birds are more sight oriented than smell oriented.

They have great sight, sure, but they are no slouches when it comes to
odor. Check out this link for more:

http://www.valleywild.org/TVfacts.htm



Note that having a sensitive sense of smell should also be a weakness in some contexts.

And then there's the "fade out with time" issue. Plus, to do it right, there'd have to be effects from perfume, rotting corpses and so on.

Both of those comments only make me like the idea better.



Finally, I think the results you're proposing are far too specific. There's already blood trails which accomplish the bulk of what's probably reasonable.

"Smell" goes right along with the theme of blood trails, but gives it
a logical boost where animals are concerned (as one would expect.)

It might be plausible to introduce some utility form with this as its utility ability, but I suspect this involves way too much work for the gain.

That completely contradicts the idea, which was to give ALL animal
forms somewhat "heightened" sense in this area, where they should
be.

Bizarro I'm helping!