They *can* -- at least the lighter ones can. But my experience with my fence is that most of them *don't*. :)
My California White flies like a helicopter and goes in and out at will. I don't worry about her because she's the only bird in the flock bright enough to fly back in instead of pacing...
What Dobie said.
I never saw the neighborhood loose dog test my fence, but my neighbor lost all but a couple of his chickens and ducks and I didn't lose any.