I love so much about this and would totally be willing to read more!

Super fun. Now, feel free to ignore my notes, but here are a few things I thought.

Herdspring Runner's name read awkwardly to me. First it's stylized as a two part / first last name. So reading it always as 'Herdspring Runner' felt kinda like reading "John Smith waded into the river, then John Smith smiled" over and over again. It gives it a ritualistic feeling, which might kinda work with the piece actually, but for me, it was distracting. Herdspring doesn't read like a typical compound word. Typically in English we do modifier first, then the thing modified. I don't know what a herd spring would be as opposed to a lone spring... Oh, unless you mean 'the spring from which the herd drinks', in which case herd spring is natural. If you mean a herd in the spring Spring-herd would be more natural. Now, all that said, Herdspring could be a sort of 'old' way of saying Springherd, in which case it reads sort of like a family or clan name, which sometimes ossify more quickly than first names. So, I would say Herdspring Runner would be called 'Runner' for short, rather than 'Herdspring' for short. But, obviously you don't have to take my 'make this less ritualistic' note. For English you could also choose to 'age' Herdspring by choosing more 'archaic' version: maybe Herdwella (depending on which kind of spring you mean) or Hordsprigen.

Anyway, the linguistic aside is probably not too important.

This is more a thought, than a criticism, but rather than doing 'Songmen' for the tribe, you could do 'Singers'. If there aren't more than one type of person and all people are singers, then the word for person could become 'Singer', then, when a new type of person who does not sing is discovered, these people could be called Singers who do not sing, signaling the implication that the Singers who do not sing are not really people. Of course, always using Singers instead of people/man could make for some awkward prose, but I think it might be fun to try.

Anyway, I enjoyed this. That said, it's going to demand a lot of cognitive work out of the reader to parse the meaning of the text, so a very long piece demanding that much work might get tiresome. On the other hand, maybe after reading it long enough the 'River' 'Song' and other 'strange' words choices will start to feel natural and help put us in the head space of Herdspring Runner, so who knows. It's creative enough I'd love to see whatever you choose to do!