The noise level varies a lot depending on the size of the drone, number and type of propellers, type of motors, etc. This one is a Mavic 3, with add-on "stealth" propellors that don't actually quiet it much, but change the pitch very noticeably to a duller lower pitch sound that is less intrusive.
For human ears, even out here in the quiet countryside, the noise is barely noticeable 100 feet away, and gone at about 200 feet. For this video, I had started higher, then slowly lowered the drone for a closer shot (about 50 feet up). As you can see, it did startle a few of them (definitely less than the kayakers do!) enough to fly up, but circle right back. I was careful not to intrude on them any closer, and then used the zoom lens in and out for the closer shots.
One does have to be careful around birds especially around predators, which will often attack the drones. The best tactic if attacked is to fly straight up, (birds can't do that), then angle away and out then lower altitude once safe.
Oh ok that makes sense with the different types of propellers, I suppose you'll never get it 100% quiet and animals can hear so much more than we do.
I didn't even think about a drone being mobbed by birds, that must be quite a thing to experience, but having an exit strategy like yours which seems pretty simple is a good tool to keep in the toolbox incase. Not many people have drones on this side, I see them occasionally but not often.
Looking forward to the next adventures you have 😁