Not too long ago I was on a beach one morning to photograph birds. It was an optimal day for it, with a very low tide the low tide occurring at sunrise, which is a few day event in any given month. There were perhaps three other photographers on the beach and a couple of bird watchers. About 20 minutes after the sun rose I was composing a photo of a heron when the bird started acting very nervous for about 2 seconds and flew off in a panic. At about the same time I heard something that sounded fairly distant but I didn't know what it was. Looking away from the camera in the direction the bird looked before it flew, I saw every bird on the beach in flight, and a drone cruising not too high and moving along the waterline. My day, and that of everyone else on the beach except the drone's owner, was over in 20 minutes. I was not happy.
Birds, especially smaller ones, view larger flying objects at bird-like altitudes as predators, because until now they usually were. They're also sensitive to noise. I don't even like the sound level of my Canon shutters around them, but I stay far enough away that it's not a problem.