I think the one with Herbert Birdsfoot singing is called "High, Middle, Low". It goes like this:
I can only sing the high part
It's the only part I know
And when I sing the high part
This is how I go:
La, la la la la la la, la
La, la la la la la la
And then Ernie and Bert sing the middle and low parts, and then they put all 3 parts together. Now it was done by Herbert Birdsfoot, Ernie, and Bert on the album Sesame Street 2. On the show, it was done by 3 anything muppets.
And I think the reason why they got rid of Herbert Birdsfoot was because people said he didn't really go anywhere, he was just like a regular school teacher, or a regular person. Apparently, nobody liked him.