Originally posted by BrianR Birds or other skittish wildlife ... if you're up close a loud whirring screw drive can set then fleeing before the loud flappy mirror sends them fleeing, which can lead to a photo of the animals bum.
It is well understood among wildlife researchers that it isn't so much the amount of noise as the nature of the noise.
Wildlife usually occupies a given territory and adapts to that territory including what it accepts as "natural" to it's territory.
So if a bird has nested and grown up in a bush 30 feet from a railway track a freight train rumbling by is just a natural part of his territory and is accepted as such.
Get withing 30 feet with a relatively quiet whirring screw drive and the bird will spook. That sound is new and different and new and different is dangerous within it's territory and is interpreted as such.