[svandamme]

Horse dung is better because a cow processes it's stomach(4 stomach lobes) content much more, as such the cow dung is more depleted of nutrients.

You can compare a cow and a horse poop side by side, cow is just yer run of the mill diarrhea ,
while horse dung is way more solid, dry and when you break it up you can find fibers (if they are grass/hay fed that is.) Most dogs love a got bite of horse dung, but you won't catch em chowing down on a cow patty.

Basically horse dung has a lot of organic content, and low nitrogen or acidic stuff
bird chit has even higher organic stuff, but also high in nitrogen and fosfor stuff, pretty good