Check Musical Traditions http://mustrad.org.uk

There's an article in there on the blue yodel in country music. Yodelling in country music more or less has its genesis with Jimmie Rodgers.

As far as I can remember the article says that Rodgers picked the habit up after hearing Tyrolean vaudeville troupes. However, since he used to knock around and play with Black country blues singers, I wonder if the blues falsettos he would have heard from some of them might already have attuned his ears.

Incidentally, I'm sure it's pure bunkum, but Howling Wolf, himself a great exponent of blues falsetto, claimed to have met Jimmie Rodgers in the 1920s. Yep, according to the Wolf, he was the one who taught Rodgers how to yodel.