I think I might be able to help, and this seemed a like a good topic to tackle.
It just so happens that while browsing past images this one struck me. I had already done the edit, and tonight framed it and added a logo.
So, first thing is - sometimes a butt shot is ok and cool.
But if what is happening is that every time you see a bird it flies away, then yeah, that's a problem.
Birds will do what they are doing until they want to do something else. If you are not there they will do their own thing. If you are there, you can either watch and see what they'd do on their own, or you can influence things and then see their reaction. The trick really is to be observant, and to tell when your presence is affecting things.
With some observation, you can start to guess at what might or might not cause the flight-response.
I've even used that simple premise as a way to get CLOSER to birds. Here's how - if I see a bird, I'm watching, and I see someone else on a trajectory towards me and the bird, I will back off. Give the bird lots of room, so I am no longer a part of the equation. Then I (sometimes, and sometimes succeeding) have tried to predict what the person approaching might do, and what the bird might react by doing. Then I've moved and sat or positioned myself in a non-threatening position, and waited. It doesn't always work out, but some times it does.
Butts shouldn't always be seen as a bad thing.
But if that's all you get to see you need to rethink your approach and try to think like the bird a little.
If you are approaching to the point that the bird always flies away, then, you need to not approach so close and learn to read the birds better. Some birds don't like friends and will always fly away (like a kingfisher) but others will tolerate you if you do it right.
Other things you can do to limit your impact when out looking for birds are:
Original content posted at natureandwildlifephotography.blogspot.com/Nikographer.com / Jon