Welcome to WAB. I've been reading this thread with interest.
I am sure that as the natural food dries up and birds become hungrier they will come to your feeders but I do feel that lack of natural cover is a problem at the moment. When ever a bird is out in the open it is vulnerable. Your apple tree will be great when it is bigger and it is good of you to take such a pro-active approach.
I wonder if I could make another suggestion though, would it be possible to add a trellis along the top of your existing back fence? A couple of feet of trellis would give birds somewhere safer to perch and also make it difficult for cats or other predators to walk along the fence top. Just a thought. If you could grow some climbers up the fence then even better as you'll provide insects for the birds to feed on as well as cover.
Susie