I have never done this, well, only for the lack of funding really, but I have read along this concept...
Take the area you want to feed the birds, or certain birds..(*most people want to feed the smaller birds, chickadees, finches, sparrows, etc.*)
But if you take say a pole or a sheppards' hook...
put it in the ground and figure out a way to put some stakes in the ground and put a circle or square around the stakes... and figure out someway to get a way in for your self...
Make the circle out of chicken wire...
**Small holes*...... now if you have that enclosed on the top, the only birds that can get in the holes are the small birds...
And inside of that area is where you've hung the feeders or whatever inside there to keep the smaller one's in and the bigger one's out!!!
Weezie
Actually when I first read about this concept,
it was to keep animals, such as cats
out of that area so they couldn't eat the birds...