Good on you. Wildlife has enough of a challenge feeding themselves every day without us interrupting them. In theory one reason bird photographers decry BB shots is to dissuade people intentionally flushing birds to get flight shots. Unfortunately we can't read their minds so we all flush them from time to time. Goodness knows in the past few days dealing with unfamiliar species I've seen seen quite a few tail feathers
You may be over complicating things. Particularly since you shoot in manual (I assume you mean REAL manual mode WITHOUT auto ISO enabled?) Knowing you're shooting white birds you can simply set up for the sunny f22 rule and fire away. Living in higher latitudes this time of year you may need to tweak the rule a bit. The sun is low enough in the sky for some light to be filtered even on (seemingly)clear days. In a situation like this where the bird is there and you're waiting for something to happen it's even easier. I just shoot a couple of sample shots, check the histogram, and adjust settings. And continue waiting....
I enjoy your posts. Love the area you live in. It's top of our list of places to retire to if/when we leave Alaska.