I wonder why they would die from fledging? I've had birds nesting in my garage and shed, and they never had any trouble fledging onto a concrete floor. Do you think something is disturbing them, causing them to prematurely fledge? I'm not sure how you could get them to chose a better place to nest.

We had a phoebe build her nest on a security light above our deck, and after finding 3 or 4 nestlings that had fallen prematurely from the nest to their deaths on the hard deck below, we put an extra dog bed (large size!) underneath the nest, and sure enough, the next day I found a live nestling on the dog bed. I couldn't reach the original nest, but put a last years' nest in a margarine container, put the nestling into it, and pushpinned the margarine dish into our wood siding about 2 ft below the original nest on the security light, which still had 2 nestlings in it. Mama fed the rescued nestling in its improvised nest along with the rest of the babies, and I was fortunate enough to see it fledge a week or so later.

So my suggestion: find something large & cushiony and put it below your phoebe's nest. :)