For a while now I have been organising some of my birdwatching so as to compare 4 different mid-price binoculars: 7x42, 8x30, 8.5x44, and 10x40. After all this I find that there isn't that much difference between the number of different birds seen with each, but I also find that if there had been a dramatic difference I would have spotted it. However if there was I would have expected bird survey instructions say more about binoculars. I have put a much more detailed writeup, complete with the raw counts, at http://www.mcdowella.demon.co.uk/BirdsByBinocular.html. Of course, these observations could well be specific to one particular birdwatcher, or one particular set of sites.
Interesting and thought-provoking ! This also seems to support the view that people should just use whatever they're comfortable with. Is this the end of the magnification debate as we know it ? (or just the beginning !).