I recently purchased a house and have decided to add two nest boxes to the back yard.
I have been doing a lot of landscaping and have added a 50'x10' prairie garden in the front, almost all native plants with many grasses. The back yard is still in progress but I have added many native plants but with a heavier focus on non-native ornamentals. There are several native fruiting trees and shrubs including native red, European, and our native Canadian Elderberry. Several red currants, Honeyberries, and two species of Service berry. I have tried to do as much as I can to provide for the birds. There is also a small Japanese style fountain with a solar pump to keep that water moving during the day. This will eventually be incorporated into the shade garden near a Pagoda Dogwood.

The questions I have are concerning my nest boxes and their placement, also deterring squirrels and House Sparrows.
I have added a 1/2" piece of wood over the entrance hole of each nest box to make it more difficult for predators to reach inside; also each box has a copper portal (1 1/4" and 1 1/8"). There are squirrels and chipmunks in the yard. We have also seen a Opossum. We don't have to worry about snakes here and the species we have are not known to climb trees or to prey on bird's eggs.
I am planning on putting one of the nest boxes on a metal pole with a PVC baffle. The other nest box will go on the side of our vinyl garden shed, about 4' off the ground. There is also a large maple tree in the yard that is in rough shape and I was considering nailing the nest box to one of the areas where a large dead limb was removed. Is this still a bad idea even with the precautions I have taken to protect the nest box entrance?

The House Sparrows use the back and front yards to dust bathe and they also use my small fountain to bathe in. I am afraid to offer nest boxes with 1 1/2" entrance holes for fear that the Sparrows will take over. Several neighbors feed the sparrows daily and there are at least five or six nest boxes close to my property that are occupies by the sparrows, including a large Purple Martin house on my neighbors front lawn.
I have yet to start feeding the birds and may wait until winter. I am afraid that the House Sparrows and squirrels are going to be a constant battle otherwise.

Any suggestions to help avoid Sparrows and squirrels, while still offering a good habitat for other birds? I was hoping to attract Chickadees, Wrens, and Nuthatches to nest. I have a Suet feeder for Woodpeckers and Nuthatches as well as a black oil sunflower feeder that is a ball shaped wire hanging feeder.

Thank you.

Mike