mahund how yo' shwing yo' tail.

My instincts tell me that either this text comes from _The Crisis_ or (more likely) they both derive from the same source. (Although one toned-up or toned-down the dialect spelling!)

What was the common source?

I am guessing this Spin article was one of Stan Hugill's.

The Johnson Girls note in their liner notes, to an album on which they sing the song (Kasin/Adriano style), that a text of the song appears in Hugill's _Bosun's Locker_, i.e. the anthology of his _Spin_ articles. And they say Hugill got the verse from Marston and Allen's _Shanties_ (1921), a little 14 pg. booklet which I also don't have! Is this the common source?