First, I guess the conditions of the world are in order. It's approximately 85% the mass of earth and there is going to be a full ecosystem with all the usual types of bugs, insects, trees, and predators. The ecosystem obviously won't be the exact same as on Earth, but there will be the usual things like pollinators, seed/fruit-bearing plants, decomposers, several trophic levels etc. Oh, and atmospheric composition will be very close to Earth too.
Now, how would birds be different, physiologically, from what we are used to seeing on Earth? I realize there are a huge variety of birds from finches to eagles, but they all share some similarities.
I believe, the lighter gravity, assuming a world approximately the same diameter as Earth, would lead to less atmospheric pressure, right? So there would be less air for their wings to find purchase on. To me, that means birds would either have to be lighter or would have to work harder to fly.
But part of me is conflicted because with less gravity there will be less strain on the bird's skeletal system and it probably would have evolved to be lighter/weaker than whatever its bird counterpart is on Earth. And that lighter skeletal system might make up for the less dense air. So I don't really know which way to write this, whether birds should look too different from their counterparts here. Maybe there would be plenty of smaller birds but not as many larger birds of prey? Or the inverse of that situation?
Could any ornithology-minded individuals chime in? Or anyone that knows the types of physics that could help me figure out if something like a ~15% reduction in g-force would result in like an 80% reduction in atmospheric desnity or whatever. That's a little beyond my education haha.