Nava insn't a spirit despite his spirit like apperence. I
think he was born with the ability to make himself one with
the tree. He must of been the shaman of the pack or
something like Silverstrike said.

Exactly what Silverstrike said. (Nice gob Silverstrike!)

No. As stated by Nava when he disappears; "I simply became
one with the tree."
This indicates that he is a
shaman, or a spiritual magician. As seen later in the movie,
he shows his living side because he is too old to follow
Alue and his old pack.

In short; No. Nava is
not a spirit, he is a shaman. He resembles Aniu a bit
because of lazy animators