- The use of a flash in TTL mode fires a pre-flash to assess the required output for the exposure. This brief pre-flash may trigger a response in subjects, that than can show in the real frame.
- The popping sound of a flash firing, just like the aperture closing down and shutter opening and closing, can cause a response in subjects. Best hummer shots I took a few years back were taking under a sound curtain of heavy metal and hard rock blasting from a boom box on the patio.
- Using flash a fill during daylight and using flash as main or only light source at night are two different beasts. The first scenario will not really affect vision of the subject. Shooting a flying owl in the dark night with a flash giving max. output will have a different effect that may result in injury or worse.
So use common sense and ask yourself how you would feel driving down a highway being "flashed" head on during the day and how it would feel in the middle of the night with vision adapted for no ambient light. In each case what would be your chances to respond to a suddenly appearing obstacle in your path.
Interesting contentions worth considering.
I don't see any way round the first point. However, my actual experience is that it's not an image issue. If I encounter an issue in the future I'll let you know.
I've never heard my own flash firing, which may be because of other noises from the camera about which I can do nothing, so not really a consideration.
I've never shot at a flying owl in the dark but pictures of landing ones in prepared situations show owls arriving accurately onto the perches aimed at. In any case, as previously stated, I prefer to understate the flash and not use max chat. In addition I'm not talking about prepared situations but active birding/mammaling at night, so I'm not in a zero light situation and nor is the subject: quite a lot of mitigation of the risk to the animal. Usually there is a crossing component to their movement so not from directly ahead of them. On top of all that, as mentioned, lightning happens to these creatures at random times and can be in considerable intensity, so sudden bright light followed by darkness is not unfamiliar to them.
I'm all for the use of common sense.