That's something that the drone world really needs to take on because there can be a world of difference in drone size, prop types and motor types to reduce impact on the wildlife we're studying.
I've had one real world experience where a Marmot about the size of a large cat was scared of a small drone (DJI Mavic Pro)
But a drone about half the size that could do the exact same work was able to fly 15 feet from the Marmot and not disturb it. If you're only needing an observation drone to take photos and videos I highly recommend the DJI Mavic Mini. I'm not a rep for them but fly their products a lot.
If you have any questions I'm happy to help.